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FOUS11 KWBC 041927
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 PM EST Tue Feb 4 2025

Valid 00Z Wed Feb 05 2025 - 00Z Sat Feb 08 2025

...Pacific Coast to the Northern Rockies...
Days 1-3...

An active period of weather will bring repeating rounds of active
wintry weather to much of the west, as a series of Atmospheric
Rivers (ARs) and varying snow levels occur through the week.

The forecast period begins with an impressive closed low
positioned just west of the WA/B.C. coast. 500mb height anomalies
within this low are progged by NAEFS to fall below the lowest ever
recorded in the CFSR climatology offshore these states by Wednesday
morning, indicating the intensity of this feature. In response,
downstream ascent will be impressive through mid-level divergence
and an intensifying jet streak exceeding 140kts Wednesday pivoting
into central CA. The overlap of this jet streak with confluent
mid-level flow downstream of the closed low will surge moisture
into the West as an AR, with both GEFS and ECENS probabilities
indicating a high risk (>90% chance) for 500 kg/m/s IVT lifting
into northern CA. With impressive mid and upper level winds
driving this AR, spillover moisture could also be significant,
aided by a wave of low pressure shifting east towards the Northern
Rockies beneath a spoke of vorticity shedding from the main low.
This will help spread an axis of moderate to heavy snowfall from
central CA through the northern Great Basin and into the Northern
Rockies D1. WPC probabilities are high (>70%) for more than 6
inches along the Sierra, across much of the northern CA ranges
including the coastal range, across the Blue Mountains, much of the
Northern Rockies, and into the Wind Rivers/Tetons. As much as 4-6
feet of snow is possible in the Sierra, with 1-2 feet possible in
the higher terrain of the other ranges. Additionally, with snow
levels crashing to the surface, lowland snow and coastal range snow
will also be impactful in Oregon.

The wave of low pressure D1 will push a cold front eastward behind
it, which when combined with the approach of the upper low will
lower heights and steepen lapse rates considerably late D1 and into
D2. This will result in snow levels falling to less than 500 ft as
far south as central Oregon, which when combined with the steep
lapse rates (dry adiabatic through the mid-levels as reflected by
regional forecast soundings) and increasing synoptic ascent, could
result in heavy snow into the coastal ranges and even the lowlands
of WA/OR and northern CA. Available moisture with this second wave
is somewhat suppressed from the prior day, but should still support
rounds of moderate to heavy precipitation falling as snow. There
is a lot of uncertainty involving the snowfall amounts due to the
likelihood for more periodic rounds of snow rather than a
continuous plume of precipitation, but theta-e lapse rates AOB
0C/Km will support convective rates which could quickly and
efficiently cause snowfall accumulations. Despite the transient
nature of heavy snow showers, WPC probabilities across the lowlands
of OR and southern WA, as well as some of the lower elevations of
northern CA reach 50-70% for 2 inches, and around 30% for the
Portland, OR metro area. The coastal ranges will likely receive
significant snowfall D2 reflected by a 50-70% chance for 4+ inches.
Additionally, the continued moisture fetch onshore will bring
heavy snow again into the OR Cascades and northern CA ranges where
WPC probabilities for 6+ inches are at least 70%.

Then, during D3 /00Z Fri - 00Z Sat/ the closed low offshore WA/OR
opens, but remains amplified in response to a  maxima and
accompanying shortwave trough rotating around it, through the base
of the trough, and onshore northern CA once again. This will be
accompanied by a renewed surge of deep layer ascent through
secondary jet intensification and downstream height
falls/divergence, to push another swath of heavy precipitation
onshore. Much of this will be in the vicinity of a surface low
which will develop just downstream of this vorticity lobe, and this
low is progged to move progressively northeast through the Great
Basin, reaching the Central Rockies by the end of the forecast
period. Snow levels will rise south and east of this low, but as it
tracks along a stationary front, there will likely be continued
low snow levels to the north, with an axis of frontogenesis and
accompanying enhanced ascent into a deepening DGZ driving a swath
of heavy snowfall across the area. A weaker AR will accompany this
low, reflected by GEFS and ECENS probabilities for 250 kg/m/s IVT
peaking around 70%, but this will still be sufficient to produce
heavy snow, especially in regions of upslope or the strongest fgen.
The global models still feature quite a bit of variability in
placement and timing for this second AR and low, so confidence in
is somewhat modest this far out. However, WPC probabilities are
already high (above 70%) for 12+ inches in the Sierra D3,
especially above 4000 ft. Heavy snow exceeding 6 inches is also
likely (>70%) for the ranges of NW WY.



...Upper Midwest/Great Lakes...
Day 2...

