Tuesday, May 06, 2008

Severe weather threat on Thursday



Parameters for severe weather are looking ominous for Thursday afternoon across North Alabama.



The above graphic illustrates what I am concerned about. It shows the potential for very high 0-3m storm relative helicity (SRH) Thursday afternoon at 1 p.m. CT. SRH is a measure of the potential of a thunderstorm to acquire a rotating updraft. This is one of the most important factors in the development of tornadoes.

It has been said that SRH values of >300 often contribute to the development of strong (EF2 or EF3) tornadoes. I highlighted the area that the NAM model suggests might have SRH values of >280 at 1 p.m. Thursday. I have seen other runs of other models that have suggested SRH values of over 500 in North Alabama on Thursday afternoon.

This doesn't mean that it is time to run to your nearest basement. It does mean that parts of the deep south may experience a tornado outbreak Thursday afternoon.

Please let me emphasize the word "may". We are talking more than two full days from now and models usually change from run to run. This trend has been showing up in the models now for the past day or so.

The timing, placement, and even the existence of this event are all still very much up in the air at this point. This is just a heads up for the possibility of a very bad day of weather in the South Thursday.


What I have done in the above graphic is show how the model progresses the areas of peak SRH and peak buoyancy through the afternoon. At 1 p.m., the 400 SRH is combined with buoyancy values of over 2000 in northwest Alabama. By 7 p.m. the areas of 400+ SRH pulls out to the northeast while the higher buoyancy values are a bit further south.

I have been discussing 0-3km helicity values. While these values are certainly high enough to support the formation of significant tornadoes, there are many other parameters to consider as well. Some of those we will not get a good idea about until the day of the event. One of those is SRH in the lowest part of the atmosphere. I will be interested in seeing whether 0-1km helicity values will be significant enough, or whether they will have to be.


The above map shows the SPC's outlook for significant tornadoes hits a max in North Alabama around mid afternoon on Thursday.

Based on this run of the NAM, the greatest risk for severe weather in Alabama will be between 11 a.m. and and 9 p.m. The area of greatest concern for tornadoes might be in North Central Alabama, between the Tennessee River and Interstate 20. Please do not take this as a specific forecast. This is just an analysis of one run of one model.


Above graphic from the SPC, valid at 4 p.m. CT Thursday May 6, 2008

11 comments:

mmmiklik said...

Look foward to alittle action again... at least we will get some good rain again.

Mike said...

I will be watching this very closely over the next few days!

mmmiklik said...

Looking at the NAM map I wonder if we will get anything here in my corner of the state???

Mike said...

Yes, I think you will. Remember this map is representing a projection of how things will be at 1 p.m. I will be interested in seeing how the timing of this thing plays out.

mmmiklik said...

I agree. Maybe we will get a chance to go spotting, or if anything maybe I can get some storm picts, either lightning or something. Sometimes I feel weird about getting excited about bad weather, but when your geeks like us I guess you can't help it. Guess it will also mean that I won't get much work done Thursday afternoon, looking at radar, reporting on the Skywatcher IM etc..

mmmiklik said...

Also just noticed that under the NWS B'ham site for current hazards they upgraded from a level 3 for severe storms to a level 1 for tornadic storms on Thursday.. I wish the HSV NWS had a page like that but they don't.

Mike said...

There are some questions about the amount of low level moisture return from the Gulf, but we shall see.

mmmiklik said...

Thanks for keeping me updated.. I will check your blog for updates as well as our 33*40 blog.. You've done a great job.

Mike said...

Thanks.

mmmiklik said...

Well based on your latest data, and the 33/40 blog, I think we may have something to do on Thursday. Also, I know James doesn't use the word"Volital"( I can't spell)that often, and he used it in his afternoon post. So it will definately be intresting.. Have a good evening Mike..

Mike said...

It looks that way Michelle...

I am interested in what shows up when I look at the SPC site and models tomorrow a.m.