FOUS11 KWBC 171052
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
551 AM EST Wed Jan 17 2024

Valid 12Z Wed Jan 17 2024 - 12Z Sat Jan 20 2024

...Prolonged Great Lake Effect Snow... Days 1-3...

Core of sprawling upper low centered south of Hudson Bay this
morning will eject east today, but upper troughing will linger
over Ontario/shift south across the Great Lakes through Friday
night, maintaining LES from all five Lakes through this time. A
cold front crosses the lakes from NW to SE on tonight into
Thursday with continued Wly flow. The aforementioned troughing
approaches Thursday night with surface low pressure to the south
causing flow to back northerly, opening new LES snow belts by late
Friday night - most notably single banding shifting down Lake
Michigan into NW Indiana. Only the margins of the Great Lakes
icing so far in this cold snap according to GLERL, with water
temperatures generally +4C. The persistent CAA across these still
warm lake temperatures will result in steepening lapse rates to
drive inversion depths to potentially above 10,000 ft, highest
east of Lake Ontario, which will support lake-induced instability
of 500-750 J/kg. Additionally, forecast soundings still indicate a
favorable cross section of ascent into the DGZ, further evidence
of potential for additional heavy snowfall. With 850mb
temperatures wavering at times between -10C and -25C, it is likely
that any bands of lake effect snow will produce heavy rates of
1-2+"/hr, and although the multiple boundary passages will cause
wavering to the Wly flow into Thursday morning before a lull when
the northerly flow sweeps through Thursday night.

Days 1 and 2 WPC snow probabilities for >6" are present in the
Keweenaw Peninsula, around Buffalo and north from the Tug Hill
with Day 2 a bit south of 1. heavy snow maximize in the favored
N/NW LES belts across the U.P. of MI, and east of both Lakes Erie
and Ontario. WPC probabilities for more than 6 inches reach
50-80+% both days in these areas, and 2-day snowfall could add up
to several feet in some areas, especially in the Tug Hill Plateau.
Day 3 probs are notable in NW Indiana for the single band off Lake
Michigan and around Cleveland given the additional fetch from
Huron/Georgian Bay (probs are very low off Lake Ontario which does
not have the added fetch that Erie has in north flow).

...Pacific Northwest through the Central Rockies... Days 1-3...

An active pattern will continue to bring rounds of heavy
precipitation to the Pacific Northwest, with heavy icing and heavy
snow both expected into or through Friday.

A compact mid-level low moving on the OR Coast early this morning
will weaken into an open wave over OR this morning and shift to
the northern Rockies rest of today and provide considerable
forcing for ascent as it races east in the left exit region of a
potent 110+kt NWly Pacific jet streak. Moisture ahead of this low
will continue to surge inland with snow levels around 5500ft over
OR and around 5000ft in the western slopes of the northern Rockies
today. Immediately behind this low is a northern stream trough
that shifts down the southern BC coast this morning and surges ESE
over northern WA/ID/MT this afternoon through tonight, providing
lift along the remaining baroclinic zone with Arctic air to the
northeast and producing a notable swath of heavy snow with 2"/hr
rates common per 00Z HREF means across the WA Cascades this
afternoon, and the ID/MT into northwest WY Rockies tonight where
Day 1 snow probs for >12" are over 70%. Pacific moisture streaming
over the northern to central Rockies also allows over 70% probs
for more than 8" over the Sawtooths, Wasatch and northern CO
Rockies. Furthermore, surface ridging extending from the lee of
the Canadian Rockies through northeast MT make for a strong low
level frontogenetical zone over north-central MT this afternoon
which the northern stream wave tracks over late tonight. Bands of
heavy snow develop this afternoon and shift southeast, weakening
as they lose topographical lift by the time they reach SD
overnight - Day 1 snow probs for more than 6" is over 70% on the
north-central MT High Plains.

Ongoing freezing rain will continue from the Portland metro up
through the Gorge and into the Columbia basin today before precip
diminishes (though briefly) from west to east as ridging builds in
behind the two aforementioned waves. Day 1 WPC probabilities D3
for more than an additional 0.1" of ice after 12Z is 30-50%
through these areas.

