Monday, March 31, 2014

Thursday Severe Weather Threat Could be Season's First Outbreak

I'm looking at Thursday for the nation's first chance at a severe weather outbreak this season.

Note: There will be some technical terminology in this post. For explanations of a specific topic, rather than copying and pasting the information in a time-consuming manner, I will paste the link to an explanation alongside the topic discussed.

TwisterData
We look to see an upper level low emerge over the Central Plains by Thursday afternoon, taking on an apparent negative tilt by the time we reach Thursday evening. The presence of a negative tilt, depicted in the 500mb vorticity image above as the maxima vorticity values pushing to the southeast, will add to the intensity of these storms, as it is known that negatively-tilted storms define the mature stage of a storm system, priming the atmosphere for a major severe weather event. (More on negatively-tilted storms in middle of this post). As the storm digs into the Plains, we will see ridge formation in the Central and East US, indicating the open flow of moisture and instability across the East, concentrated particularly in the Oklahoma, Arkansas, Missouri, and Louisiana/Texas areas.

TwisterData
A look at the jet stream for Thursday evening confirms the potential severity of the event, as we see a split jet flow across the United States. The jet stream splits over the same areas we mentioned above, as the subtropical jet stream is forced south along the Gulf Coast and the Pacific jet is pushed north into the Upper Midwest. This split indicates the presence of divergence over our severe weather area, highlighted below by the Storm Prediction Center. With divergence, we see air rising strongly into the middle and upper layers of the atmosphere, helping to form thunderstorms. (More on divergence on bottom of this post)

Storm Prediction Center outlook for Thursday.
Outlined area denotes enhances severe weather threat.
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Skew-T chart for northern Oklahoma
The latest run of the NAM model has come in quite a bit more tornadic than what other guidance suggests. This image is valid at 1 AM Central local time for early Thursday morning, not Thursday night, which we have been talking about up to this point. There is a severe weather risk on Wednesday night, and this skew-t chart shows it well. By 1 AM, we see just over 2000 j/kg of CAPE, a substantial amount of instability. We also observe the EHI, or Energy-Helicity Index to be at a whopping 7.4 for this time period. This would be a very dangerous situation, but it's not being talked about as much. The reason for that is because it looks like our capping inversion may hold through the night and kill off the risk. As the skew-t shows, we see the red temperature line not really curving to the left much from the surface to 700 millibars. This means temperatures want to stay warm, and that limits thunderstorm formation. It's possible this does end up being a substantial severe weather event, but I'm not seeing too much activity for Wednesday night. The big story is Thursday night.

---   ---   ---

TwisterData
Back to Thursday night, lower level winds look to be roaring just southeast of the storm center, as this forecast 700mb wind speed chart shows. These elevated lower level winds extend all the way down to the 925 millibar level, which isn't too high above the surface (a couple thousand feet, roughly). These lower level winds should act to strengthen the lower level jet stream, shown well on that 700mb chart above, which will then act with the rest of the environment to provoke this severe weather event.

My thoughts follow the Storm Prediction Center's outlook above, with the primary threats being hail, damaging winds, and an isolated tornado.

Andrew

Sunday, March 30, 2014

April 3rd Potentially Major Severe Weather Outlook

Confidence is increasing in a potentially major severe weather event on April 3rd.

This is an update to yesterday's post on this severe threat. A full update will come tomorrow.


The Storm Prediction Center has outlined the areas of northeast Texas, eastern Oklahoma, Arkansas, southwest Missouri, and southeast Kansas for a severe weather risk on Thursday, April 3rd. 


An upper level low will push eastward into the Southwestern states at the beginning of the workweek, reaching far northern Baja California by Wednesday evening, as the 500mb relative vorticity and height contour map shows above. We see that the maximum vorticity is not 'tilting' in any particular way, meaning it isn't pointing towards the southwest or southeast. Instead, we see the contour lines rather symmetrical around the upper level low, signifying a neutrally-tilted storm system.

USA Today
As the graphic shows, there are two types of tilts to a trough- positive and negative. The positive tilt trough sees the jet stream bending towards the southwest, as the Energy Pocket, also known here as the 500mb maximum vorticity, pushes in that direction. The negatively tilted trough indicates the system has reached maturity, as the vorticity maximum now pulls the system to the southeast. The mature storm now produces more vigorous storms, hence why we are concerned more when a negatively tilted trough comes around, compared to a positively-tilted trough.

When looking back to the GFS model forecast at the top of this post, we can now confirm that the storm is at a neutral tilt. This means it is stronger than the positively-tilted storm, but not at the mature level that is defined as a negative tilt. Because the upper level trough is of a neutral tilt, we still have to be concerned about the formation of a capping inversion that must be eroded before the storms can commence.

Example of a capping inversion

This graph might look complicated, but we're only going to look at the circled portion. The red line represents temperatures, and the green line depicts dewpoint. The lines bend and move as you move up the graph, as they show temperatures and dewpoints with height- the height legend is located on the left side of this chart, in hPa (interchangeable with millibars). If we start at the surface, at the bottom of the graph, we see how temperatures begin to decrease with height. This indicates instability, as warm air rises and cool air sinks. However, we suddenly arrive at a point in the atmosphere, around 850 millibars, when the temperature warms quickly and the dewpoint drops. This means the air from the surface that had been rising can no longer rise, because temperatures have now warmed above what they were a bit lower below this circled portion. Thus, thunderstorms cannot form. This phenomenon is the capping inversion. The capping inversion can be broken, as the warm temperatures in the circled portion are cooled down. When they cool down enough, the capping inversion is broken and thunderstorms can now freely form. A 'broken cap' is shown well in this example graph below.

