Severe Weather Threat August 22-23, 2021

[TH2002]

I hesitated to start a thread about this upcoming potential severe weather event because a lot of events this year (and in general) have failed to materialize, but it does at least provide an opportunity to analyze different forecast models.

What the models agree on is that at least some severe thunderstorms are possible over east central Nebraska and surrounding areas on August 22, possibly into the 23rd.

Here is the SPC's D2 outlook with a Slight Risk and 2% tornado probabilities:


It should be noted that the 2% tornado probabilities extend into the eastern Dakotas, western Iowa and SW Minnesota (with an unrelated 2% area over New England coinciding with the landfall of Hurricane Henri) while other models do not show a tornado threat (or only marginal) in those areas. Based on the guidance of other models I am curious to see if the SPC will upgrade to D1 Enhanced with a 5% tornado area over eastern Nebraska.

Here is Windy's outlook:

Note the area of thunderstorm development over E/SE Nebraska. Unfortunately the Windy model doesn't really go into detail about severe weather parameters but does give you an idea of where thunderstorm and potential supercell development is possible.

Forecasted sounding based on the NAM model using STP:

It is interesting to note that while the model is in agreement that there will be a severe weather risk over eastern Nebraska, the STP doesn't kick in until 03z on Monday. Parameters conductive for the development of tornadoes and other severe weather are also in a narrow window, almost completely disappearing by 06z.

The GFS model based on supercell composite tells a completely different story, only forecasting marginally severe storms:

 

[TH2002]

Interesting. The SPC did add a 5% tornado area, but in eastern SD rather than NE.

 

[andyhb]

Looks like another set up starved of moisture because of a preceding cold frontal passage.

 

[TH2002]

It will be interesting to see how today plays out.

While the NAM model focuses its tornado threat over SE NE/NE KS the SPC mentions that the evolution of a severe threat in that area is more unclear. The SPC is focusing its tornado threat over eastern SD with a 5% area while NAM only forecasts a MRGL TOR threat in that area. Windy shows the possibility of one or two discrete supercells over eastern SD through eastern NE, while the GFS model is only forecasting a MRGL TOR threat in SE NE, possibly due to greatly limited moisture return as andyhb pointed out.

 

[TH2002]

Storms have taken on a linear mode in ND, where a Severe Thunderstorm Watch is in effect. In NC SD an SVR-warned isolated supercell has developed.