Saturday, January 26, 2013

January 29-31 Potential Significant Severe Weather Event

The Storm Prediction Center has issued two separate areas of severe weather potential, with the 'D4' area outlining the potential for January 29th and the purple outline hinting at January 30 for the threat area that day.

The Storm Prediction Center did issue a smaller area for severe weather about 3 days ago for the same areas in the red outline, and was present up until yesterday. Yesterday, the SPC indicated that confidence was too low to outline a severe weather area, but as you can see today, the SPC is apparently very confident in their outlook, now issuing two separate areas of threats.

This is the 500mb vorticity forecast for the evening of January 29th. We can see a strong storm system is present in the Southern Rockies at this timeframe, with high vorticity values being found across the mountains and Plains. The system does not appear to be tilting its highest vorticity values in any particular direction, meaning this system has a neutral tilt. This is good news for those who do not welcome severe weather, as negatively tilted systems (highest vorticity values pointing to the southeast) tend to allow enhanced severe weather chances due to cold air flowing aloft, where storms can build and strengthen faster. If you wanted to be really specific, this can be classified as a slightly positively tilted storm system, but I'm not really seeing a difference because of just how slight that positive tilt is.

This is the lower level wind speed forecast, again valid for the evening of January 29. These winds are forecasted for the 900mb level, not very high above the ground. We can see very strong winds at this level across the Midwest and South Plains into the Gulf Coast. Lower level winds reach as high as 74 knots in east Arkansas. This enhances my theory that Arkansas will be the center of the January 29th outbreak. For those unfamiliar with the implications of lower level winds, tornado potential is enhanced in areas of stronger lower level winds, as wind shear is increased and thunderstorms are encouraged to form.

The last image I will show you is something called surface-500mb shear, also known as Deep Layer Shear (DLS). Deep layer shear is looked at during severe weather potential timeframes to evaluate if there really is a tornado potential. The first ingredient for making a tornado (other than having a thunderstorm) is shear. If you don't have shear, you cannot have a solid tornado. However, the deep layer shear forecast calls for shearing above 100 knots in western Arkansas, a significant value for winter. If storm activity maintains an individual cellular formation and blossoms in the areas with high shearing, tornadoes could be strong and widespread.

The first real evaluation of this threat will come tomorrow, when the Storm Prediction Center places this event in either Slight, Moderate or High risk of severe weather. Right now, I think this event will end up as a Moderate Risk event when the time comes, although that's for the SPC to define, not me.



Anonymous said...

North texas is in the danger zone of wind shear...arkansas and north texas could see widespread tornados?i see strong shear in the upper air etc...this could be a bad severe weather outbreak andrew?

Anonymous said...

Actually the SPC doesn't have the ability to issue a high risk on D3.

Andrew said...

Anonymous at 3:48: We'll know as the time draws closer.

Andrew said...

Anonymous at 5:46: It does, if you look on the bottom right corner of the image on Day 3.

Eric said...

Good post Andrew, I agree that there will be the potential for some decent severe weather, but I think one issue that has to be resolved is the amount of available instability, and models sometimes can have issues picking on this because of overrunning precipitation, which naturally delpletes the atmosphere of instability. This is a good reason why positively tilted troughs, orientated southwest to northeast, usually are not as dangerous as negatively or neutrally titled troughs, because not only do the surface and upper level winds usually go in the same direction (limiting turning of the winds with height), but also there's a strong tendency for precipitation to train over the same areas which limits instability, but also leads to flooding problems. I think it will be a wait and see on the degree of severe weather, but from what I can tell, overrunning precipitation will be a large factor in determining the intensity of this severe weather outbreak, regardless, with such a large temperature difference across the frontal boundary, there is bound to be some wind damage even with limited instability.

Anonymous said...

No it doesn't Andrew, here is the SPC's probability description link:
I'll say it again, it's from the SPC. If you actually read it, you'll find at the bottom that the SPC in fact *does not* have the ability to issue a high risk.

Andrew said...

I stand corrected. There is still no reason to be so arrogant- simply explaining the link would have sufficed rather than going on the offensive.

Anonymous said...

What can southern virginia and northern north carolina get from this? The just uped the anty on our forecast from thunder showers to t-storms with the potential for severe. What are the risks?