Example of no capping inversion
In this graph, we no longer see the sudden warming of temperatures. All we see is a steady cooling in temperatures throughout the troposphere, which tells us the cap has broken, and thunderstorms are free to form.

There actually is a very slight capping inversion in this graph, too. Can you find it?

If you guessed it was located at the very bottom of the red line, you're correct. There is a slight inversion at the surface as temperatures warm a bit before rapidly cooling. The red line breaking to warmer temperatures only slightly signifies a weak cap that should be easily broken.

With our neutrally-tilted storm system, we're going to need to break the cap, but when that does happen (which it will), the fireworks will begin.


Note: Two images will be displayed below. There will be the jet stream forecast on top, with an example graphic on bottom. The top graphic (forecast jet stream) will be discussed first, before we shift to a discussion about the different regions of the jet stream, which is when the example graphic will be used. Bear in mind the example graphic is NOT a current forecast.

Forecasted jet stream for April 3rd
Divergence circled, more on divergence will be discussed below.
EXAMPLE GRAPHIC for the topic discussed below.
NOT A CURRENT FORECAST.
The top image of the two above shows the GFS jet stream forecast for the evening of April 3rd, when we expect this severe weather event to occur. There is an area of divergence that I circled, on the right exit area of the jet stream (continue reading for explanations on those terms). In the bottom example image, I separated the jet stream into four sections. We have the 'Left Entrance' region, on the bottom left part of this diagram; the 'Right Entrance' region in the bottom right; the 'Left Exit' region in the top left, and the 'Right Exit' region in the top right area. The severe weather event looks to be located over the Right Exit region, and being in the Right Exit region is a big deal.


In the Right Exit region, we see divergence aloft, meaning air is being pushed up and outward, as the diagram above shows. This divergence acts as a helper for the formation of convection, including (but not limited to) thunderstorms. Even in winter, being in an area of divergence allows for the formation of snow. If the divergence is strong enough, thundersnow can occur as well (with other atmospheric conditions cooperating, of course). For our severe weather event here, with the divergence centered in the Right Exit region, I'm closely watching Oklahoma, Arkansas, and portions of Missouri and Texas for severe weather this Thursday, April 3rd.

Andrew 

April 9-15 Multiple Potentially Significant Storm Systems

I'm seeing the threat arise for not one, but two potentially significant storm systems.

Tropical Tidbits
The GFS model has been consistently bringing in a strong upper level low into Japan around April 4th, beginning to attain a negative tilt on the image above, valid for the afternoon of April 4th (for more information on negatively-tilted storms, please click on this link to see the post published yesterday on this topic). There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. If we take the April 4th day and extrapolate it out 6-10 days, we arrive at the April 10-14 timeframe for what could be a hefty storm. I say it could be strong, as the strength of these East Asian systems has been reflected in the resultant United States storm . For instance, a strong storm over Japan does usually result in a strong storm in the US 6-10 days later, and that's what we're looking to see in this April 10-15 timeframe.

But we're not just looking for one system. This time, there are indications we could see two systems.

Tropical Tidbits
About a full day after the original system moves out from Japan, we see another swath of significantly below-normal heights enter Japan. The GFS image above, now valid for April 5th, reflects this, and we can see our first storm system that was discussed above now located just west of the ridge in the Bering Sea. This second storm system is kind of a tricky one. I'm watching closely here, as it could end up being one storm with residual cold weather just hanging behind. However, this forecast says we are in for two storm systems, and since we're entering spring, these strong storm systems can create nasty severe weather. For that reason, I'll err on the side of caution and highlight two storms in this post, but do realize that this may switch back to one significant storm.

The pattern I had highlighted earlier last week, which showed how the Northeast was at the most risk, is now a bit more hazy than when we last analyzed this timeframe. Model guidance is no longer as favorable for an East Coast impact, but rather than drop that region from a potential impact zone, I'll still tentatively keep the Central and East US in line for this storm. We should know much more about what this storm(s) could do in about 4 or 5 days from today.

As you can tell, there's a lot of uncertainty. Let's sum up what we do know.

- There is the potential for at least one significant storm system around the April 9-15 period.
- Severe weather does look to be a potential factor in this timeframe.
- Cooler and unsettled weather can be anticipated for this timeframe.

Andrew

Saturday, March 29, 2014

April 2nd Severe Weather Event Forecast

It's looking more likely that we'll see a severe weather event over the southern Plains.


An upper level low will push eastward into the Southwestern states at the beginning of the workweek, reaching far northern Baja California by Wednesday evening, as the 500mb relative vorticity and height contour map shows above. We see that the maximum vorticity is not 'tilting' in any particular way, meaning it isn't pointing towards the southwest or southeast. Instead, we see the contour lines rather symmetrical around the upper level low, signifying a neutrally-tilted storm system.

