Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 PM EST Sat Mar 09 2024

Valid 00Z Sun Mar 10 2024 - 00Z Wed Mar 13 2024

...Eastern Great Lakes, Northeast, and northern Mid-Atlantic...
Days 1-2...

A southern stream shortwave trough over the KY/TN border this
afternoon will quickly lift up the Mid-Atlantic this evening, with
a developing surface low tracking from central NC to NJ.
Meanwhile, a potent northern stream shortwave trough over WI this
afternoon will quickly close into a low over MI this evening and
begin to interact with the southern stream wave overnight. This
will cause rapid further development of the surface low overnight
as it tracks through central New England by daybreak. A near phase
of these two waves occurs Sunday/Sunday night with the progression
of the surface low slowing as it crosses Maine through Sunday
night before slowly shifting east over Nova Scotia through Monday.

Wintry precip from this complex low pressure system comes from two
sources; from southerly flow ahead of the system this evening
across the Northeast before slowly shifting to a developing TROWAL
north of the low tonight/Sunday and then to wrap around flow
around the low from west to east late tonight over Upstate New
York, northern New England Sunday morning, then very slowly over
Maine through Sunday night. The heaviest precip will be with the
southerly flow given a sub-tropical connection to the Gulf/Gulf
stream with snow levels over the Northeast tonight generally
3000-5000ft and SLRs in the 5-7:1 range over the Adirondacks and
Greens/Whites, limiting heavy snow to only the highest peaks
there. Snow levels drop to 2000-3000ft behind the southern stream
wave, then cold air behind the northern stream low brings snow
levels to the surface starting on Sunday with lower precip rates,
but focused on terrain.

As the strong/phasing low shifts to Nova Scotia Sunday night into
Monday, strong CAA in its wake accompanied by further vorticity
streamers rotating through the flow will combine to re-invigorate
ascent within a cold column particularly north facing slopes of
the Adirondacks and far northern New England. Moisture from the
open eastern Great Lakes will increase as the strong CAA across,
driving impressive late-season LES downstream of Lakes Erie and
Ontario. Additionally, the intense flow will favorably upslope
into the Adirondacks and Greens, with Froude numbers indicating
sub-critical flow to result in the heaviest snow axes along or
upstream of the terrain crests. The SLRs will be much higher with
this secondary snowfall, especially in LES regimes, and as the
saturated column cools into the DGZ.

Day 1 snow probabilities for >8" are 40-90% over the Adirondacks,
Greens, and Whites with the note that most of the model derived
snow is during the southerly flow portion. Day 1.5 snow probs for
>8" feature the secondary wave with 20-40% probs east of Lake
Erie, and 40-80% probs from areas east of Lake Ontario through the
northern slopes of the Adirondacks. Then Day 2 snow probs for >8"
are 30-50% over the northern Greens to the Northeast Kingdom of
VT. Wrap around snow lingers over northern New England/Maine into
or through Monday.

Snow Squalls: As the northern stream upper low crosses the
northern Mid-Atlantic Sunday, a reinforcing cold front and
subsequent afternoon convective activity will produce snow
showers. A tight surface pressure gradient will results in strong
wind gusts, so a progressive snow squall with the frontal passage
and subsequent showers producing squall-like visibility concerns
and flash freeze conditions with rapidly cooling air. The
environment appears favorable for at least isolated snow squalls
from southern Upstate NY southward through PA to central WV in the
morning, then over central/eastern PA into northern NJ and
northern Maryland in the afternoon. Rapid reductions of visibility
and flash freezing of roadways (less so with flash freeze by the
I-95 corridor) in gusty conditions will make for difficult travel
conditions.


...The West...
Days 1-3...

A series of shortwaves continue to shift onshore over the
Northwest through Monday night, continuing an extended period of
unsettled weather with heavy snow each day across much of the
Northwest terrain down to the central Sierra Nevada.

The initial trough will cross the northern Rockies this evening
and Saskatchewan overnight as its amplitude decays. A more
substantial wave reaches the PacNW coast early Sunday with a
reinforcing impulse crossing far northern CA late Sunday. Stronger
height falls and downstream divergence with increasing moisture
will result in an expanding area of precipitation from the
Olympics eastward through the Northern Rockies, and diving as far
south as the Shasta/Trinity/Siskiyou region of CA, and even
reaching the northern Sierra. This second wave digs over more of
the Intermountain West (thanks to the reinforcing impulse) than
the first wave through Monday. In this progressive flow, a third
shortwave trough with an additional wave of moisture and ascent
leading to another round of snowfall in the terrain arrives to the
PacNW coast Monday night. Snow levels stay generally 2000-3000ft
over the PacNW through the forecast period and more like
3000-4000ft over the northern Rockies and 3000-5000ft over the
northern Sierra Nevada.

Day 1 WPC snow probabilities for >8" are 40-90% over the higher
Cascades, Olympics through the Shasta Siskiyou. Values generally
decrease for Day 2 between the second and third waves, but
increase over the northern Sierra Nevada with values of 30-60%.
For Day 3, the values for >8" are increased again over the length
of the Cascades and Shasta/Siskiyou, with similar values over the
northern/central Sierra Nevada to Day 2. 3-day snowfall,
especially above 3000 ft, should reach 2-4 ft in the Cascades and
northern CA ranges, with locally more than 6 ft possible on the
highest peaks. 1-2 ft can be expected over the northern Rockies of
central ID through western MT, northeast OR and northeastern WA.


Jackson