Sunday, January 27, 2013

January 29-30 Potential Significant Severe Weather Event

January 29 Severe Weather Risk

January 29 Probability of Severe Weather

January 30 Severe Weather Outline
I'm continuing to observe the potential for a multi-day severe weather outbreak across much of the nation. Below is yesterday's discussion, which remains valid today.

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.



Anonymous said...

There is a 140 mph+ jet stream for tuesday severe weather outbreak....andrew thats very strong winds in the upper air?

Anonymous said...

Not quite 140, but in excess of 100... yes, it is in the upper-air. These winds can provide a lot of volatility to the storms. When the storm's anvil reaches up into the jet stream, it can tap the strong winds and bring them down to the surface in straight-line winds and/or microbursts. Wind shear is turning winds with height, which can increase tornado potential.