SPC MD 1615

MD 1615 CONCERNING SEVERE POTENTIAL...WATCH LIKELY FOR SOUTHEAST LOWER MI...NORTHWEST OH...NORTHERN AND CENTRAL IN...AND PARTS OF CENTRAL IL

Mesoscale Discussion 1615
NWS Storm Prediction Center Norman OK
0217 PM CDT Mon Sep 04 2017

Areas affected...Southeast Lower MI...northwest OH...northern and
central IN...and parts of central IL

Concerning...Severe potential...Watch likely 

Valid 041917Z - 042115Z

Probability of Watch Issuance...95 percent

SUMMARY...Thunderstorms are expected to increase in coverage into
the early evening along a cold front advancing across southern Lower
MI, northwest OH, northern and central IN, and into central IL. 
Damaging winds and hail are expected to be the primary
severe-weather threats, resulting in the need for watch issuance by
late afternoon (by 21Z).

DISCUSSION...Visible satellite imagery showed cumulus forming across
southwest Lower MI where stronger forcing for ascent is expected to
be located (just south of the equatorward-advancing cloud shield
with the cold front).  Trends in surface analyses indicated the
front was continuing to move through Lower MI and northern IL, and
now into northwest IN.  This boundary is expected to reach southern
Lower MI, the rest of northern IN, and northeast to east-central IL
between 20-23Z, where it will encounter stronger instability. Ahead
of the front, southwesterly low-level winds have allowed greater
moisture to spread into northern IN to southern Lower MI and
northwest OH, with surface dew points rising through the 60s F. 
This combined with surface heating and steepening mid-level lapse
rates will contribute to increasing instability (mixed-layer CAPE of
1500-2500 J/kg).  Forcing for ascent along the cold front and the
leading edge of greater height falls with the approaching upstream
shortwave trough combined with the stronger instability will support
additional storm development as the cold front advances to the
south. 

The air mass across the discussion area remains capped per regional
18Z soundings, though further surface heating and an increase in
aforementioned deep-layer ascent should weaken the cap by 21-23Z for
sustained updrafts.  Linear forcing and shear vectors nearly front
parallel should favor damaging winds as the primary threat, though
steeper mid-level lapse rates suggest hail will be a threat too.

..Peters/Thompson.. 09/04/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...CLE...ILN...DTX...IWX...GRR...IND...LOT...ILX...

LAT...LON   41428338 40528475 39428726 39538826 40258864 40798845
            41608650 42168511 42658403 42998305 42778258 42428265
            41428338 

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