BIG STORM POTENTIAL MONDAY

All signs continue to point to a rather high-end severe weather event across the Great Lakes and Midwest region on Monday with a rather volatile combination of wind shear and instability, but questions remain on storm coverage and evolution through the day. To start things off a Level 4 of 5 risk, a Moderate Risk issued by the Storm Prediction Center covers much of Iowa and Wisconsin, as well as southern Minnesota.

The ensemble mean of 0-1km storm relative helicity (shaded area) and SBCAPE (contoured) off the HREF.

Looking at the environment the ensemble mean of low-level wind shear (0-1km SRH) is in excess of 100m2/s2 across Iowa, and pushing 200+ m2/s2 in northern/northeast Iowa into Minnesota and Wisconsin. Meanwhile the instability measured by SBCAPE is in excess of 2000j/kg across Iowa and over 1000j/kg farther north in Minnesota and Wisconsin. This overlap is a rather high-end severe weather parameter space and if thunderstorms do form, they can quickly go severe capable of tornadoes (some strong), large hail and damaging wind gusts.

From the Storm Prediction Center:

Therefore, any mature,
   discrete supercells which can form and maintain within the open warm
   sector on Monday late afternoon/evening will pose a threat for
   strong to potentially intense tornadoes.

Looking at the collection of high-resolution weather models we use to forecast severe weather the probabilities of rotating thunderstorms are high across Minnesota by 6-7pm Monday, but very few models actually initiate storms in Iowa. Rather perplexing at first glance given the nature of the environment in place.

Using an experimental model I have referenced in the past, the RRFS, it keeps the bulk of the activity to the north and south with Iowa actually somewhat quiet. I think the reasons for this are two fold. For one, I think the overall convergence along the dryline in Iowa may be lacking compared to farther north. Additionally, I think the models are falsely handling the amount of inhibition that might be place over Iowa that would "cap" storm development.

Looking at forecast sounding from the area of the dryline in northern Iowa Monday evening there are two things that immediately stand out - the extreme wind shear in the low/mid levels of the atmosphere as well as the capping inversion indicated not far above the surface. I cannot explain the reasoning yet for this capping indicated in the models, but I do think it is overdone and is leading to models improperly simulating thunderstorm development.

The lower-resolution global models are instead more confident in storms forming, and they latch onto the idea of two rounds of storms. There is likely an initial wave of storms that could be supercells capable of tornadoes and large hail, with a second wave of storms along the cold front/dryline that could produce damaging wind gusts.

Analogs, or historical weather events that look similar to Monday, indicate high probabilities of severe weather reports from southern Minnesota, through Iowa and into southeast Nebraska. This is another reason I think some of the higher-resolution models are not handling Monday's outbreak well.

So to summarize the severe threat Monday with a few bullets:

• Area: Great Lakes and Midwest

• Timing: 3-11 p.m.

• Hazards: Tornadoes, large hail and strong wind gusts

• Confidence: High on severe environment, moderate/high on storm formation

After a very active stretch of weather it appears things will slow down a bit in the early to mid May period. Latest guidance from the Climate Prediction Center indicates below normal precipitation for the Central US. A nice break for the farmers to hopefully catch up on the spring plant.

I'll end with a stunning launch from the Florida Space Coast this past week as the Bandwagon-3 mission separated creating a beautiful nebula in the night sky.

Check back for more updates on the severe weather risk!

-Meteorologist Nick Stewart