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Snow Magazine

Severe Weather and Excessive Heat Target the Midwest Wednesday

July 19, 2017

Over the past few days, weather models have been drawing our eyes in for what looks to be a potentially significant severe weather event in the Upper Midwest tomorrow evening- we're talking the eastern South Dakota/Minnesota/Wisconsin area and possibly northern portions of Iowa.

 

Earlier today, we mentioned to clients and on social media that an upgrade from the marginal risk was likely by tomorrow. In the afternoon SPC Day 2 update, a slight risk was issued as shown below:

 

 

After digging into the data even more this afternoon, it appears that the potential for a significant, local severe weather event is present tomorrow, especially in southern Minnesota. We think the best chance for a few tornadoes (some significant?) will be in that area, and that an enhanced risk will likely be issued.

 

 

Taking a look at some of the specifics

 

A deep low pressure system will push through the Upper Midwest tomorrow evening, with a cold front in tow. This cold front will be the focal point for storm development, supplying extra low level shear and added lift to the environment.

 

 

 

By the time storms initiate late tomorrow afternoon and into the evening, the environment will be very unstable. Some of the latest high resolution computer models suggest that CAPE (convective available potential energy) values will be upwards of 4500-5000 J/kg. This is plenty of energy to support severe weather.

 

 

Moisture will also be incredibly high tomorrow, with dew points forecast to reach the mid-70s. Along with that, PWATS (precipitable water values) will also be near or at 300% the normal. These high figures mean that many storms are likely to develop, and that storms that do develop will likely be associated with heavy downpours. Localized flooding is likely, with rainfall amounts of 3-5" possible in southern Minnesota and Wisconsin.

 

 

One last thing I want to mention about tomorrow's severe weather set up, is the impressive wind shear. In the image, below, notice the areas I've highlighted in yellow boxes. These highlight the wind speed and direction at various heights in the atmosphere, namely speed shear and directional shear. Both are very strong, with wind speeds increasing from 15 knots at the surface to 60-70 knots around 3 miles up in the atmosphere, and turning from the south-southeast at the surface to west-northwesterly aloft. This signals that any supercells that develop pose a tornado threat.

 

There are a few red flags that may decrease the severe weather threat, which include mesoscale boundaries/frontal location, morning convection, and the speed of upscale growth. Tomorrow morning, showers and thunderstorms will be moving in from the west. Sometimes in the Midwest when this happens, it continues into the afternoon hours, keeps the atmosphere stable, and the severe weather risk becomes much lower. Second, the frontal position and mesoscale features will greatly influence the severe weather potential (i.e. if the lift doesn't like up with instability, etc). Lastly, the threat for large hail and tornadoes will be dependent on storm structure. Initially, storms are forecast to be discrete/superecllular. At that time, larger hail and tornadoes will be possible. Eventually, though, the storms will merge into a line or bowing segment of storms, and a damaging wind threat will prevail as the storms move east into Wisconsin and possibly northern Illinois. These factors will have to be monitored as we approach the event tomorrow.

 

One other thing I want to briefly mention is the excessive heat risk for tomorrow as well. Excessive heat watches and warnings have already been issued as temperatures will climb into the 90s to near 100 degrees. "Feel like" temperatures will be even higher. Warnings and watches will likely be expanded tomorrow morning. The heat will be exceptionally bad where very moist soils and standing water remain from recent flooding. The sun will "fry" these areas and damage turf and crops.

 

 

As always, we will post some updates on social media, but all details and analysis are given to clients at tdsweather.com in the mornings. Inquire about our services or a free trial here.

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