FOUS11 KWBC 280709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
209 AM EST Tue Jan 28 2025

Valid 12Z Tue Jan 28 2025 - 12Z Fri Jan 31 2025

...Great Lakes, Northeast, Mid-Atlantic... Days 1-2...

Progressive pattern continues across the east as a cyclonic gyre
centered near the Hudson Bay drifts east, but vorticity lobes
shedding around it will maintain/re-amplify the upper trough into Thursday.

The first of these shortwaves will be racing east across Upstate NY
and northern New England this morning, pushing an arctic front
southeast in tandem. Scattered convective snow showers and isolated
snow squalls will remain possible along this front through the
morning hours, with briefly heavy snow rates above 1"/hr and gusty
winds combining to produce scattered impacts as far east as I-95 to
NYC and Boston. However, the intensity of these features is likely
to be much less substantial than occurring overnight into Tuesday morning.

Behind this front, a brief respite in snowfall will occur before a
second impulse digs rapidly from Alberta, Canada towards the Great
Lakes and Northeast. The interaction of this shortwave trough with
the leftover baroclinic boundary behind the cold front will yield
surface cyclogenesis in the form of an "Alberta Clipper" which is
progged to track quickly from the Arrowhead of MN this afternoon
through the Upper Great Lakes tonight, and then across New England
on Wednesday. Weak secondary development is possible east of
Massachusetts/Maine Wednesday as well, but should have marginal
impact on the winter impacts. Ahead of this clipper, intensifying
SW flow will drive enhanced WAA and moist isentropic ascent,
especially along 285-290K surfaces, to produce an expanding swath
of moderate to heavy snow. The DGZ becomes exceptionally deep
during this time as reflected by SREF probabilities of 90% for
100+mb of depth during the period of most impressive WAA. This will
create a near iso-thermal layer within the -12C to -18C
temperatures, explaining this deep DGZ. PWs are progged to exceed
the 97th percentile over MN/MI according to NAEFS, which will help
additionally enhance snowfall, and despite PWs falling to the east
with time, a period of heavy snow is likely within this downstream WAA.

Then, in the wake of this system and behind a subsequent cold front
Wednesday morning, lake effect snow (LES) will develop as NW flow 
causes additional accumulations in the favored NW snow belts. With
strong winds progged in forecast soundings, some of this could 
push as far SE as the coastal Mid-Atlantic states by Wednesday
night. WPC probabilities D1 and D2 for areas that receive both the
WAA snow and then the subsequent LES are moderate to high (50-90%)
for 4+ inches from the western U.P. southeast through Traverse City
area, east of Lakes Ontario and Erie, and into the western
Adirondacks, Greens of VT, and Whites of NH. Storm total snow of
12-18" is possible across the U.P. and the Tug Hill, with 6-12" in
lollipops elsewhere across this swath, especially in the higher terrain.

Finally, a second round of convective snow showers or snow squalls
continues to look likely Wednesday. These convective snow showers 
will occur during a more favorable time of day to support increased
CAPE forecast to reach 100-200 J/kg in the 0-2km layer, overlapping
some impressive low-level fgen from central New England, Upstate
NY, and into the northern Mid-Atlantic states. This secondary set 
of squalls will again be accompanied by strong winds and heavy 
snow rates leading to the potential for significant travel impacts 
despite modest snowfall accumulations. 

After this second front pushes east and snow squalls wind down
Wednesday evening/night, much of D3 will be quiet across the
region. A larger scale storm system is likely to approach late D3 
from the south with increasing moisture and some light mixed 
precipitation, but at this time any impacts from that event are 
most likely into D4, just beyond this forecast period, and WPC
probabilities for 0.01" of ice peak around 50% across northern PA
before the end of the forecast period.

The two rounds of potential snow squalls continue the issuance of 
Key Messages linked below.


...Southwest through the Four Corners... Days 1-3...

Modestly anomalous upper cutoff low (NAEFS 700-500mb heights
falling to below the 10th percentile of the CFSR climatology) will
roll slowly eastward from southern CA this morning. This feature
will move slowly (such is the nature of cutoffs) across the Desert
Southwest and Four Corners Wednesday before pinwheeling into the
Central Plains on Thursday. The exact track and placement of this
upper low remains uncertain due to vorticity lobes which will
periodically dance around the central gyre, tugging it in subtly
different directions at different times, but the large scale
pattern is well agreed upon at this time by the various global models.

This evolution will result in pronounced synoptic forcing across
the region from west to east, primarily due to height falls,
divergence, and periods of PVA. However, other forcing at play 
will enhance deep layer ascent. This includes increasing LFQ
diffluence as a subtropical jet streak intensifies downstream of
the closed low, especially Wednesday into Thursday across NM/CO,
and periods of upslope flow embedded within isentropic ascent.
There has also been an increase this morning in post-system
deformation as a surge of wrap-around precipitation develops over
eastern CO Thursday morning and then pivots southward. These
parameters together should produce sufficient lift in an area of
increasing moisture to produce warning-level snow above generally
4000-5000 ft in the Sangre de Cristos, San Juans, and Jemez
Mountains. However, there remains some uncertainty as to the
movement of an accompanying dry slot, which could lower
accumulations in some areas.

WPC probabilities for 4+ inches on D1 are minimal, but by D2 expand
considerably and amplify into the San Juans and Sangre de Cristos,
where they reach as high as 70-90% for 6+ inches, with locally over
1 foot possible in the highest terrain. Additionally WPC
probabilities are high (70-90%) for 4+ inches across the Raton Mesa
and higher elevations of I-25 near the NM/CO border. By D3 the 
event begins to ramp down, but WPC probabilities for 4+ inches 
above 30% continue in the Sangre de Cristos, and expand into parts 
of the Palmer Divide as well.


...Pacific Northwest... Day 3...

A trough moving eastward across the Northern Pacific will generate
an intensifying jet streak downstream, pivoting ascent and moisture
into the region after 00Z Friday. This will manifest as a corridor
of enhanced IVT as high as 500 kg/m/s (above the 90th CFSR
percentile according to NAEFS) reaching as far inland as Idaho
before the end of the period. This will drive an expanding area of
precipitation spreading eastward, but with the accompanying WAA
driving snow levels to as high as 5500 ft, much of this will fall
as rain. However, in the Olympics and Cascades, a heavy wet snow is
likely, which could accumulate to more than 4 inches before 12Z
Friday as reflected by WPC probabilities reaching 30-50% in these areas.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Weiss

...Winter Storm Key Messages are in effect. Please see current 
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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