USA Today
As the graphic shows, there are two types of tilts to a trough- positive and negative. The positive tilt trough sees the jet stream bending towards the southwest, as the Energy Pocket, also known here as the 500mb maximum vorticity, pushes in that direction. The negatively tilted trough indicates the system has reached maturity, as the vorticity maximum now pulls the system to the southeast. The mature storm now produces more vigorous storms, hence why we are concerned more when a negatively tilted trough comes around, compared to a positively-tilted trough.

When looking back to the GFS model forecast at the top of this post, we can now confirm that the storm is at a neutral tilt. This means it is stronger than the positively-tilted storm, but not at the mature level that is defined as a negative tilt. Because the upper level trough is of a neutral tilt, we still have to be concerned about the formation of a capping inversion that must be eroded before the storms can commence.

Example of a capping inversion

This graph might look complicated, but we're only going to look at the circled portion. The red line represents temperatures, and the green line depicts dewpoint. The lines bend and move as you move up the graph, as they show temperatures and dewpoints with height- the height legend is located on the left side of this chart, in hPa (interchangeable with millibars). If we start at the surface, at the bottom of the graph, we see how temperatures begin to decrease with height. This indicates instability, as warm air rises and cool air sinks. However, we suddenly arrive at a point in the atmosphere, around 850 millibars, when the temperature warms quickly and the dewpoint drops. This means the air from the surface that had been rising can no longer rise, because temperatures have now warmed above what they were a bit lower below this circled portion. Thus, thunderstorms cannot form. This phenomenon is the capping inversion. The capping inversion can be broken, as the warm temperatures in the circled portion are cooled down. When they cool down enough, the capping inversion is broken and thunderstorms can now freely form. A 'broken cap' is shown well in this example graph below.

Example of no capping inversion
In this graph, we no longer see the sudden warming of temperatures. All we see is a steady cooling in temperatures throughout the troposphere, which tells us the cap has broken, and thunderstorms are free to form.

There actually is a very slight capping inversion in this graph, too. Can you find it?

If you guessed it was located at the very bottom of the red line, you're correct. There is a slight inversion at the surface as temperatures warm a bit before rapidly cooling. The red line breaking to warmer temperatures only slightly signifies a weak cap that should be easily broken.

With our neutrally-tilted storm system, we're going to need to break the cap, but when that does happen (which it will), the fireworks will begin.


This image shows the GFS jet stream forecast for the evening of April 2nd, when we expect this severe weather event to occur. In this image, I separated the jet stream into four sections. We have the 'Left Entrance' region, on the bottom left part of this diagram; the 'Right Entrance' region in the bottom right; the 'Left Exit' region in the top left, and the 'Right Exit' region in the top right area. The severe weather event looks to be located over the Right Exit region, and being in the Right Exit region is a big deal.


In the Right Exit region, we see divergence aloft, meaning air is being pushed up and outward, as the diagram above shows. This divergence acts as a helper for the formation of convection, including (but not limited to) thunderstorms. Even in winter, being in an area of divergence allows for the formation of snow. If the divergence is strong enough, thundersnow can occur as well (with other atmospheric conditions cooperating, of course). For our severe weather event here, with the divergence centered in the Right Exit region, I'm closely watching Oklahoma, Arkansas, and portions of Missouri and Texas for severe weather this Wednesday, April 2nd. The Storm Prediction Center had outlined that area yesterday, but revoked the outlook today due to model disagreement. Despite this, the indications are there that we will see the season's first formidable severe weather event on April 2nd.

Andrew

Thursday, March 27, 2014

April 9-13 Potentially Significant Winter Storm

I'm looking at a potentially significant storm system around the April 9-13 timeframe.

Tropical Tidbits
The image above shows the GFS model forecast valid on April 3rd, depicting 500mb height anomalies over the western Pacific. Warm colors depict high pressure and quiet conditions, while cool colors indicate the presence of cool and stormy weather. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. If we see the presence of a storm system over Japan on the evening of March 3rd, we find that we can expect a storm system in the April 9-13 period. What's more, the storm is clearly of a very strong caliber, indicating that the resultant storm in the US should be of a strong caliber as well.

If we take a look at the image above, we see forecasts of four teleconnections: the Pacific-North American (PNA) index on the top left, the North Atlantic Oscillation (NAO) on the top right, the West Pacific Oscillation (WPO) in the bottom left, and its close neighbor, the East Pacific Oscillation (EPO) on the bottom right. We're going to pay close attention to the two top panels, the PNA and the NAO.

The Pacific North American index deals with 500mb height anomalies in the West US and into the Northeast Pacific. When we see high pressure dominating this area, we deem it to be a positive PNA. When we see storminess over the aforementioned regions, a negative PNA is in place. Looking at the forecast graphic out until about April 9-13 timeframe, we see that the PNA is projected to transition from negative to positive. If we remember that the positive Pacific North American index phase tends to enhance the risk of cold and stormy weather in the Central and East US, this adds to the potential for a pretty dynamic storm, not only in the winter sector, but the severe weather sector.

The North Atlantic Oscillation (NAO) is an index that can help us calculate the risk of wintry weather impacting the United States. When there is strong high pressure over Greenland, the NAO is said to be negative, and the jet stream buckles south to send cold air into the East US. The jet stream also bends north along the East Coast to help out the prospects for coastal storms. In positive NAO scenarios, strong low pressure dominates Greenland, leading to a mainly west-to-east jet stream, not too wavy, lowering the risk of significant cold or warm weather intrusions. We see that latest forecasts have the NAO staying negative throughout this timeframe, enhancing the risk that this storm will have a wintry component to it. Additionally, the 'wavy' jet stream may also enhance the risk for severe weather, should it arise to be a threat with this system.


