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Mesoscale Discussion 17
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   Mesoscale Discussion 0017
   NWS Storm Prediction Center Norman OK
   0345 PM CST Tue Jan 03 2017

   Areas affected...The Sierra of northern and central CA and vicinity

   Concerning...Heavy snow 

   Valid 032145Z - 040345Z

   SUMMARY...Snowfall rates of 2-3 inches per hour will likely develop
   over the higher elevations of the northern CA Sierra around 23Z-01Z,
   with similar rates spreading southward across the Sierra through the
   evening hours. This will delineate the initiation of a long-duration
   heavy-snow event, which will continue into the overnight hours.

   DISCUSSION...A 700-mb speed maximum continues approaching the coast
   of northern and central CA, and the BHX VAD wind profile is already
   indicating southwesterly winds increasing to around 55 kt at 3 km
   AGL in association with this speed maximum. As the speed maximum
   eventually impinges upon the western slopes of the Sierra,
   precipitation rates will increase in response to strengthening
   upslope flow. Furthermore, GPS imagery depicts a plume of enhanced
   moisture -- reflected by PW around 1.0-1.1 inch -- approaching the
   Pacific coast, which will contribute to efficient precipitation
   generation. These factors will support a long-duration heavy-snow
   event for the higher elevations, continuing into the overnight
   hours. Strong winds may also combine with the snow to result in
   areas of restricted visibility in blowing snow.

   While warm advection, and offsetting adiabatic cooling accompanying
   the upslope flow, will contribute to spatiotemporal variations in
   snow levels, present indications are that heavy snow with rates of
   2-3 inches per hour will be likely for elevations at or above
   6500-7500 feet. Ample ascent through saturated dendritic growth
   zones will support such rates amidst the approaching moisture plume.
   Extrapolation of the speed maximum and the latest model guidance
   suggest that 2-3-inch-per-hour snowfall rates will likely develop
   around 23Z-01Z across northern CA, with similar rates spreading
   southward through the evening. Furthermore, model forecast soundings
   indicate narrow, elevated CAPE profiles evolving during the event.
   This could offer weak convective enhancements to snowfall rates, and
   perhaps support an isolated lightning strike or two with the highest
   rates.

   ..Cohen.. 01/03/2017


   ATTN...WFO...VEF...REV...HNX...STO...

   LAT...LON   36641864 37481942 39062064 39802120 40122142 40382147
               40502113 39372024 38011924 37421878 36821841 36641864 

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Page last modified: January 04, 2017
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