SPC AC 170426

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1126 PM CDT Sat Oct 16 2021

   Valid 171200Z - 181200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Little thunderstorm activity is expected across the U.S. today
   through tonight.

   ...Discussion...
   A blocking mid-level ridge centered near/southwest of Greenland will
   maintain considerable influence across eastern North America and the
   Atlantic.  Large-scale troughing to its south-southwest appears
   likely to progress only slowly eastward, but the base of this
   feature  is forecast to gradually shift east of the southern
   Atlantic Seaboard.  An associated surface cold front is expected to
   clear Downeast Maine by 12Z this morning, and advance through the
   central and southern Florida Peninsula later today through tonight. 
   Warm mid-level temperatures evident in forecast soundings across the
   Florida Peninsula are expected to generally suppress deep convective
   development ahead of the front, but the maintenance of somewhat
   better boundary-layer moisture across the Florida Keys vicinity
   might contribute to isolated thunderstorm development late this
   afternoon and evening.  Otherwise, dry and stable conditions will
   prevail across most areas east of the Rockies, in the wake of the
   cold front, with generally negligible risk for thunderstorms through
   this period.

   Upstream, a significant mid-level short wave trough is forecast to
   progress inland of the Pacific coast late today through tonight.  It
   appears that this will include a 90-100 kt 500 mb jet streak digging
   across the San Francisco Bay vicinity into the southern Sierra
   Nevada, with a developing low to its northeast.  This is forecast to
   be accompanied by substantive mid-level cooling (500 mb cold core of
   -24 to -28C) across much of northern California, but mostly after
   dark, with limited destabilization.  While the development of some
   convective cells briefly becoming capable of producing lightning
   might not be out the question, thunderstorm probabilities are being
   maintained at less than 10 percent, at least for now.  

   Farther east, beneath increasingly cyclonic mid-level flow ahead of
   the digging impulse, southerly low-level return flow will be
   accompanied by at least modest moistening across the northern
   Mexican Plateau into the southern Rockies.  Associated weak
   destabilization across the higher terrain of northwestern New Mexico
   into the San Juan Mountains may become supportive of a few
   thunderstorms by this evening, perhaps aided by forcing for ascent
   associated with a perturbation emerging from the subtropical eastern
   Pacific.

   ..Kerr/Wendt.. 10/17/2021

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