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ACUS11 KWNS 200604
SPC MCD 200603

Mesoscale Discussion 1661
NWS Storm Prediction Center Norman OK
0103 AM CDT Wed Sep 20 2017

Areas affected...Minnesota

Concerning...Severe Thunderstorm Watch 488...

Valid 200603Z - 200730Z

The severe weather threat for Severe Thunderstorm Watch 488

SUMMARY...The ongoing line of storms probably will tend to weaken
diminish as it progresses east of watch 488, but it is still
possible that another watch will be needed to account for a
lingering risk of potentially damaging wind gusts.

DISCUSSION...The more vigorous southern flank of the pre-cold
frontal squall line is progressing east northeastward around 40 kt,
and appears likely to progress east of the watch into east central
portions of Minnesota (including the Minneapolis/St. Paul metro
area) during the 07-09Z time frame.  However, even within the narrow
corridor of higher pre-frontal surface dew points (mid 60s),
forecast soundings suggest that activity is likely rooted above a
fairly deep (1-1.5+ km) surface based stable layer, supported by
low-level warm advection.  While the stronger mid/upper forcing for
large-scale ascent pivots northward into the international border
area, and inflow of drier more stable air becomes an increasing
issue with eastward extent, general weakening trends seem probable
within the next few hours.  Even now, it is not particularly clear
that the risk of strong downdrafts reaching the surface is
particularly high, but until more substantive weakening of
convection begins, heavy precipitation could continue to aid at
least the downward transfer of higher momentum to the surface,
accompanied by localized potentially damaging wind gusts.

..Kerr.. 09/20/2017

...Please see for graphic product...


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