Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Fri Apr 5 2024

Valid 12Z Fri Apr 05 2024 - 12Z Mon Apr 08 2024

...Northeast...
Days 1-2...

Sprawling upper vortex and mid-level low centered over New England
this morning will continue to wobble more or less in the same area
for the next 36 hours as spokes of vorticity rotate around the
center. Surface low in the Gulf of Maine will remain mainly
stationary today before finally dislodging from the region on
Saturday as the final vorticity spoke tugs the entire circulation
southeastward late Saturday. This will maintain a rather unsettled
pattern for the eastern Great Lakes eastward into the Northeast
with scattered rain/snow showers (at lower/higher elevations,
respectively), but with a continued focus over central/northern New
England for additional snowfall. Though the best dynamics to
support heavier snowfall have largely ceased/exited, the
combination of upslope flow an continued in situ moisture will
promote light to modest snowfall over the northeastern Adirondacks,
Presidential Range in NH, and across northern Maine where WPC
probabilities for another 4 inches of snow are around or higher
than 50%. Snow coverage and accumulations will be much lighter on
Saturday but a few inches of snow are likely at the highest
elevations.



...Central Appalachians...
Day 1...

On the southwest side of the broad upper vortex will be NW to W
flow across the central Appalachians, promoting continued upslope
flow over eastern WV and the western MD Panhandle. Last vort max
will swing through this evening, and much of the snowfall will
occur today and tonight before slowly waning on Saturday. WPC
probabilities for an additional 4 inches of snow are moderate
(40-70%) over eastern WV.


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

Deep trough moving through WA/OR/CA this morning will continue
eastward today, but with the southern portion of the trough moving
quicker than the norther, resulting in a negatively-tilted trough
by Saturday morning as it reaches the Four Corners. Thereafter, it
will close off into a upper low as it moves onto the central High
Plains beneath broad upper divergence on the poleward side of the
upper jet across the Southern Plains. This will favor induced
cyclogenesis over eastern CO that will lift northeastward into
central Nebraska early Sunday as it reaches peak intensity (mid
980s mb; records are <980mb). As the system then stretches out
eastward, it will start to weaken with the loss of upper support as
a post-mature occluded system.

The evolution of the upper trough will bring a few phases of
precipitation to the West over the period. The first will be with
the brunt of height falls beneath upper divergence D1 in the Sierra
and Great Basin D1. Farther north, troughing will remain mostly in
place for the next 24 hours, resulting in light snow for the
Cascades. Over eastern OR into Idaho, under a stream of PVA,
snowfall will be more vigorous with >6" likely for the Blue
Mountains and central ID ranges. Back to the south, incoming height
falls will spread snow into the Four Corners region as the trough
begins its next phase.

Into D2, the negative tilt will shift the focus of snowfall
northeastward, along the vorticity gradient over WY and along an
inverted trough to the northwest of the developing surface cyclone
over eastern CO. Gulf moisture will try to surge northward, but
will mostly be too late and north to fully tap it. However, a
somewhat narrow band of PW anomalies >+0.5 sigma will be evident
from the middle MS Valley northward then westward into/around the
low via a developing TROWAL as the system deepens Saturday night.
Temperatures will be >32F over the Plains with the exception being
the Black Hills in SD, where snow will accumulate starting Saturday
afternoon. The focus on D2 will be over WY, specifically over the
Big Horns into the Laramie Mountains, near and west of the
convergent lower levels and near the LFQ of the northern stream jet
that will curve back to the NW across eastern MT by Saturday
afternoon. There, WPC probabilities for at least 6 inches of snow
are >50%, with >50% probs for >12" over higher elevations of the
Bighorns and Laramie Mountains.

By D3, the cyclone will be weakening but troughing will remain
over the Black Hills westward across eastern WY, maintaining
snowfall through the period Sun into early Mon. WPC probabilities
for another 6 inches of snow on D3 alone are >50% for the same
regions as D2. Two-day totals may be well over 1-2ft in the higher
elevations. Modest amounts are likely back into Yellowstone as
well, mostly driven by the first phase of the upper trough (D1-2).


Fracasso