MD 0814 CONCERNING SEVERE POTENTIAL…WATCH UNLIKELY FOR PARTS OF THE UPPER MISSISSIPPI RIVER VALLEY
Mesoscale Discussion 0814
NWS Storm Prediction Center Norman OK
0218 PM CDT Thu May 18 2023
Areas affected...parts of the Upper Mississippi River Valley
Concerning...Severe potential...Watch unlikely
Valid 181918Z - 182115Z
Probability of Watch Issuance...20 percent
SUMMARY...Thunderstorm development appears likely across Wisconsin
and adjacent parts of Minnesota and Iowa this afternoon as
destabilization continues. Damaging winds, and perhaps large hail
and a tornado, will be possible. However, this threat will likely
remain fairly limited in coverage and intensity; watch issuance is
not anticipated.
DISCUSSION...Over the past hour, GOES visible and daytime RGB
composite imagery have shown steady vertical development within a
pre-frontal cumulus field across central IA into southeast
MN/western WI - indicative of gradual destabilization. Recent
forecast soundings and RAP mesosanlysis estimates show limited, if
any, mixed-layer buoyancy, which appears to be in conflict with the
noted satellite trends. Recent ensemble guidance also shows some
uncertainty regarding afternoon destabilization with a narrow
corridor of SBCAPE values between 250-1000 J/kg across
western/central WI to southeast MN/northeast IA (though values near
500 J/kg appear most probable). A narrow swath of low to mid-50s
dewpoints advecting northeast ahead of the front supports this idea
of a spatially-limited warm sector and cast uncertainty onto the
coverage/intensity of robust convection.
Latest hi-res guidance suggest convective initiation should occur
between 20-22 UTC, though the noted thermodynamic discrepancies cast
some uncertainty on storm timing (especially with a few deeper
convective showers noted in KMPX imagery). Initial storm modes will
likely be a mix of semi-discrete cells and clusters, and more
intense storms may pose a large hail risk given 30-35 knots of
effective bulk shear. However, quick upscale growth is anticipated
with a corresponding increase in damaging wind potential. Forecast
wind profiles and recent KARX VWP observations show somewhat strong
shear (20-30 knots) in the lowest 2-3 km, but may be oriented
largely along the evolving line. More meridionally oriented sections
of the line may see sufficiently strong line-orthogonal low-level
shear to support brief mesovorticies (though confidence in this
scenario is fairly low). The greatest severe threat may emerge
across portions of western to central WI where ensembles suggest
SBCAPE should be maximized this afternoon/evening. The spatially and
thermodynamically limited nature of the threat will likely negate
the need for watch issuance.
..Moore/Grams.. 05/18/2023
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...MQT...GRB...MKX...DVN...DLH...ARX...MPX...DMX...
LAT...LON 42889325 43249314 45149144 45989019 46088927 46068888
45918863 45698856 44698880 43698944 42699052 42529096
42469252 42479288 42589314 42669323 42889325
