Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Sat Mar 25 2023

Valid 12Z Sat Mar 25 2023 - 12Z Tue Mar 28 2023

...Pacific Northwest, Northern & Central Rockies, High Plains...
Days 1-2...

Broad troughing over the West into the Plains will start to
finally break down over the weekend as strong upper ridging builds
into Alaska. Last mid-level vort max over western Washington this
morning will move southeastward into ID by Sunday morning, then
across Wyoming. The surface cold front has long passed through the
Rockies though a stationary boundary remains astride the northern
Rockies and will act as the low-level focus helping to promote
modest to heavier snowfall today across south central Montana into
the Wyoming ranges in concert with the approaching upper low.
Farther southeast, vort max over CO will move into KS later today
as a 130kt jet streak develops across northern NM into OK, placing
the area from eastern CO into NE beneath some divergence. Coupled
with lower-level convergence on the northwest side of a weak
surface low along a trough axis, another area of light to moderate
snow will form today as the surface low wobbles/pivots to the
east. Several inches of snow are possible in a NW to SE band this
afternoon. Here, WPC probabilities for at least 4 inches of snow
Day 1 are moderate (>40%) but high (>70%) over much of south
central MT into WY where 6-12" are likely in areas above about
5000ft. To the west, snow over the Great Basin will be generally
light while even farther west the OR Cascades will see one more
day of appreciable snow, but with most amounts less than 6 inches.

By Day 2, mid-level low over WY will slowly move
eastward/northeastward with continued snow for the south central
ranges along the stationary surface boundary. Another vort max
will slip into CO and provide another opportunity for light snow
across both the terrain and into the eastern CO Plains where WPC
probabilities of at least 4 inches are 10-20%. Lighter snow will
diminish over the Great Basin through the day.


...Great Lakes...
Day 1...

An intensifying wave of low pressure over the Midwest this morning
will track across eastern Lower Michigan and into Ontario by early
evening, with a swath of heavy, wet snowfall from eastern IA,
northwestern IL, southeastern WI, and into northern Lower MI.
S-shaped dueling upper jets have combined to maximize upper
divergence as robust PVA at 500mb out ahead of the upper trough
moves overhead. A 60 knot low level jet over the Mid-South will
wrap a warm conveyor belt of moisture on the north side of the
850mb low track this morning as the low pressure reaches peak
intensity. High pressure to the north is anchoring just enough
cold/dry air in place over the region to support snow (low 20s Tds
on the north side of the precip shield early this morning),
particularly once intense vertical velocities aloft cause
subsequent dynamic cooling in the atmospheric column. Deformation
axis supported by 850-700mb frontogenesis will favor 1-2"/hr
snowfall rates this morning over WI and into the afternoon over
northern Lower MI. Progressive motion will limit the snow
accumulation period to the first 12-18 hours of the Day 1 period
before winding down by midnight. WPC probabilities for at least 4
inches of snow are >50% from northern IL northeastward to northern
Lower MI. Probabilities for at least 8 inches are >30% from
southeastern WI northeastward where they peak around 70%.


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

The warm front associated with the storm system tracking through
the Great Lakes will advance northward through the Mid-Atlantic
this morning and early afternoon. Strong 290K isentropic ascent
will be a catalyst for precipitation to expand into the Northeast
today with 50-60 knot southerly flow at 850mb, supporting a warm
nose at low levels. Low-level/surface temperatures near to well
below freezing will make for a variety of p-types as some valley
locations lag behind some exposed higher elevations while the
higher peaks that are most below freezing slowly rise at a
different rate. Resultant icing is forecast to be on the light
side (<0.10")  but more stubborn/sheltered areas could see just
over that. Most of the region south of the Adirondacks and
northern VT/NH will see a changeover to rain as the parent low
moves into Ontario, but triple point low will start to take over
as it crosses over southeast Mass later tonight. This will leave
northern Maine with predominantly snow where WPC probabilities of
at least 4 inches of snow are >50%. Snow will slowly diminish on
Sunday with additional light accumulations over northern New
England but especially northern Maine.


...Oregon/California...
Day 3...

Vort max/mid-level trough currently over Alaska will drop
southward this weekend along 130W west of WA by the start of Day
3. Secondary system to its southwest will push eastward along 40N
and the two may interact on Monday and deepen into a strong closed
low and surface low. Frontal boundary associated with this system
is forecast to move eastward into southwestern OR and northern CA
which will bring a plume of modest moisture into the Coastal
Ranges, Shasta-Siskiyou/Trinity/Klamath and northern Sierra.
Models show a large degree of uncertainty in this interaction
especially into Day 4, but consensus is for at least modest
amounts of rain/snow into the region starting early Monday. For
the Day 3 period, WPC probabilities of at least 6 inches of snow
are >50% above about 3000ft with continued lower than normal snow
levels.


Fracasso