Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
338 AM EST Sat Mar 02 2019

Valid 12Z Sat Mar 02 2019 - 12Z Tue Mar 05 2019

...Central Plains through the Ohio Valley, Mid-Atlantic, and
Northeast...

Day 1...
A rapidly deepening low pressure will move quickly from the
Mid-Atlantic coast to the Gulf of Maine today, spreading snow from
eastern PA into eastern New England. As the low intensifies, a
mid-level deformation band and accompanying frontogenesis will
produce a band of heavy snow, likely focused across eastern
Massachusetts. Guidance has come into better agreement with the
heaviest QPF focused just along the coast and offshore, and WPC
probabilities are over 50% for 4 inches in the vicinity of Boston,
with lesser amounts onto the coast of Maine.

Days 1-3...
A more significant low pressure system will develop in the Lee of
the Rockies tonight and move through the eastern 2/3 of the
country into early next week bringing a large swath of snow from
the Plains through the Northeast. This low will develop in
response to a tightening baroclinic gradient as a cold front drops
southward, height falls ahead of an ejecting shortwave from
California, and increasing jet level diffluence within a coupled
jet structure.

As the surface low develops and then pivots across the Central
Plains and into the Mid-Atlantic, mid-level confluence will drive
moisture into the region aided by an open Gulf from high pressure
over the eastern Gulf. Increasing isentropic ascent on low-level
warm advection and intensifying mid-level frontogenesis will
interact with the mid-level impulse and result with a deep DGZ
allowing bands of heavy snow to develop and intensify as the push
east from eastern CO and cross KS tonight. This low will then lift
across the Carolinas to offshore the VA coast and then into the
Gulf of Maine by Day 3. This occurs as northern and southern
stream jets phase over New England, allowing for rapid deepening
near the Benchmark Monday morning. The flow remains progressive as
the 700mb wave stays open, and this will allow the system to eject
quickly, limiting somewhat the snowfall accumulations. The trend
overnight has been for a slightly weaker and further SE solution,
even the 00Z/2 ECMWF dropped well south from its previous runs.
This suggests the heaviest snow will occur from the mountains of
WV to just inland from NYC and Boston, with enough mixing
preventing the heaviest snow from reaching the I-95 corridor. At
this time, the highest WPC probabilities for 8+ inches are from
the Poconos of PA into central New England. However, if the SE
trend continues the major cities from DC to Boston may get into
these higher probabilities.

Isentropic lift atop the cold surface high will likely produce
some light freezing rain south of the heaviest snow, and low
probabilities for 0.1 inches of accretion exist in Oklahoma, as
well as a stripe from WV into Southern New England.


...California across Great Basin to Central Rockies...
Days 1-3...

Spokes of shortwave energy and associated jet maxima will pivot
onto the California coast today and Sunday spreading moisture and
snowfall through the Great Basin and into the Colorado Rockies.
The nearly zonal flow will keep the funneling of moisture confined
to a narrow corridor between approximately 35 and 40 degrees N
latitude through the period. The heaviest snow today is likely in
the Sierra, with 2-day maximums expected in the terrain of
Colorado including the Rockies and San Juans where moisture
transport will occur over a longer duration into Sunday. With the
relatively flat flow, snow levels will be moderately high,
4000-700 feet, keeping the heaviest snow to the higher terrain.

WPC probabilities are high for 12 inches in the Sierra and over
Colorado on Day 1, with still high potential for 8 inches on
Sunday in Colorado. Total accumulations may exceed two feet in the
highest terrain above 7000 feet, with 6-12 inches possible in the
higher elevations of Nevada, as well as the Wasatch of Utah. The
development of mid-level ridging finally cuts off moisture inflow
on Monday.


Weiss