Mesoscale Discussion 1854
NWS Storm Prediction Center Norman OK
1257 PM CST Wed Dec 16 2020
Areas affected...Northeast West Virginia...northern
Virginia...western Maryland...and central to eastern Pennsylvania
Concerning...Heavy snow
Valid 161857Z - 170000Z
SUMMARY...Heavy snowfall rates around 2 inches per hour are expected
to increase in coverage through the mid to late afternoon hours
across portions of the Mid-Atlantic. Heavy snowfall will likely
continue into the overnight hours for portions of
eastern/northeastern Pennsylvania.
DISCUSSION...Precipitation continues to gradually increase in
coverage across much of the Mid-Atlantic region within a broad swath
of strong warm air advection between 850-700 mb (per recent RAP
soundings and mesoanalysis). Mixed precipitation types continue to
be reported within the precipitation transition zone along the
Roanoke to Fredericksburg, VA line. While a wintry mix, including
pockets of freezing rain, will continue in the near term, this zone
should become more limited spatially through the mid/late afternoon
hours. Further north, moderate, to at times heavy, snowfall rates
have been observed (as evidenced by visibility reductions to
one-quarter mile and observed snow rates around 2 inches per hour
for some locations in northeast WV).
Heading through the afternoon, the low that is currently advancing
northward along the Carolina coast will continue to deepen. As this
occurs, mid-level winds will strengthen and begin to tighten the
850-700 mb baroclinic zone and augment warm advection across the
region. As isentropic and frontogenetical lift increase, snowfall
rates will see a corresponding uptick in intensity. Lift within the
dentritic growth zone and weak instability closer to the coast will
help further augment snowfall rates. Widespread snowfall rates above
1 inch per hour over the region are likely, and the potential for
widespread snowfall rates exceeding two inches per hour will
increase through 00 UTC as the potential for organized
frontogenetical/deformation-driven snow bands increases.