The lead shortwave ejecting downstream of the closed low west of
WA/OR will spin east out of Montana, reaching the Northern Plains
by Thursday morning and then racing east within pinched/progressive
flow to the Great Lakes Thursday night. This feature will help
spawn a weak wave of low pressure moving from the Northern Plains,
with accompanying downstream WAA and isentropic ascent resulting in
an expanding area of precipitation falling as snow on Thursday. The
overlap of synoptic lift with this isentropic ascent could result
in axis of heavy snow, especially along the international border
with Canada in MN and into WI and the U.P. of MI where the DGZ
depth is greatest as reflected by moderate probabilities (50-70%)
for at least 50mb of DGZ depth. The system is progressive which
will limit overall accumulations, but heavy rates of 1"/hr at times
will still produce heavy snow exceeding 4" as reflected by WPC
probabilities of 30-70% in the Arrowhead of MN, as well as the
Western U.P. of MI and Bayfield Peninsula of WI.


...Midwest to Northeast...
Days 2-3...

A fast moving upper-level disturbance over the Central Plains
Wednesday AM will be the primary feature in providing upper-level
ascent from the Midwest Wednesday evening to the Northeast on
Thursday. The progressive 500mb shortwave trough will be located
beneath the divergent left-exit quadrant of a roaring 150kt 250mb
jet streak located over the Rockies. At lower-levels, an 850mb
ridge over the subtropical Atlantic will work in tandem with
longwave 850mb troughing over the Northern Plains to create a 50kt
850mb jet over the Mississippi/Ohio River Valleys Wednesday night
that will act to deliver modest low-mid level moisture. NAEFS shows
IVT values by 06Z Thursday are topping 750 kg/m/s over the OH
Valley, which is above the 97.5 climatological percentile in the
CFSR climatology. In addition, 850-700mb WAA and 290K isentropic
glide amidst SWrly flow will lead to an gradual incline of the FGEN
axis over the OH Valley on east to the Mid- Atlantic Wednesday
evening. Last but not least in the overview of the key contributors
in this icy setup is the presence of a colder/dry air-mass that
will be anchored initially by a 1028+mb high that is above the 90th
percentile according to NAEFS. The high will be moving well
offshore by Thursday morning, but the linger cold-air damming
signature (CAD) will stick around for much of the event.

In these classic "overrunning a CAD wedge" setups, most guidance
tends to erode the <32F near-surface wet-bulb temperatures too
quick. Timing-wise, an icy wintry mix looks to get started in
northern IL around 00Z Thurs with some freezing drizzle, sleet, and
even light snow breaking out across the Ohio Valley and Mid-
Atlantic between 00-06Z Thurs. The heaviest FZRA occurs in the
Lower Great Lakes states between 03-09Z Thursday, then from the
central Appalachians to southeast PA and northern MD between
06-15Z. Sub- freezing surface temperatures will stick around
longest in the Laurel and Potomac Highlands, the Blue Ridge
Mountains, and into parts of the Shenandoah Valley where sub-
freezing air is harder to erode. By midday Thursday, a coastal low
begins to take shape off the Mid-Atlantic coast that would allow
for some Atlantic moisture to be advected into the Northeast
Thursday afternoon. The air-mass is more supportive of snow in
southern New England and as far north as the Adirondacks. While
most totals should generally be in the 2-4" range in these areas,
should the coastal low strengthen sooner, some mesoscale banding to
the north of the coastal low could occur over southern New
England.

Most guidance has come into better agreement on the timing
and amounts with a swath of >0.1" of ice accumulations from central
IL and across the Lower Great Lakes states to southeast PA. WPC
probabilities have focused on the Laurel and Potomac Highlands, as
well as parts of the Alleghenies and Blue Ridge as having the
better odds of seeing >0.25" ice accumulation. The Laurel Highland,
most notably, sport moderate chances (40-60%) for ice accumulations
>0.25". Most ice accumulations that are observed will generally
fall below 0.25" but even amounts around 0.1" can result in
hazardous travel conditions. WPC probabilities show at least
moderate chance probabilities (>50%) for >0.1" of ice for northern
IN, northern OH, much of western PA (excluding areas south of
Pittsburgh), western VA, and along the Mason-Dixon line on east ot
I-83. There could be some portions of the western DC/Baltimore
suburbs that receive as much as 0.1" of ice as well for the
Thursday morning rush hour. The WSSI does depict Moderate Impact
potential (hazardous driving conditions and disruptions to daily
life such as closures/delays) in the DC/Baltimore metro areas,
particularly west of I-95 and north of I-66. Moderate Impacts are
also shown in the Laurel and Potomac Highlands, the Blue Ridge
Mountains, and the northern extent of the Shenandoah Valley. All
other areas previously mentioned from northern IL on east through
the Lower Great Lakes, central PA, and into New England are
forecast to see Minor Impacts from this event, highlighting the
potential for some slick roads and troublesome travel conditions.

Given the relatively large geographic coverage of at least 0.10"
ice potential and the increasing chances for over 0.25" of ice
accretion in parts of WV, MD, VA, and PA... Key Messages continue
for this event.

Weiss/Mullinax




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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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