We also see the tropics playing into this storm threat. Looking at this map, we see a body of divergence and negative outgoing long wave radiation (-OLR, indicative of enhanced tropical convection) placed south of India. The placement of this enhanced tropical convection is in the El Nino phases of the Madden Julian Oscillation (MJO). In other words, the enhanced tropical convection placed south of India will induce weather conditions similar to an El Nino, which favors a storm track along the East US, with cold weather also in that area. This tropical forcing, combined with the states of the PNA and NAO, tells me that we should be watching the East US for this storm threat.

Here's a map outlining my thoughts on who may be affected, based on the data presented above.



To summarize:
- There looks to be a storm in the April 9-13 period.
- Indications are this storm may be of a rather strong caliber.
- The projected atmosphere tells of an East US storm threat.

Andrew

Tuesday, March 25, 2014

2014 Severe Weather Season Outlook

Hello everyone, this is the 2014 Severe Weather Season Outlook, made by The Weather Centre. In this outlook, we'll go over current atmospheric factors playing into the upcoming severe weather season, go into a few patterns we saw during the winter that are likely to repeat themselves this spring, and look over analogs that we can use to help forecast this severe weather season.

Notice: If you don't want to read the technical discussion, you can scroll down to where it says 'Stop!' to get down to just the map and summary outlook.

We'll begin with an analysis of sea surface temperatures across the world.



If we take a look around the world on a sea surface temperature anomaly (SSTA) map, we find three areas of concern that we'll address today. We will discuss the temperature anomalies over the Northeast Pacific, anomalies over the eastern Pacific, and anomalies in the northern Atlantic.

We'll begin by looking at the temperature anomalies over the eastern Pacific, offshore of Ecuador. Looking over there now, we see widespread below normal water temperatures, with a few swaths of above normal water temperatures along the Ecuador. Everything seems normal, nothing really out of place, but there is quite a disturbance brewing just under the surface.


(Refresh page if animation stops looping)
The animation above shows a depth-by-latitude cross-section of the waters just below the eastern Pacific along the Equator. In simpler terms, this shows you temperature anomalies underwater, just below the Equator, if you were to look at the water from the surface down to about 450 meters. If we look at this animation, we find an enormous swath of anomalously warm waters about 100 to 200 meters under the surface, moving towards the surface. This swath of waters is called a Kelvin Wave (you can learn more about the Kelvin Wave by reading these two posts: Post 1 and Post 2), and is known as a kick-starter to an El Nino event.




An El Nino is one of two phases of the El Nino-Southern Oscillation (the other phase is a La Nina), also called the ENSO phenomenon. In an El Nino (La Nina), we see warmer (cooler) than normal temperatures placed in the waters along the Equator, west of Ecuador. These warmer than normal temperatures in an El Nino then create an overall pattern similar to the one pictured above. We see the main Polar jet stream displaced north, allowing warm air to flow into the North US, and also see a rejuvenation of the Pacific jet stream, which then flows into the Gulf Coast and along the South US, creating cooler and stormier conditions. The strong Pacific jet stream is what helps enhance severe weather during an El Nino.

Right now, I would expect the Kelvin Wave to hit the surface and initiate the El Nino in about 4-8 weeks, meaning we should start seeing the effects of an El Nino take shape around May and into June, thus possibly enhancing severe weather prospects for that time period, especially in the South.

The second area we want to look at is the Gulf of Mexico.



If we look over the Caribbean and Gulf of Mexico, we see slightly above normal SSTAs across the entire Caribbean and into most of the Gulf of Mexico, the exception being the coastline waters. For severe weather, we see the strongest systems pull up massive amounts of moisture and create immense instability thanks to above normal water temperatures in the Gulf of Mexico. The warmer than normal waters instigate tropical convection more often, leading to more instability and available moisture for passing storm systems, thus raising the severe weather threat. Right now, I'm only seeing meager above normal water temperatures, with a serious buzzkill in the cold waters along the Texas coast. If we saw warmer waters in the open Gulf, I would be more optimistic, but most of the above normal waters are only 0.0º to 1.0º C above normal- that's not as much as you'd think it is. Thus, I would temper my severe weather expectations accordingly.

Lastly, let's go over the Northeast Pacific.

In the Northeast Pacific, we currently see dominating above normal sea surface temperatures. This has been a major factor this winter, helping to keep high pressure in place in the NE Pacific, leading to the jet stream plummeting south and allowing Arctic air to flow freely south into the United States. We call this sort of set-up a northwest flow situation, because the air is coming from the northwest. This graphic from WHAS11 shows a typical northwest flow set-up well.


This winter, the ridge of high pressure was located a bit further west than what is depicted in the above example image, which allowed cold air to flow well into the Plains, Midwest, Great Lakes and all the way to the Mid-Atlantic.

If we see this body of above normal sea surface temperatures (SSTs) in the NE Pacific sustain itself into spring, it means we might be at risk for more than a few northwest flow severe weather events.



