Sunday, May 1, 2016

La Nina Expected for Winter 2016-2017

A La Nina is expected to develop this summer, and should reach maximum intensity during the winter of 2016-2017.

A look at sea surface temperature anomalies across the central Pacific reveals a change in progress. Over the last several months, we have been in a notable El Nino event, which is characterized by above to well-above average temperature anomalies in the waters along the Equator, west of Ecuador. However, in the last few weeks, we've started seeing those warmer water anomalies wane, and even be replaced by cooler than normal anomalies. This switch is occurring mainly in the waters immediately offshore Ecuador, as well as a little farther out to sea, between the 120W and 110W longitude lines.

We can trace this cooling to an upwelling event ongoing across the Equatorial Pacific. Shown is a cross-section diagram of the ocean at the Equator, from a depth of the surface to 450 meters. Ever since February, note how we've been seeing well-below normal water anomalies pushing towards the surface, and recently breaching the surface near Ecuador. This is the root cause behind those cooling anomalies on the surface image discussed earlier in this post, and so long as this upwelling of cooler waters continue, the rest of the Equatorial Pacific should continue to cool down from this El Nino.

But what about the future... specifically the winter ahead?

A chart showing the forecasted sea surface temperature anomalies in a portion of the Equatorial Pacific known as Nino region 3,4 shows our decline from an El Nino into 'ENSO-Neutral' conditions around May and June. You may recall that Neutral conditions are when the waters are too warm for a La Nina, but too cool for an El Nino, and this is defined when water temperatures in Nino region 3.4 are between +0.5 degrees anomaly and -0.5 degrees anomaly. We begin to enter a La Nina around August or September, when the average of model guidance dips below -0.5 degrees anomaly, the threshold for a La Nina. You'll also notice it reaches maximum intensity as we head into the winter of 2016-2017.

Now the big question: what does this mean for winter 2016-2017?
The big answer: we don't exactly know. But we do have an estimate of what things will look like if this forecast holds.

The above graphic shows typical weather conditions in a La Nina pattern. In the U.S., La Nina's tend to bring cooler than normal conditions to the Pacific Northwest, northern Plains, and Midwest. Wetter/snowier than normal conditions also tend to evolve in the Midwest, Ohio Valley, and portions of the Great Lakes. The Southern U.S., however, generally can expect warm and drier than normal winters out of La Nina's. This isn't a set-in-stone interpretation of what the coming winter will be like, but we could see something similar to this graphic set up.

To Summarize:

- A La Nina is expected to develop this summer, and intensity into the coming winter.
- As is typical in La Nina winters, the Ohio Valley may be wetter/snowier than normal, while cooler weather overspreads the North U.S. The South may remain warm and dry.

For more weather updates, follow me on Twitter: @akracki and @TheWxCentre.


Thursday, April 21, 2016

April 26 Potentially Significant Severe Weather Event

A potentially significant severe weather event continues to show up in model guidance for Tuesday, April 26th over the Central and Southern Plains.

Storm Prediction Center
The Storm Prediction Center's long range outlook for Tuesday places a 30% chance of severe weather within 25 miles of a given point across northern Texas, much of central and eastern Oklahoma, and southern Kansas. A 15% chance of severe weather within 25 miles of a given point is situated from central Texas north into Oklahoma (outside of the panhandle), much of central and eastern Kansas, and extreme western portions of Missouri and Arkansas.

This event will begin taking shape on Monday, April 25th, when the storm system that will act to ignite this potential outbreak makes landfall on the West Coast. 500-millibar speed max near 100 knots barreling into southern California and western Arizona will be associated with a vorticity max rounding the base of the main trough, whose energy will be extended from northern California offshore Oregon and Washington by Monday morning. This speed max will shunt the aforementioned vorticity max east, where it will then round the lee side of the trough and act to pull the main trough northward by Tuesday morning.

The main energy associated with the trough will continue moving southeast into the Southwest U.S. and strengthen as another speed max rounds the base of the trough, this time making it negatively-tilted. Recall that a negatively-tilted trough means the highest wind speeds are 'pointing' the trough in a southeast direction. This indicates the trough is mature, and is optimal for severe weather.

