Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Mar 23 2020

Valid 12Z Mon Mar 23 2020 - 12Z Thu Mar 26 2020


...Northeast...
Days 1/3...

On Day 1/today...low-mid level warm advection occurs across PA/NY
and into New England. This results in a period of snow.
The warm advection results in snow changing over to rain in most
of PA and southern NY. 
Steadily deepening low pressure will develop off the mid Atlantic
coast and then lift rapidly to the northeast, passing near the
Benchmark (40N/70W) overnight into Tuesday.  Enhanced upper
diffluence and divergence maxima aloft causes precipitation to
increase in coverage/intensity as the 700 mb wave crosses New
England.  Strong mid level deformation crossing from eastern NY
through southern VT/NH/southern Maine tonight focuses the heavy
snow threat in these regions.

A band of frontogenesis/coastal boundary will likely setup and
enhance snowfall totals into southern NH and southern Maine. WPC
probabilities for 6 inches are moderate in the Catskills, becoming
high in the southern Adirondacks, Berkshires, Greens, Northern
Worcester Hills, and much of the terrain of NH and southern Maine.

On Day 3/Wed...another shortwave will race across the Ohio Valley
Wednesday, with an accompanying surface low moving beneath it.
This will move eastward and transfer its energy to a secondary low
developing near Delaware which will become dominant and lift
northeast south of Long Island NY on D3. This will again spread
warm advection precipitation into the Northeast, but the
antecedent airmass is a bit warmer than with the first system. 
WPC probabilities are low for 4 inches, and again confined to the
higher terrain of upstate NY and interior southern to central New
England.

...Mountains of California, the Great Basin and CO Rockies...
Days 1-3...

A 700 mb shortwave departing southern CA this morning will slide
east Monday while slowly deamplifying. The moisture plume crosses
the ranges of central and southern UT and then western CO with
modest lift. Several inches of snow are expected in elevated
terrain, but modest lift limits heavy snow potential. Longer
duration leads to slightly higher amounts in the ranges of western
CO.

Tuesday into Wednesday, a trough will deepen off the West Coast
and then move east onshore. 700mb flow is confluent across
CA/NV/UT/western CO, driving 700mb RH above 90 percent across
these areas. Vertical velocity maxima occurring in the terrain
from the CA Sierra Nevada across the ranges of eastern NV, the UT
Wasatch/Uintas, and ranges of northwestern CO will result in
periods of snow, which are expected to be locally heavy.  WPC
probabilities become high once again for 6 inches in the CA Sierra
Nevada and the ranges of northern Utah, with 12-18 inches possible
in the Sierra.


...Mountains of the Pacific Northwest to the Northern Rockies...
Days 1-3...

A closed mid-level will drop slowly southward off the Washington
state coast today into Tuesday, before slowly filling and
advecting southeast onshore Oregon on Tue night-early Wednesday.
Moisture streams onshore and into the WA/OR cascades, with periods
of enhanced upper diffluence aiding topographic lift in producing
ascent. Increasing 700-500mb RH and ascent along a mid-level front
will drive increasing precipitation across this area, with
snowfall developing and persisting today into Tuesday.
The upper jet crosses the northern Rockies, bringing snow inland
to the ranges of ID/northwest MT, and then into WY Tue into Wed as
the jet persists across the region, allowing multiple upper
divergence maxima to produce lift.  
WPC probabilities through Tuesday are high for 6 inches in the
Olympics, Cascades, Northern Rockies, Sawtooth, and terrain of NW
WY including the Tetons, and 1 ft of accumulation is possible in
many locations.

...Northern Plains to upper MS Valley and Upper Great Lakes...
Day 3...
A band of snow is possible from the upper MS valley into the upper
peninsula of MI as a mid level wave approaches the region.  Light
snow is possible as low pressure develops along the frontal
boundary.  The UKMET/ECMWF/NAM/ECMWF Ensemble Mean indicate
post-frontal precip occurs with a change from rain to snow.  The
primary uncertainty is precip type as along the low track and
points south/east, it would likely be too warm for snow. The NAM
is typically too high with QPF in the cold sector but each of
these model solutions

The probability of greater than 0.25 inches of freezing rain is
less than 10 percent.

Petersen