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FOUS11 KWBC 070816
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Wed Jan 7 2026

Valid 12Z Wed Jan 07 2026 - 12Z Sat Jan 10 2026


...Northeast...
Day 1...

Amplified but progressive shortwave will track over New England 12Z
Wed - 00Z Thu, exiting quickly to the southeast of Cape Cod
tonight. The two distinct vorticity lobes embedded within this
shortwave will drive the two surface lows (one moving across far
southern Canada while a secondary low develops off the New England
coast), with a wedge in-between the two features supporting cold
air and overrunning precipitation. The primary mechanism for
precipitation development will be the PVA/height falls and
increasing isentropic ascent, especially as the secondary low
development occurs, before the flow shifts to produce NW winds and
CAA. There is still some uncertainty into exactly how strong and at
what speed this transition to CAA will occur, but the models
continue to support slightly more cold air, keeping snow the
primary p-type for northern New York and northern New England, with
a mix including freezing rain as far south as CT/RI, but even here
precipitation may end as light snow as the column cools.

Overall ascent is transient and moisture is just slightly
elevated, so total accumulations are expected to be modest. This is
reflected by WPC probabilities for an additional 2+ inches after
12Z Wednesday that are 30-50% in the highest terrain of
northern/central ME, and reach above 50% for 0.01" of ice, focused
across the Worcester Hills and Monadnock Region of NH.


...Pacific Northwest, Great Basin, Northern/Central Rockies...
Days 1-3...

Deep trough over the Pacific will shift its axis inland by Friday,
with longwave ridging expanding across the Pacific Coast on
Saturday. As this ridge blossoms, precipitation will finally wane
across the area, but until then an extended period of active
weather with significant snowfall will continue.

The strong jet currently in place will dig slowly southward in
tandem with an embedded mid-level impulse tracking towards the
Great Basin. This impulse will be generally within zonal flow,
although as the trough kinks to become more amplified, mid-level
divergence will also increase. Despite a weakening of the onshore
mid-level flow, the upper jet will maintain its intensity of around
100 kts, at least early D1, before weakening, providing sufficient
IVT to fuel heavy precipitation from the Pacific Northwest into the
northern Great Basin and northern Rockies. Ascent will be aided
both by upslope flow and a fast moving surface cold front. Although
forcing looks generally transient, WPC probabilities D1 into D2 are
high (>70%) for more than 4 inches of snow from the Cascades
through the Northern Rockies and into the Tetons, Wind Rivers, and
Uintas, as well as the Ruby Mountains of NV. During D2, forcing
along the front pushes into UT/AZ where additional snowfall
exceeding 4" is likely (>70%) in the Wasatch and Mogollon Rim.

Behind this cold front, snow levels will crash, potentially falling
to below 1000 ft across the Pacific Northwest, while a second surge
of moisture occurs. This second moisture plume is driven by the
upstream jet streak from the primary trough axis, with the
impressive LFQ diffluence overlapping a secondary vorticity lobe
for ascent. As this rides into the Cascades, additional forcing for
lift created by upslope flow will wring out moisture as additional
heavy snowfall in the Olympics and Cascades, with moisture spilling
into the Northern Rockies as well. Notably, with this second
impulse, lapse rates are exceptionally steep beneath the core of
the mid-level trough, so any heavier rates will likely pull snow
down to around 600ft (NBM 10th% snow level), suggesting even some
light accumulations are possible in the foothills surrounding
Seattle and Portland (although the true lowlands will likely miss
out on any snow this time around). Still, the greatest impacts will
again be across the Cascades and the Passes where WPC probabilities
D2 into D3 are high (>70%) for an additional 4+ inches, with 2-day
total snowfall of 2-3 feet likely in the higher terrain, with even
1-2 feet possible at the passes.


...Four Corners into the High Plains...
Days 2-3...

Elongated vorticity lobe will spin southeast from the Pacific
Northwest and amplify into deep longwave trough as it approaches
the Four Corners Thursday night. The intensity of this trough
remains uncertain as it amplifies towards the region around 12Z
Friday, but in general the trends have been for a deeper amplitude
while remaining positively tilted. At the same time, the resultant
jet streak downstream of this trough axis will deepen and pivot
northeast, reaching as high as 140 kts across the Southern Plains,
leaving favorable LFQ diffluence into the High Plains and Four
Corners. Where this jet streak overlaps with the most robust height
falls, lee cyclogenesis will likely result, likely in the northeast
plains of New Mexico as reflected by most ensemble members.

This low will move slowly Friday morning before elongating into
faster flow to the northeast by the end of the forecast period.
Before this occurs, a cold front will dig southward through the
High Plains, with the resulting wind field becoming more E/NE and
upsloping into the terrain of CO and NM. This will provide
additional ascent to the region, which will already be influenced
by favorable synoptic lift, suggesting a period of moderate to
heavy precipitation Thursday night through Friday aftn. Most of
this will fall as snow, especially as the cold front sags south and
cools the column through favorable CAA. However, there is still
uncertainty into how much snow will fall, especially from the
Palmer Divide northward where a significant 850mb moisture gradient
is expected. The ECMWF and its ensemble members are much drier
farther north than the other camps, but locations across southern
CO and northern NM have higher confidence in impactful snowfall,
including along the I-25 urban corridor. While the Palmer Divide
will likely experience significant snowfall due to the higher
elevations and more pronounced upslope, areas north of there,
including Denver, may experience lesser snow. However, in an area
that has seen very little snow so far this winter, this could
still be an impactful event for much of the region, with heavy snow
pulling northeast into OK/KS by the end of the period beneath the
upper jet streak.

Although confidence is modest for this time range, current WPC
probabilities are high D2 for at least 6 inches of snow across the
CO Rockies and into the White Mountains of AZ. Late D2 into D3 as
the surface low consolidates and upslope flow improves, WPC
probabilities indicate a greater risk, (50-70% chance), for at
least 6 inches of snow in the Front Range, Palmer Divide, Raton
Mesa, Sangre de Cristos, and into the high plains of southeast CO.


Weiss


$$

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