As mentioned above, by Tuesday night at 7 PM Central Time, we see a speed max just breaching 70 knots traversing the Central Plains, indicating the trough is now negatively-tilted and at max strength. The trough here is in prime positioning for a severe weather event to unfold across the Central U.S. in the evening and overnight hours on Tuesday, and will set the stage for continued opportunities for severe weather as the week continues.

Strong moisture flow prior to the trough's entrance to the region will allow surface dewpoints to surge into the 70s by Tuesday evening, as the above graphic shows. As such, instability on the order of 4000 j/kg to 5000 j/kg is anticipated across Oklahoma into northern Texas, approximately in line with the 30% delineation in the Storm Prediction Center graphic at the beginning of this post. This ample moisture will allow for cloud bases to drop below 1000 meters- soundings from points across central and northern Oklahoma suggest cloud bases (also known as LCLs) will be around 600-800 meters off the ground, which can be conducive for tornado development. 800 meters off the ground is a little high for prime tornado formation conditions, but ample instability may act to offset a portion of this.

Lastly, supercell composite forecasts off the 18z GFS suggest the environment most conducive for supercell formation will be from southern Kansas across central Oklahoma, just into north Texas, again in line mainly with that 30% severe threat corridor. Although supercell composite values are high in central Kansas, the best axis of instability will be displaced in extreme southern Kansas and central Oklahoma, and it is this latter region where the best potential for severe weather lies. In this region, taking current model guidance at face value, all hazards of severe weather would be possible. This would include a threat for strong, possibly long-track tornadoes, very large hail, and damaging winds.

To Summarize:

- A severe weather event is being forecasted for April 26th.
- Substantial uncertainty exists regarding storm initiation, threat level, and placement of best environment, among other things.
- As such, confidence is low with this event.
- Going by current model guidance, a potentially dangerous severe weather situation could evolve across northern Texas, Oklahoma, and southern Kansas on April 26th.


Wednesday, March 30, 2016

Severe Weather Discussion for March 30, Posted March 30

A severe weather event is forecasted to unfold across the Plains and Mississippi Valley today, March 30th.

           Large upper level low currently stationed across the Western US will provoke a potential severe weather event in the Plains and lower Mississippi Valley today. Situation is complicated as convection later in the day will be heavily influenced by morning convection, residual cloud cover, as well as dryline placement later in the day.

               Stout upper level low centered across Nevada and Utah will begin to flatten as energy currently located in the Dakotas is ushered northeastward in pace with the jet stream, which will be arching north of mid-level ridging currently stationed in the lower Mississippi Valley. This energy in the Dakotas will be drawn towards another system in south-central Canada, resulting in a band of elevated vorticity extending from Canada back to Nebraska at 12z today. Further, the vorticity max currently in southern California, extending in a semicircle offshore the West Coast into Oregon, will keep the upper level low positively tilted and aid in this flattening-out.
    Moisture flow will continue to amplify across the Southern Plains, with water vapor imagery showing the subtropical jet feeding a moisture plume into the region. As such, cloud development occurred across the area earlier Tuesday and continues to persist at the time of posting. To this extent, continued moistening of the planetary boundary layer through isentropic ascent, notable 850-millibar moisture plume advection, and substantial low-level warm air advection will result in areas of drizzle to showers overnight, particularly across Texas and Oklahoma. 3/30 00z KOUN sounding showed strong temperature inversion between 800-millibar and 700-millibar levels, and I suspect this inversion will be strong enough to suppress anything more than broad, generally-weak precipitation. Surface plots at 0700z show areas of mist and light rain across central and eastern Texas, in line with favored model guidance. Model guidance troubles will be discussed more in depth in the following sections. Uncertainty with this particular subject of morning precipitation will feed into the morning and afternoon discussions below.

