Sunday, July 9, 2017

Wind Threat Sunday Night into Monday Morning

11:10pm Update: Strong thunderstorms continue to move south out of Minnesota late Sunday evening and into northern Iowa.  Additional thunderstorms have developed over west-central Wisconsin, but for the time have mostly remained below severe limits.  Looking over some of the current data and we are beginning to get a draw of moisture northeast from Iowa, but the highest instability remains over northern Iowa.  The increase in moisture to the northeast is likely what's helping thunderstorms develop over west-central Wisconsin late this evening.  But the bowing line segment in Minnesota is moving more due south, than southeast.  While a shift to the southeast is possible - as the low level jet continues to strengthen - the highest severe risk may occur outside of the immediate Stateline.  Still there will be a threat for localized strong winds, especially closer to 4am/5am, but the overall severe threat as the line approaches will be low.

9:40pm Update: New Severe Thunderstorm Watch issued for parts of southeast Minnesota and north-central Iowa until 3am.  Thunderstorms over Minnesota continues to develop Sunday evening, but may take more of a south/southeast track versus southeast track overnight.  The reason for that is because of where higher moisture and instability are located in the atmosphere currently.  Now as we head through the night the higher moisture and instability may be pulled a little further to the east thanks to the arrival of the low level jet.  Storms are still expected over northern Illinois and southern Wisconsin, but could actually be arriving in more of a weakening trend.  I still do believe isolated hail and localized strong wind gusts will still be a threat, but the highest risk for severe weather may be more centered over to our northwest.  Arrival time for storms still appear to be roughly between 2am and 5am.  Pushed back slightly due to slower storm development over Minnesota.

8:00pm Update: Enhanced Risk for severe weather has been shifted further northwest, while a Slight Risk covers all of northern Illinois and southern Wisconsin.  Not too much has changed with the forecast for the night, however, the strongest storms may occur closer tied to where the highest instability is located.  This, right now, appears to be over southeast Minnesota and northeast Iowa.

Original Post:
The quiet weather from the weekend will be coming to an end Sunday night as northern Illinois and southern Wisconsin return to a very active weather pattern for the next several days.  The first round of thunderstorms will arrive late Sunday night - after Midnight - into very early Monday morning.

HRRR Model
08Z - 3am Monday Morning
Most of the evening will be dry, other than maybe a passing sprinkle or two.  Cloud cover will be gradually increasing as a decaying complex of storms in Wisconsin moves into northern Michigan.  Low pressure moving out of the Dakotas and into western Minnesota Sunday evening (between 7pm and 9pm) will allow thunderstorms to quickly developing in central Minnesota around or just shortly after sunset.  At the same time, winds within the lowest level of the atmosphere will be increasing and pulling an unseasonably humid air mass northward.  Once thunderstorms develop over the upper Midwest they are expected to quickly develop into a line of storms, reaching southeast Minnesota between 10pm and 11pm.  These storms will then feed off of the humid air being pulled north by the
RPM Model
0830Z - 3:30am Monday Morning
low level jet and continue their movement southeast - reaching southwest Wisconsin and northeast Iowa between 12am and 2am and then south-central Wisconsin and northern Illinois between 2am and 4am.

Damaging winds will be the biggest concern, followed by hail and heavy rain.  The two images on the left are from our short-term weather forecast models the RPM and HRRR.  Both show somewhat similar scenarios between 2am and 3am Monday morning - a line of thunderstorms with a few stronger embedded bows.  Most of the activity should be moving east between 8am and 9am, with the rain wrapping up after 10am Monday. 

Before going to be Sunday night, make sure you have a way to receive watches and warnings should they be issued.  Bring in any loose objects that could fly around outside and make sure you know where your safe place is should you need to take shelter. 

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