The image above shows five examples from the Storm Prediction Center of northwest flow set-ups, and their corresponding severe weather outbreak locations in the hatched areas. Let's analyze each set-up.

Top left: The top left set-up involves high pressure presiding over the Plains, with deep low pressure protruding into the Northeast. This results in severe weather for the Upper Midwest. This type of set-up is not expected to be dominant this spring, but will happen from time to time.

Top center: This set-up sees high pressure centered in the Southwest, with strong low pressure dipping south into the Great Lakes, producing severe weather in the Ohio Valley. Again, this set-up doesn't look to be dominant, but I could see it happening a few times.

Top right: This set-up is brought on by the "summer polar vortex" dropping anomalously far south, resulting in ridging along the West Coast and producing severe weather in the Central Plains. This set-up is likely to be seen multiple times this spring and summer.

Bottom left: In this scenario, we observed ridging over the Rockies producing a northwest flow over the Central Plains and Great Lakes, leading to severe weather in the Southern Plains. This set-up is not expected to be dominant.

Bottom right: In this scenario, we see strong ridging offshore and pressure from strong negative heights in Canada leading to a severe weather event in the Midwest. The expected ridging in the NE Pacific will likely see this set-up play out numerous times.

In summary, northwest flow events typically see severe weather pushed further north than the typical alignment of Tornado Alley, which is shown below.



Let's now move on to my analog set for this severe weather season. I created these analogs based on the observed and predicted conditions of the ENSO phenomenon, sunspot values and anomalies, the Quasi-Biennial Oscillation (QBO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multi-Decadal Oscillation (AMO). Let's begin by looking over all years that placed into at least two of the categories that I outlined above.


The image above shows a composite of all the years that fit into at least two of the outlined categories. This composite shows you 500mb height anomalies, where the warm colors depict high pressure/ridging, and the cool colors indicate low pressure/troughing. In this image, we see the general idea is for this spring severe weather season to feature strong ridging in the northeast Pacific, possibly indicative of those warmer than normal sea surface temperatures we're seeing at the top of this post in the SST images. This strong ridging then leads to deep troughing over the Western US, as the widespread deep blues and purples show. We then see the emergence of ridging along the Southeast, as this pattern is exhibiting a typical negative Pacific North American (PNA) index pattern, as is shown below.



This sort of analog composite really fits the typical storm that favors tornadoes in Tornado Alley. If you take a look at that Tornado Alley graphic we looked at a little bit up from here, the jet stream is outlined as diving south through the Southwest before shooting north again as it enters the Plains. That tells me that the 'typical' Tornado Alley storm usually occurs in a negative PNA pattern, which my analog set says is likely to happen throughout the winter... or will it?


This composite, showing the same years as the one in the first composite year, but now projecting temperature anomalies at the surface, tells me we can also expect more than a few instances of the northwest flow regime. Why? Take a look at the deep negative temperature anomalies out West. We see the negative temperature anomalies along the West Coast (though I'm a bit skeptical of those, due to how high pressure has prevailed this winter in that area), but we also see negative temperature anomalies in the northern Plains, with warmer than normal temperatures in the East Coast. This temperature pattern is much more similar to this winter than the cold weather out West, and is indicative of a northwest flow pattern. This is why I still believe we will see a few northwest flow severe weather events, as well as the typical southwest flow events (the southwest flow pattern is depicted well in the 500mb composite image two images above this temperature composite).


Lastly, my analog set does keep the drought conditions around in the West, unfortunately.

I refined those analogs to a set that I felt fit the upcoming year better than all of those years jumbled together. The following images will show you my 'preferred' analogs.


This image shows 500mb height anomalies for my five preferred analog years. Once again, blues show negative height anomalies/stormy weather, while warm colors depict quiet, warm weather. We still see the general southwest flow regime outlined in my preferred analog set, with some troughing along the West Coast, and a stronger Southeast Ridge in place. We still see dominant high pressure out near the Bering Sea. This pattern actually tells me the severe weather season might be a little quieter than initially projected. The reason? This sort of pattern would likely produce a few instances of zonal flow, where we don't see any prominent high or low pressure systems- just plain west-to-east flow, nothing exciting. However, considering this map is made of five years of data averaged out, I'm not too concerned about that prospect for zonal flow ruining the season. 


The temperature composite for my five preferred analog years shows a cold spring ahead for those in the far north US, with warmer weather along the Gulf Coast. This sort of set-up tells me we'd be looking at a lot of cold air available for severe weather events, as well as abundant warm air from the south. All of this could add to the severe weather dynamics, and enhance the event's strength and expanse. Overall, these temperature boundaries could act to strengthen any severe weather events that hit the US, though we'll have to deal with each storm on a case-by-case basis, of course.

(If you scrolled down to see my severe weather season map and summary, STOP here!)

With all of this accounted for, let's look over my severe weather map for this season.



• I predict we'll see a higher than normal severe weather threat along the Central Plains and into the Midwest, thanks to instances of both southwest flow severe weather events and northwest flow events that would likely target this area. The temperature contrast along the jet stream (which is predicted to set up just south of this outline area, based on analog images I did not show here) looks to be pretty tight, enhancing the severe weather threat for that area.