Early Morning to Noontime…
         Convection-allowing model guidance has been promoting the formation of showers and thunderstorms along the Red River, most notably in southeast Oklahoma in the mid-morning hours today, following expected overnight precipitation and continued moistening of the lower levels. Not completely convinced we see hail threats as advertised by some, including the threat of up to tennis ball-sized hail as per NWS OUN in their hazardous weather outlook posted yesterday. However, as the atmosphere continues to become more conducive to severe weather through continued warm air and moisture advection overnight, will not rule out severe weather tomorrow morning, especially given expected values of elevated instability that may favor stronger storms. I do believe hail is a primary threat with these storms, especially in their elevated nature, but again, I am not completely sold on the idea of severe and/or significant hail.
         Primary point of this timeframe discussion is to what extent precipitation is ongoing by and after 12z. Model guidance has been hinting at potential convection across portions of Oklahoma, especially south and east in the morning hours, but has also left the door open for weaker, more broad precipitation in central and northern Oklahoma. Regardless, expect overcast cloud cover given continued lower-level warm air and moisture advection, as well as taking into account 3/30 00z soundings across Texas and the western Gulf Coast. I am not taking the HRRR model into account for this discussion, as although it is currently handling surface precipitation (or lack thereof) rather well, it brings surface temperatures across Oklahoma and Texas into the 80s and 90s, well above current and more-realistic temperature forecasts introduced by the NWS. Additionally, am not using the NCAR ensemble, as all members have weak, broad, and scattered precipitation at initialization (3/30 00z) across Texas, which was proven to not be the case when compared with 3/30 00z radar. In terms of morning precipitation, am currently favoring 00z 4km WRF-NMM model, as well as SPC SREF system, as both models handle initialization conditions at the surface with regards to precipitation coverage well, and perform well with expected high temperatures later in the day. Given high model uncertainty with this event, however, making these two forecasts ‘favored’ is not exactly as affirmative a word as it is made out to be.
          In all, expectation is for precipitation, possibly convective in nature, to be ongoing in portions of Texas and Oklahoma by daybreak today and in the morning hours in general. Most favored region for this activity, as promoted by model guidance and SPC probabilistic thunderstorm outlooks, is southeast Oklahoma into northeast Texas, east into Arkansas. Weaker and more broad precipitation also possible for central Oklahoma, where uncertainty is greatest.

Afternoon and evening…
        Remarkably complex situation will unfold for the afternoon and evening hours. It has been noted that the upper level low in question has been progressing east more slowly than anticipated yesterday, and this has been feeding into a further-west trend with the dry line positioning later on in the afternoon today. Case in point, 4.0km WRF-NMM guidance shows convective initiation in the afternoon hours along the dry line, which is projected to be planted almost immediately east of the Oklahoma panhandle in due north-south orientation. This is the most westward solution I’ve seen, and while it has handled overnight precipitation well so far as of this posting, and the upper level low has been progressing slower than thought, the far westward positioning of this dry line is something to monitor. To speak more frankly, this solution is suspect, although this model still remains among favored guidance as of posting time. 3/29 21z SPC SREF takes a nice middle ground between this westward solution and further-east guidance, placing the dryline approximately along a north-south line of Lawton, Oklahoma to Burlington, Oklahoma by the afternoon hours. As such, thunderstorm initiation along the dryline would threaten the Oklahoma City metro area, and eventually may threaten the Tulsa region.
        Concerns have been brought up frequently in discussions around the lack of convective initiation on the dry line in some model guidance, despite the presence of an uncapped, supercell-favorable storm environment. This would be the result of a lack of trigger mechanism, it appears, as some guidance shows weak convergence along the dry line. I am personally opposed to this solution, and while it may very well pan out, I am more pressed to believe favored guidance, which does initiate convection along the dry line in the afternoon hours.
In the event convection does initiate along the dry line, it is expected that thunderstorms would quickly become supercellular, with all modes of severe weather possible. The new SPC Day 1 outlook has placed central and southern Oklahoma in the Enhanced Risk of severe weather, primarily for potentially significantly large hail, in addition to damaging winds and a non-zero, yet relatively low tornado threat. Although it still retains unrealistically-high surface temperatures tomorrow afternoon, new runs of the HRRR model continue to gradually increase the coverage and intensity of convective initiation along the dry line in central Oklahoma. Initiation here is along the Lawton-Burlington line previously mentioned, but I am not willing to put stock into the HRRR currently given its unrealistic surface temperatures.
       Outside of Oklahoma, best tornado dynamics outlined by SPC Day 1 projection and SPC SREF guidance will be placed in Louisiana/Arkansas/Texas area, where a 10% hatched delineation has been posted in the Day 1 outlook. Despite this, as mentioned previously, non-zero tornado threats will exist in Texas and Oklahoma in addition to these more favorable dynamics.

