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Mesoscale Discussion 1661
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   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
   continues.

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

   ATTN...WFO...DLH...MPX...FSD...

   LAT...LON   45499468 46269454 46209348 45339305 44129320 43539442
               43619518 44469484 45499468 

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