FOUS11 KWBC 030900
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EST Fri Jan 3 2025

Valid 12Z Fri Jan 03 2025 - 12Z Mon Jan 06 2025

...Western U.S....=20
Days 1-3...

Precipitation will continue to spread inland across western Oregon=20
and northwestern California ahead of an occluded low approaching the=20
coast this morning.  Deep, moist southwesterly flow ahead of the=20
boundary, along with strong forcing will support an uptick in=20
precipitation, with orographically focused moderate to heavy amounts=20
forecast to develop along the coastal ranges into the southern=20
Cascades and northern Sierra Nevada.  However, high snow levels are=20
expected to confine any heavy snow accumulations to the highest=20
peaks early on.  Then, as the upper trough and associated cold front=20
begin to move inland, snow levels will begin to lower, expanding the=20
potential for heavy snow along the southern Cascades, the=20
northwestern California ranges, and the northern Sierra later today.=20
Meanwhile, a well-defined warm front, lifting across the northern=20
Cascades, Intermountain West, and Rockies, will be a focus for=20
organized precipitation and locally heavy mountain snow as well=20
today.=20

Meanwhile to the east, moisture focused by low-to-mid level=20
convergence along the western periphery of strong high pressure=20
extending from central Canada into the Northern Plains will support=20
a swath of light to moderate snows extending from central Nebraska=20
northwestward through the High Plains and into the northern Rockies.=20
 Upslope flow is expected to help enhance amounts across the central=20
to the northwestern Montana ranges.=20

By early Saturday, the upper trough is expected to bring high=20
elevation snow into the central Rockies as it moves across the Great=20
Basin into the region.  This will be followed by a second system,=20
which is forecast to bring additional orographically focused heavier=20
amounts into the Olympics and the northern Cascades Saturday into=20
early Sunday.

Conditions will remain unsettled across the Northwest into Monday,=20
as a third shortwave impacts the region.  However, a building=20
upstream ridge will preclude any long fetch of deep moisture and the=20
threat for widespread heavy amounts across the region.

WPC probabilities indicate that three day totals exceeding 8 inches=20
will be mostly confined to the higher elevations of the Cascades,=20
northern Sierra Nevada, and portions of the Rockies from=20
northwestern Montana to northern Utah and Colorado. Widespread, but=20
lighter amounts are expected across central and southeastern=20
Montana, with high probabilities for accumulations greater than 4=20
inches covering much of the region.=20=20


...Great Lakes and Northeast...=20
Days 1-2...

The strengthening -NAO block over Greenland and the North Atlantic
is aiding a mature surface cyclone to stall over Quebec and help
produce persistent cold northwest flow between it and a strong
surface high over Saskatchewan. This will prompt favorable LES on
northwesterly flow along with a few upper level disturbances
traversing the Great Lakes over the next few days. The Great Lakes
will gradually cool as a result, but the lapse rates will remain=20
steep enough over and downwind of the lakes to where lake-induced=20
instability could top 500 J/kg on occasion. The snow belts of the=20
Michigan U.P., northern portion of Michigan's Mitten, from=20
northeast Ohio on north east along the Chautauqua Ridge, and from=20
the shores of eastern Lake Ontario to the Tug Hill will be most=20
favored for heavy snow through early Sunday AM. Favorable fetch
connecting a band from lakes Superior, Huron, and Ontario will help
produce the most significant amounts (1-2ft+) in Upstate NY between
Syracuse and the Tug Hill. Snow could also be measured in feet=20
from northeast OH, northwest PA, and western NY (south of Buffalo).=20
Snowfall rates in these areas of 1-2"/hr are expected and could
come in waves over the next few days. The WSSI shows Major Impacts
for these areas with even a small Extreme area (extremely=20
dangerous to impossible travel and widespread closures) in the Tug=20
Hill through Sunday AM.=20


...Central Appalachians and Mid-Atlantic...
Day 1...

A compact wave of low pressure moving across the Ohio Valley early
this morning will move progressively eastward beneath a modest=20
shortwave racing southeast within the broad trough amplifying=20
across the eastern CONUS. Immediately in the wake of this=20
shortwave, a potent jet streak reaching 150kts will dig rapidly out
of the Northern Plains, producing a narrow but intense corridor of
overlapping ascent through height falls, PVA, and LFQ diffluence.=20
This will help at least subtly deepen the low and the accompanying=20
baroclinicity/attendant fronts as it moves eastward to off the VA=20
coast Friday night.

The system is compact and progressive, so snowfall in general is=20
expected to be low outside of the Central Appalachians. Here,=20
despite the modest forecast depth of the DGZ, appreciable upslope=20
flow will drive ascent favorably into the DGZ, while some moisture=20
enhancement occurs on post-frontal NW flow from the Great Lakes.=20
This suggests an extended period of moderate to heavy snow,=20
especially D1, with the higher elevations receiving the most=20
significant snow accumulations. Event total snow here could reach=20
double- digits in the higher terrain.

Finally, although total accumulations are expected to be minimal, some
snow showers or convective snow showers moving along the cold front=20
may crest the Appalachians and move across the Mid-Atlantic states
Friday afternoon as the vorticity max dives southeastward. The=20
late afternoon timing of this vort max allows for steep lapse rates
from the sfc all the way up into the DGZ, which could promote brief
snowfall rates above 1"/hr as shown by low probabilities in the WPC
snowband tool. Low-level temperatures will be slightly above=20
freezing from Richmond, VA to Philadelphia, PA, but wet-bulb
temperatures below 0C could allow any heavier rates to accumulate
on roads and elevated surfaces, causing hazardous travel despite=20
minimal accumulations, during the Friday aftn/eve commute.
Additionally, where low-level instability is the greatest,
especially overlapping the higher moisture content focused along
the MD/PA line, isolated snow squalls are also possible.