        To summarize, potential severe weather event is on the table today for the Southern Plains into the lower Mississippi Valley, with high uncertainty still included in the forecast. Broad, weak precipitation expected to develop and move into Texas and Oklahoma overnight into the morning hours will likely temper a full-on severe weather outbreak for the Plains, but afternoon clearing and favorable dry line positioning may allow the formation for strong to severe storms in the afternoon across central and eastern Oklahoma. Widespread convection is expected for the eastern Texas / northern Louisiana / Arkansas region, where tornado dynamics will be best. Large hail will be a day-long threat for the Oklahoma area, particularly with a round of thunderstorms in the morning that may form in the midst of the weak, broad precipitation to produce hail, and then again in the afternoon when potentially-supercellular development occurs along the dry line.

Friday, March 4, 2016

Severe Weather Discussion for March 7th, Posted March 4th

This is a discussion for the potential severe weather event on March 7th, 2016 in Oklahoma.

Models Used: 18z GFS, 12z ECMWF, 15z SREF

    Energy currently elongated near the Gulf of Alaska, beginning to round the base of a strong upper level low in the Gulf, will continue to be pushed east as the strongest portion of the upper level low will retreat north and west over the Aleutian Islands by Sunday. System of interest will make landfall in California around 15z Sunday, digging as it does so to form a ridge across the Northern Plains. Small piece of energy will be sheared off into the Aleutian Islands upper level low as this landfalling occurs, and this will weaken the primary energy enough until it begins to re-organize while progressing into the Four Corners region. By 00z Tuesday, this strong shortwave will be located over western Kansas, with a much stronger upper level trough digging well south into Baja California, and eventually into Mexico. This stronger upper level feature will force a slight ridge over the Four Corners region, and also act to pump a large ridge over the Midwest and Eastern U.S., in response to the general long wave trough pattern in the Western U.S. into the Plains. As the shortwave rides the western fringe of this ridge, weakening will occur before the energy is ingested by another vorticity maxima riding the Canada / United States border.

    Potential severe weather event will become set up with the strong shortwave moving into western Kansas, attaining what will pass as a negative tilt in the process. 500-millibar jet streak AOA 60 knots will form by 18z 3/07 over the Texas and Oklahoma Panhandles into western Kansas as the shortwave moves into the Plains. Jet streak will move east by 00z, with speeds weakening slightly to around 55 knots out of the southwest over much of Kansas and Oklahoma. Best wind features on the 700-millibar level at 00z 3/08 will be in the terms of a jet streak positioned in central / eastern Kansas, Oklahoma (outside of the panhandle and southwest portion of the state), northwestern Arkansas and western Missouri, with speeds up to 55 knots out of the west-southwest. 850-millibar wind field finds southerly winds across the Plains, with speeds of 40 to 50 knots in the Oklahoma area. Also will make note of sustained lower-level warm air advection, most intense from about 00z 3/07 until the time of the severe weather event. This advection will see 850-millibar dewpoint temperatures rise to the 10-12 degree Celsius mark, enabling moistening of the PBL. Most intriguing feature is 925-millibar wind direction at 00z 3/08 out of the south-southeast at speeds AOA 40 knots for much of Oklahoma, and slightly higher into Kansas. This veering wind profile is confirmed when observing projected surface winds at the same time to be almost out of the southeast, if not south-southeasterly.
    Surface low with strength of approximately 992 millibars in south-central Nebraska by 00z 3/08 will enhance moist airmass fetch from the Gulf of Mexico, tracing surface and 850-millibar wind fields back to the Galveston, Texas to Mobile, Louisiana region. Surface dewpoint projection from the 18z GFS show values nearing 60 degrees Fahrenheit by 12z Tuesday, although 00z Tuesday values see a lower, but still impressive swath of dewpoint numbers AOA 55 degrees.