• I also shaded in the Gulf Coast for an enhanced severe weather threat, as I expect we'll see an increase in activity as we move towards the spring and summer months in response to the expected emergence of an El Nino. If you recall, El Nino events help to strengthen the jet stream that runs through the South, thus strengthening severe weather prospects for that area.

• I colored in the New England area for a quieter than normal severe weather season. At this point in the game, I'm thinking much of the Northeast goes without more than a handful severe weather events this season, but it's the far Northeast I think will see the brunt of this quiet severe weather season. 

If you have any questions about this outlook, you may ask them in the comments below. 

Don't forget to share this outlook using the social media sharing tools just below this post!

Andrew

Monday, March 24, 2014

Stratospheric Final Warming Occurring; Sustained Warmth Just 2-4 Weeks Away

In an update to an earlier post I wrote, where I evaluated the state of the stratosphere and possibilities for the final warming occurring, I believe we are indeed seeing the final warming event, meaning spring weather could be as little as two weeks away.

*Note: In previous posts, I addressed the potential of seasonal temperatures returning in just a handful of days. That forecast still stands- I am looking solely at the stratosphere in this post.

The image here shows a black line, which tells us observed temperatures in the 30 millibar level of the atmosphere, and also shows a superimposed gray line, which depicts the average temperature at the 30 millibar level at a given time. If we look at observed temperatures right now, where the black line stops, we see that they are well above the gray line, as much as 20 degrees above normal. The 30 millibar layer is located very high up in the atmosphere, especially when you consider we live on the surface level, located at the 1000 millibar, and considering planes usually fly at 35,000 feet, or about 250 millibars. Now, this massive warming event is not an odd occurrence in the stratosphere. Every winter, we usually see a few instances where the stratosphere suddenly warms up, which we refer to as a sudden stratospheric warming (SSW), or major stratospheric warming, which is a weaker SSW. However, at the end of winter, the stratosphere flips from having persistent low pressure dominate the upper millibars (we call this the polar vortex (yes, it's actually located very high up in the atmosphere, NOT on the surface)) to persistent high pressure, as the stratosphere basically shuts down for the winter. This flip from low to high pressure is natural, and happens every year, usually in the early spring months.

How do we know when this flip occurs? We look for a big SSW event that reverses wind direction in the stratosphere. In the Northern Hemisphere, positive zonal winds are also known as 'westerlies', as they move to the east in a counterclockwise formation. That is why we look for areas of positive zonal winds to identify the polar vortex in the stratosphere, because the polar vortex is essentially just one big low pressure system. In the same sense, negative zonal wind anomalies define 'easterly' winds, as they blow towards the west. Recall that high pressure winds spin to the west in a clockwise motion, providing the reason why we look for negative zonal winds to tell if the polar vortex has weakened.

Let's apply that rule on zonal winds to this observed chart of 10 millibar winds. Oranges show positive zonal winds, or westerly winds, while blues define negative zonal winds, or easterly winds. We observed strong westerly winds in the upper stratosphere in December and into January, indicative of a strong polar vortex. However, as we began to move into February, we began to see weakening of these westerly winds, and perturbations of easterly winds, the result of brief sudden or major stratospheric warmings. These have usually seen the polar vortex weaken but then recover. However, at the bottom of the image, we see that there has not been a recovery yet. Based on the previous SSW events, the polar vortex should have recovered by now. Despite this, we see no resurgence of westerly winds. Considering it's now late March, it's time to face the facts: we're probably looking at the Final Warming event, thanks to the strong 30mb warmings and lack of a resurgence of positive zonal winds.

We can also take a look at past and present zonal winds across the whole stratosphere and troposphere, not just the 10 millibar layer. The image above shows a cross-section of latitude-by-height zonal winds, where positives are in oranges and negatives are in blues. They go from least recent on the top left, to most recent on the bottom right. You can also identify the images by the dates just above each image. Note how in each image, since March 16 to March 21, we have seen the lack of positive zonal winds in the upper stratosphere, shown at the top of each panel. We have also seen a weakening of positive zonal winds that still remain across the stratosphere, and even into the troposphere. Once again, the lack of positive zonal winds coming back after the SSW event looks to have ended tells me not only has the polar vortex essentially collapsed in the upper stratosphere, but the final warming is occurring.

This can be further confirmed by the presence of dominating high pressure at the 1 millibar level of the stratosphere. Note the lack of a sustained polar vortex...


To summarize:
• We are looking at the Final Warming occurring in the stratosphere.
• This event signals the end of winter in the stratosphere.
• Since the effects of stratospheric events take 2-4 weeks to propagate to the surface, we should see a cold shot followed by potentially sustained warmth roughly 2-4 weeks from now.

Andrew

Saturday, March 22, 2014

Strong El Niño May Be Materializing Faster Than Predicted

The strong El Nino that was discussed in a recent post, where we analyzed the possibility of an El Nino that may rival the 'Super El Nino' event(s) of the past, may be forming now- far sooner than originally thought. Note: This post is an update to my previous post on the impending El Nino, which can be accessed by clicking here.

Let's refresh our memories about the El Nino-Southern Oscillation phenomenon.


The image above shows sea level anomalies associated with the El Nino phase of the El Nino-Southern Oscillation (ENSO) phenomenon, as well as the La Nina phase. As is shown, an El Nino sees substantially higher than normal sea levels in the eastern Equatorial Pacific, while a La Nina sees below normal sea level anomalies. In the same sense, an El Nino observes warmer than normal waters over those same areas, and a La Nina depicts colder than normal waters in the same blue and purple areas shown.