...Northern High Plains...
Days 1-2...

The forecast remains on track and very similar to the previous
issuance as an amplified upper level trough moving into the=20
Pacific Northwest this evening will spread downstream ascent=20
through increasing divergence into the Northern High Plains by=20
Saturday morning. This feature is progged to split into dual=20
vorticity lobes, with the primary one swinging into the Central=20
Rockies, while a secondary impulse lifts into Alberta/Saskatchewan.
Downstream of this trough, weak shortwave ridging will initially=20
be placed over the region, but this will gradually become displaced
by the approaching trough, leading to increasingly strong deep-=20
layer ascent. This forcing will act upon a saturating column as=20
290K isentropic ascent maximizes in a narrow corridor from NW to SE
from the foothills of central MT east into the High Plains of SD,=20
which will effectively overlap with a stripe of robust 700-600mb=20
fgen. The overall setup is not ideal for extremely heavy snow, but=20
an expanding footprint of snowfall is likely, and rates will be=20
enhanced by a cold column with very fluffy SLRs expected.=20
Additionally, the guidance has become more excited about snowfall=20
potential as reflected by an EFI of 0.8 to 0.9, primarily due to an
extended period of moderate to at times heavy snow. WPC=20
probabilities have increased as well, and now feature a 70-90%=20
chance for 4+ inches across central MT, with locally up to 10=20
inches possible in some of the higher terrain, and a 30-50 chance=20
for 4+ inches across far NW SD.=20


...Central Plains through the Mid-Atlantic...
Days 2-3...

Guidance remains aligned on a significant winter storm to impact a
large area spanning from the Central Plains across the Ohio Valley
and to the Mid-Atlantic Saturday night through Monday. While there
continues to be considerable temporal and some latitudinal spread=20
in the models, the overall evolution and footprint is quite=20
similar, with impactful snow, sleet, and freezing rain expected.

Starting Saturday afternoon, a deepening shortwave will eject from
the Central Rockies and move into the Southern Plains by Sunday=20
morning while taking on a negative tilt. At this point there is=20
already considerable differences in the timing and intensity of the
upper low, with the EC/UKMET and accompanying ensemble the=20
fastest, the GFS/GEFS making up the slow/strong end of the=20
envelope, and the CMC/CMCE in the middle. That being said, the
answer likely lies somewhere in between but could lean towards
either camp. The ensemble sensitivity analysis from 12z 1/2 would
suggest the differences begin this afternoon as the trough moves
ashore the West Coast, so these differences should be brought
closer together in a few more forecast cycles. The WAA regime in
place ahead of the storm without a strong surface high to the=20
north- northeast would support some of the more northern solutions,
but the upper- level configuration in the western Atlantic limits=20
this potential. Regardless, significant spread in timing and=20
intensity results in lower than typical confidence for D3.

Despite uncertainty in timing and placement, confidence remains in
an impactful winter storm. Regardless of the exact placement, the=20
setup is favorable for a widespread wintry precipitation from the
Central Plains/Mid-Mississippi Valley by the end of D2 through the
Ohio Valley D3 and eventually Mid-Atlantic D3-D4. As the upper low
tilts negatively over the central U.S. and then closes off, a=20
subtropical jet streak will sharpen and rotate around the base of=20
this trough leading to coupled downstream ascent. At the same time,
increasing downstream moist advection on low-level flow from the=20
Gulf of Mexico will rapidly saturate the column as isentropic=20
ascent along the 295K surface lifts northward and then pivots=20
cyclonically into a TROWAL around the developing cyclone.=20

As this low intensifies, in conjunction with the strengthening=20
WAA, the setup seems to conceptually support a laterally=20
translating band of heavy precipitation from west to east. With=20
DGZ depth probabilities rising to above 30% for 100mb of depth, and
a an overlap of folding theta-e with -EPV in cross sections
supporting CSI/CI, an impressive band with snowfall rates at least
1-2"/hr appears likely. Additionally, a secondary axis of=20
deformation could lengthen the heavy snow in some areas with=20
more impressive snowfall rates.

There is still uncertainty as to where the transition zone between
heavy freezing rain, sleet, and snow will occur but the EFI is=20
highlighting values exceeding 0.9 with an SoT of 2 from eastern KS
through southern IN, and the WSSI-P is suggesting a greater than=20
70% chance for moderate impacts in this same geographic window.
Given the strong WAA and deep low-level cold airmass in place,
sleet could become a dominant ptype from parts of eastern KS
through southern IN/northern KY, with over an inch of sleet
accumulation possible for some areas.

In addition to impacts, although confidence in exact placement is
uncertain, WPC probabilities current reflect a high risk (>70%) for
8+ inches near the NE/KS border through northern MO and into
central IL, with a stripe of high probabilities for 6+ inches=20
extending east along I-70 through IN and west-central OH. On the=20
south side of the system, WPC probabilities indicate high chances
(70-90%) for at least 0.1" ice accretion in a stripe from eastern=20
KS through southern MO and into southern IL, southern IN and much
of KY. 0.25" ice probs of 40-60% are found from southern MO to KY.

This system has prompted the issuance of key messages which are=20
linked below.


Pereira/Snell




...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!5H5hnZtkp-rhOKjNWO-8xlb6XuPlMQYf5zBVWlPiut_jM=
3Voua0gBjlHmCCAm4LHwDzSYF2hgcUK7B6VKkxupQIWpz0$=20

$$

=3D =3D =3D
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