    Storm Prediction Center currently outlooks western Oklahoma into central Kansas and eastern Nebraska, as well as much of central Texas for a 15% chance of severe weather on the long range Day 4-8 outlook for this event. Latest SPC SREF run shows a maxima of over 50% probability of supercell composite values AOA 1.0 occurring simultaneously with over 0.01” of precipitation falling - basically the probability of strong thunderstorms occurring - along the Red River south into the region immediately northeast of Abilene, Texas at 03z, with values gradually increasing and pushing almost due north towards Woodward, Oklahoma by 06z 3/08. The presence of this 50% to 70% maxima advecting northward during the night conveys uncertainty exhibited by the SPC SREF members, but also the confirmation that strong thunderstorms are possible in this environment. Projected 0-6 kilometer shear will increase past 00z Tuesday, as the nocturnal lower-level jet kicks in and the aforementioned veering wind profile continues to take shape across Oklahoma.
    GFS suite has taken a more aggressive tone in this event since a particularly bullish 00z run on 3/04, which triggered a potentially tornadic environment signal for a good portion of Oklahoma for this Monday night event. Analysis of forecasted soundings using the 18z GFS continue to indicate a potentially tornadic environment near Lawton, Oklahoma with SBCAPE exceeding 1800 j/kg amidst close to 60 j/kg CIN. Surface to 6km shear forecasted at 50 knots, combined with 0-3km storm-relative helicity values near 300 m2/s2 only acts to confirm this strong thunderstorm environment for the southwest Oklahoma area, the same region highlighted by the latest run of the SPC SREF suite for potential strong thunderstorms. Examination of forecasted soundings over Norman shows lower instability on the order of 800 to 900 j/kg SBCAPE, although values slightly higher for MLCAPE on the order of AOB 1000 j/kg. Storm-relative helicity and shear values generally consistent with Lawton numbers, although weaker instability and still-present PBL temperature inversion may pose problems for convection in central Oklahoma. Despite this, veering wind profile affirms potential for strong thunderstorms in the general state of Oklahoma, particularly if the inversion can be broken. Environment in Norman becomes far more favorable for convection by 06z 3/08, with SBCAPE and MLCAPE both taking a significant jump to AOA 1200 j/kg, although the presence of a now-weakened surface temperature inversion on the magnitude of 10-20 j/kg CIN still presents some concern. Additionally, substantial lowering of bulk shear and storm-relative helicity values, despite maintaining a veering wind profile on this forecasted sounding, highlights how factors may line up for strong to severe thunderstorms, but not necessarily an explosive severe weather event according to current projections.
    In terms of tornado potential- lowering LCLs as convection approaches, combined with sufficient instability and SRH / shear lead me to believe this event will pose the first notable threat for rotating thunderstorms this spring season. Not incredibly impressed with the potential for a large-scale - nor a large-number - tornado event, but the environment should be supportive of potentially tornadic thunderstorms. This will be further addressed as the timeframe of this event approaches.

    Overall, this event is showing signs of supporting strong to severe thunderstorms over Oklahoma. Highest concern from most recent model runs rests with western Oklahoma, particularly along a corridor of Eldorado to Comanche, to Buffalo to Medford. It is this corridor that should see the best forcing for thunderstorms, and will likely have the best threat for tornadic thunderstorms. Further east, in the Oklahoma City / Norman region, severe thunderstorms will still be possible, albeit higher uncertainty is present with potential capping and forcing concerns. Will re-evaluate this portion of the state in the next discussion to try and clear up this uncertainty, should successive model runs follow suit.


Sunday, February 28, 2016

March 1, 2016 Severe Weather Discussion for Oklahoma

This is a severe weather discussion valid for the morning of March 1, 2016 for the state of Oklahoma.

Models used: 00z NAM, 00z NAM-4km, 00z GFS, 21z SREF

Energy that will trigger this event is currently pushing into the Pacific Northwest / British Columbia region, moving directly into a small ridge positioned immediately due east of the system. Expectation is for the main body of energy to ride down on the lee side of a ridge that will build in across the Pacific Northwest as the energy continues moving eastward. Immediately south of this main piece of energy, at approximately 18z 2/29, emergence of higher mid-level vorticity values will commence across the Southwest, immediately ejecting southeastward into the Southern Plains. It will be this new piece of energy that will help to trigger the potential severe weather event.

