SPC MD 17

MD 0017 CONCERNING HEAVY SNOW FOR THE SIERRA OF NORTHERN AND CENTRAL CA AND VICINITY

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

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

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

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

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