The image above shows observed values of the Southern Oscillation Index, also known as the SOI. The SOI is calculated by observing pressure differences between Darwin and Tahiti near the Equator. Negative values of the SOI, namely below -8, indicate the dominance of El Nino-like conditions, while positive values above +8 show the presence of La Nina conditions. This graph goes back to January 2012, when we began to see a consistent negative SOI values. However, we then returned to typical fluctuations of the SOI in 2013. It is only now, in March 2014, that we have observed an absolute crash of the SOI in recent days, well into definitive El Nino territory. This collapse of the SOI well into negative values tells me the atmosphere is priming itself for the onset of the El Nino that we discussed a few days back. We've known for a while that the El Nino is on the way, but the SOI crash indicating an El Nino arising is happening far faster than originally predicted. But how can we know this is an actual El Nino emerging, and not just your normal fluctuation of the SOI?

The graphic here shows water temperature anomalies by depth over the eastern Equatorial Pacific, more specifically from the 140E longitude line to the coast of Ecuador, right along the Equator. Let's take a look at the top panel of this graphic, which shows the anomalies by depth (the bottom panel shows temperatures by depth, not anomalies). What we see is quite startling- there is a body of extremely warm water just under the surface of the Equator, controlling the waters 100 meters to 200 meters below the surface.  This swath of warm water tells us that there is a Kelvin Wave afoot.




This graphic, drawn up by Mike Ventrice, shows the sort of situation we're experiencing with the Kelvin Wave. As the Kelvin Wave pushes east, we see sea level anomalies rise in conjunction with the anomalous warmth. Ahead of the Kelvin Wave, we then see downwelling, which lowers sea levels and cools sea surface temperatures, leading to a 'false Nina', where the SSTAs might tell us there's a La Nina, but it's actually just a byproduct of the Kelvin Wave beginning to push east. 

This Kelvin Wave has been sitting in the waters below the Equator for some time now, but it is only recently that we're seeing the Kelvin Wave actually start to impact the surface waters. We can see how this is happening, as the temperature anomaly by depth image above shows two arms of warmer than normal waters stretching from the Kelvin Wave up to the top of the image. It's no surprise that the Kelvin Wave is impacting the surface. What is surprising, though, is how fast this is happening- the formation of an El Nino was expected to be around a 2-4 month timeframe, but now we could be looking at formation in just a matter of weeks.


We're also seeing skyrocketing upper ocean heat anomalies, confirming the idea that this is a true El Nino, and not just some random SOI fluctuation or the odd, small body of warm water pushing to the surface.

A more startling feature of this phenomenon is, even though we're seeing the warm water push to the surface, the Kelvin Wave is getting stronger rather than weaker.

This image shows three panels of observed sea surface temperatures on March 20th. We see climatology of sea surface temperatures (SSTs) on the top panel, actual observed SSTs in the middle panel, and anomalies of the observed SSTs on the bottom panel. We're going to be focusing on the bottom panel, where we can see the full strength of the Kelvin Wave. If you go back to the Super El Nino post, we saw the last observation of the Kelvin Wave at abut +5.35º C above normal. However, this image shows how the Kelvin Wave has strengthened to +5.64º C above normal- a +0.29º C increase in just a week! When you take into account that the Kelvin Wave that produced the 1997 Super El Nino was only +4.3º C above normal at the same time as this image (in March of 1997), it looks like we might be looking at the strongest Kelvin Wave observed to date. This would help the idea that we're looking at a Strong El Nino to evolve in the next few months (or weeks depending on how fast the Kelvin Wave propagates to the surface).

All of this tells me we are looking at the El Nino arriving much sooner than first predicted. Rather than the original 2-4 month timeframe I had been planning, it's possible we see an El Nino in just 4 to 8 weeks. I wouldn't be surprised if we saw the emergence of a moderate El Nino in about 6-12 weeks from today, with a strong El Nino arising in as soon as 12-16 weeks. This may seem like a long time, but when you consider my original projection for a potential strong El Nino was after August 2014, this is certainly a much sooner projection, reflective of what the Kelvin Wave has been doing in the last week or so.

Impacts on the United States include a potentially more active severe weather season, which I will address in my 2014 Severe Weather Season Outlook, to be issued tomorrow (Sunday). We may also see a more suppressed hurricane season, and this will be addressed in my 2014 Atlantic Hurricane Season Outlook, which will come out later on in April.

Andrew

Friday, March 21, 2014

Seasonal to Above-Normal Temperatures Just Days Away

Though one of the harshest winters to hit this nation in recent memory is still leaving its mark to this day, with below normal temperatures continuing to persist across the country, I'm finding new, concrete data indicating that warmer weather is as little as 10 days away - and it could be here to stay.