Concentrated lower-level warm air advection will commence in the morning hours on Monday, with 850-millibar winds out of the west by 12z. Winds shift to southwesterly in the late morning, marking the warm air advection as high pressure shifts to the east, away from the Plains. Warm air advection aloft intensifies by 21z, and continues to build in magnitude through the late evening on Monday, up until the early morning hours on Tuesday when the potential severe weather event begins to unfold. Overall wind field for this event is messy. 500-millibar jet streak will round the base of the main trough as the event unfolds, reaching speeds AOA 80 knots in Colorado, as per latest model guidance.

Despite muddying of synoptic environment from previous discussion to this one, overall severe weather set-up has not deteriorated, and has actually strengthened in a handful of regards. Thunderstorms in this event will need to be elevated, due to strong temperature inversion consistent on forecasted soundings around the 900-millibar level. Surface CINH depicted at approximately -265 j/kg on latest NAM guidance, certainly conducive to a stable environment. However, 850-millibar to 500-millibar lapse rates of 7.8 degrees Celsius/km show a much more favorable environment above the PBL. As such, MLCAPE of just below 1400 j/kg with 87 j/kg MLCINH displayed at 06z Tuesday over KOUN. Wind profile shows substantial veering, perhaps with slight backing near 500-millibars, but overall much improved from previous model runs a few days ago. Surface to 3km storm-relative helicity values over KOUN progged at 435 m2/s2 from latest guidance, in addition to 0-6km shear around 50kts exemplify this improved wind field from previous model runs. LCLs outlooked at around 700 meters, certainly supportive of tornado formation, though perhaps slightly high in my opinion for any real tornado potential. SARS product over KOUN at 06z from 00z NAM depicts 65% chance of tornado, utilizing favorable MLCAPE and improved wind fields. Still skeptical of PBL temperature inversion, as well as depicted isolated storm coverage on 00z NAM-4km.

Most concerning point about this event, in addition to PBL temperature inversion, is projected coverage and evolution of convection. 00z NAM-4km projects evolution of thundershowers in extreme northern Texas, intensifying into either a group of strong cells or a single severe thunderstorm by 04z Tuesday, crossing into Oklahoma by 05z. Complex appears to strengthen and begin to bow out by 07z Tuesday, and begins to impact OKC/OUN at 08z, albeit in a slightly degraded severity. Complex continues to bow out and degrade through 12z, turning to the right and bowing out in the process as it moves into southeast Oklahoma. Scattered strong thunderstorm cells begin to fire at approximately 10z in eastern Oklahoma out ahead of the weakened segment of thunderstorms, and these could also pose a risk for marginal severe weather, in line with the latest Storm Prediction Center outlook (Slight Risk across most of central Oklahoma). In terms of probabilities, latest SREF suite places a 50% to 70% likelihood of thunderstorms occurring in an area with a supercell composite at or above 1. SREF members notably less enthusiastic with MLCAPE values over Oklahoma for this potential event, on the order of AOA 500 j/kg from about KOKC south. This uncertainty calls into question the bullish appearance of the 00z NAM, but will wait for the 03z SREF suite to see if uncertainty holds or is reduced using new 00z data. Reduction in uncertainty is certainly a possibility, as SREF suite will likely be influenced by more aggressive 00z NAM solution.

Overall, situation is a difficult one to assess. Not entirely convinced mixed layer will be conducive to severe weather event, especially given unfavorable temperature inversion around 900-millibars. In addition, very scant coverage by 00z NAM-4km only adds to concerns about any convection actually forming to begin with. Do believe that if convection is able to form, strong thunderstorms will become a good probability given favorable wind fields and potentially-favorable mixed layer environment. Severe thunderstorm formation also possible in the event convection can form and be sustained. Tornado risk somewhat low but not entirely zero in my opinion, given slightly high LCLs despite favorable wind profile, including veering winds, rather high 0-3km SRH values, and good surface to 6km shear values. Best threats will include hail and strong winds, with perhaps a low threat for a tornado if mixed layer environment can be fully realized.