Thur-Sat, June 17-19th Severe T'Storm Event Update #3

(11:00PM - 6/17/21) Multiple disturbances will move through the region through Saturday, bringing periods of severe t'storm potential to portions of the state tonight through Saturday night.

Summary...
Multiple disturbances will eject out of the Plains, and push eastward across the Midwest and Great Lakes through Saturday. These disturbances will bring periods of severe t'storm potential to portions of the state, tonight through Saturday night. With this update we have trimmed back severe t'storm probabilities for tonight/early Friday morning based on observational trends. The forecast for Friday through early Saturday morning needed little to no change.

Forecast...
Tonight-Friday Morning...
A large complex of severe t'storms developed this evening across portions of Southeast Minnesota into Central Wisconsin. This activity developed a bit further north than originally anticipated, due to a slightly stronger cap than previously modeled. Additional back building has continued to occur across Southern Minnesota and Northern Iowa, with the main complex then located to the east from the Minnesota/Wisconsin border on east into Central Wisconsin. This complex will continue to push east-southeastward overnight, and still has the potential to affect portions of Northern Illinois overnight and into very early Friday morning. Ahead of this activity, an environment supportive of a severe t'storm threat continue to advect in, as it has through the day today. This environment features moderate instability of 2,000-4,000J/KG, dew points well into the 60's, good shear, good forcing, and high lapse rates of 7.5-8.5C. This environment will be supportive of upwards of an isolated to widely scattered severe t'storm risk (level 1-2) across portions of Northern Illinois during this time. Damaging winds and hail will be the main severe weather threats.

​Friday Afternoon-Friday Night...
The complex(s) of t'storms from tonight/Friday morning is expected to clear the state by mid Friday morning. With another disturbance or two expected to move through the area and a frontal boundary expected to be draped across the heart of the state, additional rounds of showers and t'storms are expected to develop Friday afternoon and evening across portions of Northern and Central Illinois. This activity will then push southeastward Friday evening into Friday night, affecting the heart of the state. An environment supportive of a severe t'storm threat will be in place. This environment will feature moderate to high instability of 3,000-5,000J/KG, dew points in the mid 60's to low 70's, good shear, good forcing, and high lapse rates of 7.0-8.0C. This environment will be supportive of upwards of a widely scattered to scattered severe t'storm risk (level 2-3) across the heart of the state. Damaging winds and hail will be the main severe weather threats, though an isolated tornado cannot be ruled out.

Saturday Afternoon-Saturday Night...
With a frontal boundary expected to be draped across the heart of the state and another disturbance or two set to move through, another round of showers and t'storms will develop Saturday afternoon across the heart of the state. This activity will spread eastward across portions of Central and Southern Illinois, Saturday afternoon through Saturday night. An environment supportive of a severe t'storm threat will be in place. This environment will feature moderate to high instability of 3,000-5,000J/KG, dew points in the mid 60's to low 70's, good shear, good forcing, and high lapse rates of 6.0-7.5C. This environment will be supportive of upwards of a an isolated severe t'storm risk (level 1) across the heart of the state. Damaging winds and hail will be the main severe weather threats, though an isolated tornado cannot be ruled out.

​Below is the severe t'storm forecasts for this event.

JP