Friday, June 13, 2014

Multi-Day Severe Weather Discussion for June 17 - 20

This is a severe weather discussion concerning the potential multi-day severe weather threat from June 17 to 20.

COD
The severe weather potential kicks off on the evening of June 17th, when this instability image above is valid for. In this forecast, we see a reservoir of extremely unstable air extending from the Gulf Coast to southern Canada. At the heart of all of this instability, we see convective available potential energy, or CAPE values exceeding 6000 j/kg, an absolutely massive amount of fuel for thunderstorms. This kind of extreme instability finds itself from Nebraska to Minnesota, down through Illinois and Missouri. All of these areas may be in line for the first of potentially multiple rounds of severe weather on the back end of this upcoming workweek.

COD
Fast-forwarding to Thursday evening, we see lower-level wind fields are suddenly much more supportive for severe weather than they had been on Tuesday and Wednesday. This comes as a low pressure system slowly meanders across the Plains gradually attaining a negative tilt in the process. A negative tilt means the contour lines appear to be digging to the southeast direction, which then enhances the risk of severe weather. In this case, we see low-level wind speeds exceeding 40 knots in the Midwest, and this is where I believe we may see some organized severe weather on Thursday evening, as continued extreme instability and now-supportive wind fields combine to create for what could be an impressive environment.

CIPS/SLU
We can also use analog dates to predict the future. This method of forecasting takes decades of weather observations and matches a handful of select dates up that are closest to the forecasted conditions. This image shows compiled severe weather reports from the top 15 analog dates, basically the 15 days that were the closest to projected conditions on Wednesday evening. On this image, we can see that the majority of severe weather events that happened under similar conditions as this coming Wednesday occurred in the Plains regions. We see an anomalous swath of hail, wind and tornado reports throughout the Dakotas, Nebraska, Kansas and Colorado. We then see an area where strong tornadoes hit under similar conditions in central Oklahoma into Texas. While this coming Wednesday won't exactly follow this set-up and put tornadoes/hail/damaging winds down in these exact areas, it does give us a good glimpse at some regions that might be affected by potentially tornadic weather. While I don't have images for Thursday's threat and onward, it can be anticipated that the threat would shift east.

Andrew

Severe Weather Discussion for June 14

This is a severe weather discussion concerning the expected severe weather outbreak on June 14th.

SPC
The Storm Prediction Center has issued a Moderate Risk of severe weather for Saturday, June 14th, affecting eastern Nebraska, western Iowa, far north central Kansas, and even extreme southeast South Dakota. Moderate Risks aren't issued too often by the SPC, and it's even more uncommon to see them issued a full day in advance. Typically, a Moderate Risk will be posted on the day of the severe weather event. Posting the risk a day in advance of the event means the severe weather threat is likely more threatening, or confidence in the event is higher.

COD
Model guidance forecasts of 500mb wind speeds on Saturday evening show an upper level low located over northeastern Montana into southern Canada producing an extended trough over the northern Plains. This is defined well by the mid-level jet maximum developing in the overnight hours of Saturday into Sunday, when the above image is valid. Wind speeds at the base of this trough look to exceed 75 knots, a beneficial environment for creating convection. The trough pressing eastward into the dominating ridge, pulling up moist and unstable air in the process, is likely to create opportunities for severe weather on Saturday. This mid-level jet and supportive lower-level wind fields confirm this possibility.

COD
The forecast for instability on Saturday evening shows multiple opportunities for severe weather, as well as a few caveats. This projection of convective available potential energy, or CAPE, shows values exceeding 3000 j/kg, indicating the presence of a very unstable environment. These high CAPE values, combined with the supportive wind fields, will likely play a significant role in producing and sustaining the expected convection. Also of note is how the uncapped instability is located in the Moderate Risk, while areas with a capped atmosphere (a thunderstorm suppressant) are not in the Moderate Risk area. This tells me that the capping inversion that prevents thunderstorms may be strong, but areas that manage to 'break the cap' will experience some intense weather.

COD
A zoomed-in forecast of the Supercell Composite on Saturday night also shows the potential for supercell thunderstorms, considered to be some of the most violent thunderstorms. We see the elevated Supercell Composite values from Oklahoma and Texas to Iowa, but again, due to the capping inversion, we'll be focusing our attention mainly on southeast Nebraska and Iowa, where potential seems the greatest for this event.

Andrew