Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Mon Nov 18 2024

Valid 00Z Tue Nov 19 2024 - 00Z Fri Nov 22 2024

...Pacific and Interior Northwest

Days 1-3...

***Impressive and long-lasting atmospheric river event to bring
 heavy snow to the Northwest***

Broad cyclonic flow across the Northwest will amplify impressively
as a strong closed low digs off the British Columbia coast and
drifts southward west of WA/OR through Wednesday. This intense
cutoff low is progged to create height anomalies that fall below
anything in the CFSR climatology (in this 3 week period) at both
500 and 700mb heights. This will drive incredible onshore
advection of wind and precipitation as an intense surface low
develops beneath this cutoff and shunts a cold front southeast
towards the coast. This front is progged to stall as the upper low
pivots in place through Wednesday before finally beginning to fill
and retrograde back to the northwest, leaving persistent and
impressive IVT surging onshore. IVT probabilities from the
ECENS/GEFS are above 60%, and from the West-WRF above 90% for
750 kg/m/s lifting onshore near 40N, with plumes suggesting the
potential for IVT reaching 1000, highest on D2. This impressive and
long-lasting IVT surge /AR/ will push multiple rounds of heavy
precipitation onshore, especially beginning early D2, and notably
where intense upslope flow will enhance ascent leading to snowfall
rates of 2-3"/hr at times.

Snow levels will begin the forecast period quite low, only around
200 ft east of the Cascades to around 2000 ft in northern CA.
However, the WAA accompanying the AR will raise snow levels
steadily D2 and D3, reaching as high as 4000 ft in northern WA and
the Northern Rockies, and to 8000 ft across northern CA. Despite
this climb, impactful snow is likely at many passes before a slow
transition to rain occurs during D3.

WPC probabilities for 6+ inches D1 are above 70% along the spine of
the OR Cascades and into the Olympics, with additional low
probabilities below 40% encompassing parts of the interior
including the Tetons, Big Horns, Northern Rockies, and even parts
of the CO Rockies. However, the more significant event is D2-D3
when WPC probabilities for more than 6 inches each day are above
90% from the Shasta/Trinity region northward along the OR Cascades
and into the WA Cascades/Olympics, and then east into the Okanogan
Highlands and the Salmon River/Sawtooth/Blue Mountains. 2-day
snowfall will likely reach 1-3 feet in many of these areas, with
snowfall of 6 or more inches likely at many of the passes as well.


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

The mid-level pattern across the CONUS begins to sharpen today,
becoming impressively amplified by mid-week. This begins with a
500mb shortwave trough over southern Alberta sinking southward
through Tuesday while deepening into a closed low over North Dakota
by the end of D1. Throughout D2 this low strengthens further into a
large gyre spinning over the Upper Midwest in response to repeated
vorticity lobes shedding through the flow and phasing into the more
pronounced system. This development will pair with a southward
advancing jet streak out of Canada and a surface cold front moving
eastward to result in deepening cyclogenesis across the Upper
Midwest. Moisture advection reflected by PWs surging to as high as
+4 sigma Tuesday downstream of this low will begin to pivot
northwest as a theta-e ridge lifts into a robust TROWAL, especially
late D1 into D2 across ND, with the greatest theta-e advection
merging into the DGZ.

As the TROWAL pivots southward it will interact with modest
deformation to produce a swath of heavy snow, which despite strong
winds beneath the DGZ to fracture dendrites, will likely result in
snowfall rates above 1"/hr at times. These rates combined with
fluffy SLRs and strong winds will produce significant blowing snow
impacts, regardless of how much snow accumulates. However, at this
time, significant snowfall is appearing more certain, noted by WPC
probabilities for more than 4 inches reaching 20-30% in NW ND D1,
and expanding more impressively into eastern ND and far western MN
D2, peaking above 60%. Locally 6-8" of snow is possible across some
of the higher terrain surrounding the Red River Valley of the
North.


...Central Appalachians...
Day 3...

An expansive closed 500mb low will be positioned over the Upper
Midwest/Western Great Lakes at the start of D3, resulting in a
vertically stacked and occluded low pressure. Throughout D3
/Wednesday night into Thursday/ this closed low will wobble
eastward as spokes of vorticity maxima rotate cyclonically around
this feature, tugging it slowly eastward, while also pushing a
secondary surface low, which is extended from the occluded front,
across the Mid-Atlantic and towards the Northeast. This low will be
trailed by dual cold fronts to cause rapid cooling of the column,
and the resultant post-frontal flow will result in heavy snow
developing in the upslope regions of the Appalachians, with
additional enhanced lift occurring in the vicinity of a secondary
wave dropping south out of Michigan Thursday.

There is still some uncertainty into the timing of the secondary
cold frontal passage, and how impressive ascent will be in the
vicinity of the occluded low dropping into the OH VLY. A faster
progression or deeper low may result in more widespread moderate
snow as dynamic cooling can overcome marginal thermals, but a more
south or west low could reduce upslope potential especially across
the higher terrain of PA. At this time the greatest threat for
significant accumulations continues to focus in the high terrain
of WV, the panhandle of MD, and into SW PA. Here, recent WPC
probabilities for more than 4 inches of snow are 50-70%, with more
significant accumulations likely into D4.


The probability of significant ice across the CONUS is less than 10
percent.

Weiss