Severe Storm Outlook Narrative (AC)
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ACUS11 KWNS 202017
SPC MCD 202017

Mesoscale Discussion 1662
NWS Storm Prediction Center Norman OK
0317 PM CDT Wed Sep 20 2017

Areas affected...northeast Iowa through central Wisconsin

Concerning...Severe potential...Watch possible

Valid 202017Z - 202215Z

Probability of Watch Issuance...40 percent

SUMMARY...Storms may become capable of producing a few strong to
locally damaging wind gusts and hail this afternoon into early
evening from northeast Iowa into southern and central Wisconsin.
Trends will continue to be monitored.

DISCUSSION...Cold front extends from western WI through northeast
and southwest IA. Upper 60s F dewpoints have advected through
pre-frontal warm sector which, in conjunction with diabatic warming,
have boosted surface temperatures to 85-90 resulting in 2000-3000
J/kg MLCAPE. Despite weak forcing aloft and a gradual weakening of
the low-level jet in wake of a shortwave trough lifting north
through Manitoba and western Quebec, convergence and a destabilizing
boundary layer are sufficient to initiate storms along the front
this afternoon. While the stronger winds aloft remain post frontal,
moderate flow with 40-50 kt in the 700-400 mb layer resides in
frontal zone, but vertical shear remains modest and supportive of
mostly multicells and possibly some marginal supercell structures.
Primary limiting factor for a more robust threat is the weak
mid-level lapse rates with warm temperatures aloft (-6 to -7 C at
500 mb). Nevertheless, overall parameter space appears sufficient
for at least a modest risk for isolated strong to damaging wind
gusts and some hail.

..Dial.. 09/20/2017

...Please see for graphic product...


LAT...LON   43339151 44479055 44898969 44348877 43398889 42608981
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