Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
110 PM EST Mon Mar 04 2024

Valid 00Z Tue Mar 05 2024 - 00Z Fri Mar 08 2024

...West Coast, Northern Great Basin, & Northern/Central Rockies...
Days 1-3...

A broad mid-level trough encompassing much of the West will be
driven by split-flow coming in from the Pacific Ocean. Within the
northern stream, a potent but strung-out vorticity lobe will
stretch NW to SE from British Columbia into WY on D1, while a
closed southern stream low spins well west of CA. Between these
two features, confluent flow will develop as the westerlies become
pinched, surging moisture eastward as a modest IVT extends into
the northern Great Basin D1 into D2. This will result in a narrow
corridor of enhanced mid-level RH, which will be acted upon by PVA
and modest upper diffluence as the tail of an upper jet streak
arcs eastward. Together, this will drive expansive precipitation
from northern CA almost due east into the NW WY ranges D1 and D2,
with snow levels generally ranging between 2000-3000 ft, but as
high as 5000 ft in the Sierra. However, a slowly southward sinking
cold front across this area will likely drive some enhanced fgen
and more intense snowfall rates, which could cause some
accumulation below these snow levels, and as this front sags,
lowering snow levels will also occur from north to south. This
suggests that while the heaviest snowfall is likely in the
terrain, especially where upslope flow is favored on the nearly
zonal flow, the heavy snow footprint may not follow the terrain
exactly due to the areas of more intense ascent.

This is reflected by WPC probabilities for more than 6 inches of
snow reaching above 80% D1 from the OR Cascades and
Shasta/Siskiyou region eastward into the Salmon River range and
Tetons, but including some lower elevations of eastern OR. During
D2, the overall footprint of high WPC probabilities for 6+ inches
remains mostly unchanged, but the focus shifts into NW WY
including the Tetons and Wind Rivers, while continuing further
west. 2-day total snowfall of 2-4 feet is possible in the highest
Cascades, northern Sierra, and Tetons, as well as around Mt.
Shasta.

During D3 the primary overlap of moisture and ascent weakens and
shifts east, but a secondary surge of moisture and southern stream
lift downstream of the CA closed low will spread some moderate
snowfall into the Sierra where WPC probabilities for more than 6
inches reach 20-30%.


...Northern Plains...
Days 2-3...

A closed mid-level low moving across southern Alberta and
Saskatchewan will shed a lobe of vorticity through the base of the
accompanying trough and into the Northern Plains Wednesday aftn,
with this impulse deepening into a negatively tilted shortwave as
it ejects quickly into Ontario by Thursday morning. Despite the
rapid progression, the subsequent quick deepening will result in a
surface wave moving eastward as ascent is aided by modest upper
divergence. Downstream of this surface low, a modest baroclinic
gradient along the warm front will help drive some WAA/fgen, with
downstream moist advection supporting an expanding area of
precipitation which should fall primarily as snow in ND/MN. The
regional forecast soundings indicate a deepening DGZ to support
intensifying snow rates, and as the theta-e advection wraps
cyclonically into a weak TROWAL, a swath of heavy snow
accumulation is becoming more likely, and WPC probabilities for
more than 4 inches have increased to 20-40%, highest near the
northern ND/MN border.


...New England...
Day 3...

An amplifying shortwave lifting along the Mid-Atlantic coast will
take on a subtle negative tilt and combine with the RRQ of a
departing upper jet streak to produce surface cyclogenesis lifting
up the eastern seaboard and then off New England late Wednesday
into Thursday. PW advection will be pronounced downstream of this
trough and a plume of higher moisture will likely lift northeast
on increasing 295-300K isentropic lift into New England. The
column initially appears too warm for anything but rain, but a
southeast advancing cold front will serve to cool the column and
also enhance an axis of deformation into which this moisture will
converge. This could result in a rapid transition from rain to
heavy snow, especially across northern ME and the higher
elevations of NH/central ME, but confidence at this time is low
due to a lot of model timing spread. Current WPC probabilities,
however, reflect an increase in the chance for more than 4 inches
of snow, rising to as high as 10-20% in northern/eastern ME.


Weiss