The lull in activity over the Pacific Northwest is short as
moisture ahead of the next system reaches the OR/WA coast on SWly
flow late tonight. Snow levels surface up to 7000-8000ft over OR
and western WA Thursday, though cold air remains entrenched over
the Columbia Basin. Day 2 snow probs for over 8" are generally
40-70% for the WA Cascades.

However, surface cold air lingers west of the Cascades and streams
down the Columbia Gorge to reignite freezing rain chances midday
Thursday into Friday. Day 2 ice probs for over 0.1" are 30-60%
over portions of the Columbia Basin, the Gorge, the Portland
metro, with lower probs north up through the Seattle metro.
Probabilities decrease for Day 3, but linger near the Columbia
Gorge.

...Northern/Central Plains... Days 2-3...

The aforementioned northern stream wave that crosses north-central
MT late tonight shifts southeast over the central Plains Thursday.
While weakening of the related snow bands is expected as they come
out of Montana late tonight, a boost from waves rounding the low
over Ontario and the increasingly diffluent left exit of a NWly
upper jet streak over the northern Rockies that strengthens to
130kt+ Thursday morning. This will yield a shallow wave of low
pressure moving across WY, with modest downstream WAA along a
leading warm front surging moist isentropic ascent for SD/NE into
IA through the day Thursday. While available moisture is modest,
there may be some enhancement due to the fgen response to the
favorably placed upper jet streak above the low-level fgen along
the sloped warm front. Given the in place Arctic-sourced air, deep
DGZ is expected with a fast moving band of very high SLR snow
which could accumulate to several inches as it advects east. Day 2
WPC snow probabilities for more than 2" are 40-60% over central SD
to eastern Neb.

...Mid-South... Days 2-3...

A weak shortwave and accompanying PVA will slide southeast from
the Missouri Valley late Thursday into Friday bringing weak height
falls and PVA as the shortwave fills with time. Modest moist
advection on increasing isentropic ascent downstream of this
feature will surge PWs favorable to support a stripe of light to
moderate precipitation from the Mid-South east over the southern
Appalachians. Due to shallow saturation, there is potential for
freezing rain in this stripe with Day 2 ice probs for over 0.01"
are 10-30% with 40-60% probs in western TN.

...Ohio Valley, Mid-Atlantic into Northeast... Days 2/3...

A fast moving shortwave trough shifts east below the dislodging
low core over Ontario and through the Midwest/OH Valley Thursday
night and over the Mid-Atlantic Friday. The flow is fast, and the
shortwave is of modest intensity, but it will be efficiently
topped by a potent Wly 150+kt jet streak diving through the trough
to enhance deep layer ascent. At the same time, a cold front will
make its way eastward from the Great Lakes to the Mid-Atlantic
states and the associated fgen along this front could be enhanced
by the upper jet streak pivoting overhead. This will enhance
ascent and will result in a coastal low developing east of the
Carolinas Thursday night and then racing east-northeast Friday.
PWs downstream of the best ascent are modest within the generally
zonal and fast flow. The CMC remains the most bullish with this
wave. Day 2 snow probs for over 2" are 30-60% from central IL
through OH and over eastern KY into WV. Day 3 snow probs for over
4" are 40-70% over the central/WV Appalachians into the Laurels of
PA where colder temperatures and some upslope flow should result
in efficient snowfall accumulations and generally 10 to 30% over
PA/MD/DE up through southern New England.

Jackson


***Key Messages for Northwest Winter Storm***

--Active Pattern Continues
An upper trough crosses Washington this afternoon and Montana
tonight, while a second storm system directs another round of
Pacific moisture across the Pacific Northwest Thursday into Friday.

--Continued Ice Threat for Columbia Basin/Gorge
Freezing rain continues today east from Portland with ample cold
air draining from the Columbia Basin through the Gorge. Freezing
rain is likely over portions of the Cascades and Columbia Basin
with the next round Thursday and Friday. An additional inch of ice
is possible around the Columbia Gorge.

--Heavy Snow into Thursday
Particularly heavy snow ahead of the upper trough crosses the
Washington Cascades today with snowfall rates exceeding 2../hr
expected above the 2500ft snow level. This impactful heavy snow
then crosses the Idaho, Montana, and northwest Wyoming Rockies
tonight with continued risk for 2../hr snowfall rates. Bands of
locally heavy snow also develop over the north-central Montana
High Plains tonight.
$$
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