Mesoscale Discussion 1959
NWS Storm Prediction Center Norman OK
0333 PM CDT Wed Sep 11 2019
Areas affected...Southern Wisconsin...southern Lower Michigan
Concerning...Severe Thunderstorm Watch 645...
Valid 112033Z - 112230Z
The severe weather threat for Severe Thunderstorm Watch 645
SUMMARY...Damaging wind gusts will continue to be a threat into the
early evening across WW 645. Isolated large hail may occur,
particularly with more discrete activity in southern Lower Michigan.
Convective coverage may yet increase this evening as the front sags
south. Potential appears low for new WW to the south of WW 645, but
trends will continue to be monitored.
DISCUSSION...The main area of convective activity in WW 645 is
currently across southeastern Lower Michigan where stronger heating
has occurred. Mid-level ascent continues to be strongest over
southern Wisconsin, though convective coverage is more limited
currently. By late afternoon/early evening, convergence along the
cold front is expected to increase as the surface anticyclone in
Ontario strengthens. This would promote further thunderstorm
development, particularly in western portions of the Watch where
clouds have cleared out and MLCIN has been decreasing with time.
With primarily a linear storm mode anticipated, damaging wind gusts
will continue to be the primary threat with much of the activity
late this afternoon into the evening. Deep-layer shear across Lower
Michigan should improve as the mid-level wave continues eastward.
Given the more discrete storm mode ahead of the slowly advancing
cold front, isolated large hail would be most likely in this area
with stronger/rotating storms. The tornado threat, while nonzero, is
expected to remain low and decrease as 850 mb flow veers/decreases
with time and linear forcing for ascent increases. Southward extend
of severe convection is uncertain given the drop-off in deep-layer
shear. It remains possible that a few strong storms will impact
northern Illinois/Indiana. Confidence in a new WW south of the
current activity is low, though observational trends will continue
to be monitored this evening.
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 42459050 43289008 43828705 43858400 43608250 41988282
41428357 41388418 41348454 41588793 41929000 41999027