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

Valid 00Z Sun Mar 03 2019 - 00Z Wed Mar 06 2019

...California across Great Basin...Central Rockies...and Central
Great Plains...
Days 1-2...

An expansive trough spans the CONUS around a deep cold core low
over Ontario through Sunday. A low spins off the western side of
the trough over BC Sunday night and persists through Tuesday. This
directs shortwave energy across the CONUS through Monday/Day 2.
The main shortwave is low pressure that opened off CA this morning
and is now a wave train getting sheared apart from a 130kt jet
streak over the Desert SW. The main mid-level trough axis in this
train will speed across the central/southern Rockies tonight and
be across the Central Plains Sunday morning.

As the primary shortwave train pushes onto the Central Plains
tonight, Arctic air spilling south down the Plains enhanced
frontogenesis with bands of moderate to heavy snow expected over
KS. Day 1 WPC probabilities for 6 inches are moderate across
west-central KS with moderate probabilities for 4 inches extending
east into MO. The sheared aspect of this system has limited the
snow potential from prior day runs that had more developed
mid-level forcing over the Plains.

The nearly zonal flow keeps Pacific moisture and shortwave forcing
confined to a narrow frontal zone that starts around 40 degrees N
and drifts south as flow slowly veers as the low develops over BC
and lingers around WA through Monday when ridging reaches the west
coast and cuts off the moisture.

Heavy snow shifts along this frontal zone from the Sierra Nevada
to the terrain of Colorado including the Rockies and San Juans
into Sunday. Snow levels will be moderately high, 4000-7000 feet,
with Day 1 probabilities for 18 inches high over CO and moderate
for 8 additional inches from the Sierra to the Wasatch.

The next low approaches CA Tuesday with a narrow corridor of high
moisture air reaching the Sierra Nevada late in the day. Day 3
probabilities are moderate for 8 inches above 7000ft for the
southern Sierra Nevada.

Isentropic lift atop the cold surface high will likely produce
some light freezing rain south of the snow, and low Day 1
probabilities for 0.1 inches of accretion exist in central
Oklahoma.


...Ohio Valley, Mid-Atlantic, and Northeast...

Days 1-2...
A mid-level trough currently over the Great Basin will scream east
from the Central Plains Sunday morning as it rounds a very deep
cold core low over Ontario. Surface low pressure develops over
east TX below diffluence at the head of the southern stream jet
streak as it begins to connect to a northern stream jet streak
southeast of the parent low center. This surface low slowly
develops as it moves fast, reaching eastern NC Sunday evening. The
low then turns up the coast, pushing east of Maine as a rapidly
developing low on Monday. Gulf moisture ahead of the trough lifts
over the reinforcing cold air from the sprawling surface high over
the north-central CONUS with snow starting in the lower OH Valley
Sunday morning and quickly shifting to the central Appalachians by
Sunday evening.

The speed of the system limits the total snow potential at first
with moderate probabilities for two inches over the lower and
central OH Valley. Terrain enhancement, the addition of Gulf
Stream moisture and the pivoting system allows moderate
probabilities for 6 inches in the higher terrain of the Allegheny
mountains of WV/MD on Day 1 and moderate probabilities for six
inches from the Shenandoah Valley northeast to the northern
Mid-Atlantic coast and southern/eastern New England on Day 1.5.

Day 2 QPF was a compromise between the farther west 12Z ECMWF and
the farther east 12Z NAM/GFS/UKMET. This is farther east than
prior day runs with the main snow axis along the urban corridor
from northern NJ to Boston. The developing surface low shifts
north from eastern Maine Monday afternoon.

Isentropic lift atop the cold surface high will likely produce
some light freezing rain south of the snow, and low Day 1/1.5
probabilities for 0.1 inches of accretion exist northeast from the
Shenandoah Valley to the southern edge of the heaviest snow axis
up the northeast corridor.

Jackson