Tropical Tidbits
There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. In this sense, we can use the long-term weather pattern over Japan to predict when some seasonal, warmer weather will arrive in the US. Take a look at the graphic above. We see 500mb height anomalies from the GFS Ensembles over the Western Pacific. I use the ensembles rather than the GFS model itself, as I find the ensembles to be more useful in determining the synoptic (large-scale) pattern than the one model (remember the GFS Ensembles have multiple 'members' that add credibility and accuracy to its forecast). The image here shows 500mb height anomalies valid for March 24th, and looking towards Japan, we see oranges over Japan, indicating the presence of high pressure and warmer weather. Using the 6-10 day correlation, we can thus expect warmer, quieter weather in as soon as March 30th. We're about 9 days away from that today, and it might seem like one of those passing warm shots that then leads to another cold blast. The truth of the matter is, this warm-up looks to be sustained.

Tropical Tidbits
Fast-forward to the GFS Ensemble forecast on March 27th. Once again, we're looking at 500mb height anomalies over the Western Pacific. Looking at Japan, we see sustained high pressure and warmer than normal weather still over the area. At this point, if we go out to the end range of our 6-10 day gap from March 27, we might expect this warm-up to go as far out as April 6th. Bear in mind that this would be a relatively uninterrupted warm-up, as ridging would stick over Japan during this entire timeframe. No big storm systems, only quiet, warm weather. So, using this tool, we've established a warm-up timeframe of March 30th to April 6th. That's a full week of potentially non-stop seasonal to above-average temperatures!

But the warmth just keeps coming.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies over the Western Pacific, now valid on April 1st. We are still seeing strong ridging over Japan, meaning our warm-up here in the United States could extend as far out as April 11th, the end range of our 6-10 day correlation tool. Beyond this timeframe, the ensembles get too spread out and we begin to see less and less confident forecasts in terms of projecting 500mb height anomalies in above or below normal categories. This is natural, as the ensemble members typically diverge in more noticeable features as the forecast period goes on. Despite that, the agreement of ensembles on continuous ridging over Japan is remarkable, and would give us at least 2 weeks of sustained warmth.

That's not the only item arguing for a end to winter in the next several days- the stratosphere is also doing its part.


The image above shows two panels of observed temperatures over the North Pole. The left image gives you observed temperatures at the 10 millibar level of the stratosphere, with the average temperature line in gray. The right panel depicts observed temperatures at the 30 millibar level of the stratosphere, with the average temperatures for that level shown in gray. The 10mb and 30mb levels are pretty high up in the atmosphere, when you consider that we live at the surface level of 1000 millibars. They seem even higher up when you consider most airplanes only fly up to the 250 millibar level or so, which is around that 35,000 foot mark. You can see on both panels how temperatures have been really bouncing around in recent months, until the last several days, when temperatures have skyrocketed to well above normal levels. After a relatively calm November through January in the stratosphere, what's going on up there?

The polar vortex is collapsing.

In my post the other day, I discussed how we were seeing a reversal of zonal winds in the upper levels of the stratosphere, indicating the collapse of the upper stratospheric polar vortex, and the significant weakening of the overall polar vortex. I talked about how this phenomenon was likely the result of the Final Warming, which is a massive stratospheric warming event in the end of winter that dethrones low pressure and instates high pressure in the stratosphere, a natural occurrence that happens each year. These Final Warmings usually signal the end of the winter in the stratosphere, and typically result in the end of winter 2-4 weeks later here at the surface.


The image above shows a six-panel graphic, each marking a day's observation of zonal wind anomalies by latitude, as the legend on the bottom shows, and by height in millibars, as the legend on the left shows. Going from the top row of panels to the bottom row, from least to most recent, we see a notable and rather sudden drop in positive zonal wind anomalies in the upper right-hand corner. Looking closer, we find this drop in anomalies over the 1 to 10 millibar level, in the highest reaches of the stratosphere (remember that we are based at roughly the 1000 millibar level, so 1 millibar is far higher than even what planes cruise at). Now, this transition from orange colors to blue colors in the top right hand corner of the most recent observation, marked under March 19 2014, means that the winds have reversed. In the Northern Hemisphere, positive zonal winds are also known as 'westerlies', as they move to the east in a counterclockwise formation. That is why we look for areas of positive zonal winds to identify the polar vortex in the stratosphere, because the polar vortex is essentially just one big low pressure system. In the same sense, negative zonal wind anomalies define 'easterly' winds, as they blow towards the west. Recall that high pressure winds spin to the west in a clockwise motion, providing the reason why we look for negative zonal winds to tell if the polar vortex has weakened. In this case, rather than the positive zonal winds just weakening a bit, it looks like they completely reversed in the far upper stratosphere, marking a collapse of the polar vortex at that level. We saw that reversal of winds in the upper stratosphere a few days ago, but it's only now that we see a rapid decline in oranges and reds across the middle and lower stratosphere, as well as the staying power of the negative zonal winds in the upper stratosphere. This all tells me that we are indeed in the process of the Stratospheric Final Warming, and we should see the end of wintry weather in the next 2-4 weeks, when the effects of occurrences in the stratosphere usually impact the surface. There may be a chill in that timeframe, as this is still, to some degree, a stratospheric warming event, which can result in chillier than normal weather. However, this would likely be short-lived, as our East Asian tool tells us warmer than normal weather is the new norm in coming days and weeks.

To summarize:
• There are increasing amounts of data that suggests we will see sustained warmth to start off April.
• The stratosphere indicates we'll be seeing an end to wintry weather in just 2-4 weeks.
• Other indications suggest sustained warmth may be just 10 days or so away.

Andrew