Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 PM EST Thu Feb 15 2024

Valid 00Z Fri Feb 16 2024 - 00Z Mon Feb 19 2024

...Northwest to North-Central Rockies...
Days 1-2...

An upper level low that makes up a rex block over the Pacific
Northwest northward to eastern Alaska will rapidly dissipate over
the next 24 hours. The easterly flow at the base of the strong
ridge over British Columbia will contrast with increasing westerly
flow as a shortwave in the subtropical jet moves ashore into the
Pacific Northwest. These 2 competing forces will effectively tear
the newly formed upper level low near Vancouver Island apart. The
result will be rapidly improving conditions across much of the
Pacific Northwest as the ridge over British Columbia dislodges and
moves southeastward into Montana by late Saturday night. Thus, the
heaviest snow over much of the region is ongoing, with a steady
improving trend forecast into tonight. The heaviest snowfall
remains forecast for the northern Oregon Cascades and the
Yellowstone, Tetons, and Wind River Ranges through late Friday
afternoon. Additional snowfall in these areas is expected to be
between 12 and 18 inches. Expect 6 to 12 inches into much of
central ID and Bitterroot Ranges of far western MT. The valleys in
between these ranges are likely to only see modest snowfall
amounts, generally under 6 inches. The forecast WSSI-P indicates
the best chances for major impacts will be in the OR Cascades,
with moderate impacts for the other mountain ranges, and
widespread minor impacts into the valleys in between.


...Northeast...
Day 1...

Low pressure moving through southwest Ontario this evening into
western NY will continue to spread snow eastward ahead of the
occluded/cold front in broad WAA as the mid-level vort max
congeals into the Adirondacks around 06Z, helping the low to
continue to deepen. Decent FGEN across northern NY/New England
with upslope enhancement will favor modest snow totals in the
Adirondacks and Green/White Mountains into the northern Berkshires
with some rates over 1"/hr per the HREF/WPC Snowband tool. By 12Z,
low pressure will move quickly into the Gulf of Maine as all but
light snow will diminish. WPC probabilities for at least 6 inches
of snow are highest (>60%) in the Tug Hill and western Adirondacks
into the hills of VT/NH.


...Great Lakes...
Days 1-3...

LES across Lake Superior into the U.P. will translate across the
rest of the Great Lakes into Friday in the wake of the departing
D1 sfc low and in conjunction with height falls out of Ontario.
Into Saturday and Sunday, additional vorticity will sweep through
the Great Lakes with periods of snow for the lake belts (esp off
Superior but also into NW Lower MI and into western NY). Highest
probabilities for at least 8 inches of snow through the period
lies over the eastern U.P. and into the Porcupine Mountains, as
well as east of Ontario into the Tug Hill as the WNW flow becomes
W to WSWerly.


...Plains to Mid-MS Valley to Central Appalachians/Mid-Atlantic...
Days 1-2...

Combination of a mid-level trough extending southwest out of the
Upper Midwest and a shortwave exiting the central/southern Rockies
will carry a surface low out of the southern Plains eastward
across the Mid-South tomorrow evening and off the Mid-Atlantic
coast by 12Z Sat. To the north of the low, transition from rain to
snow will occur deeper into the colder air, with banded snow aided
by an advancing arc of FGEN will promote light to locally moderate
snow, extending from the SD/NE border tonight across the Corn Belt
and along the I-70 corridor from MO eastward to Ohio. Highest
totals will likely fall along the SD/NE line this evening then
skipping across the Midwest to the central Appalachians where
upslope into eastern WV may promote totals in excess of 6 inches.
SLRs will trend from near/below 10:1 on the southern side to over
15:1 on the northern side, with higher values where more energetic
lift into a favorable DGZ occurs. SREF probs of >50mb in the DGZ
shows this nicely along the axis of highest snow totals. D1 probs
of >4" snow are highest over southern SD into northern NE, though
generally <60%. Second area with >10% probabilities of at least 4"
lie from eastern MO eastward to the southern half of Ohio
(10-30%), but then much higher probabilities into eastern WV, the
MD panhandle, and into the Laurel Highlands where values exceed
50%. Low to moderate probabilities (20-50%) extend eastward along
the PA Turnpike (more or less between I-70 and I-80) to eastern PA
following along the highest probabilities of favorable FGEN and
snow growth. Lower probabilities continue eastward into NJ.


...West Coast...
Day 3...

As the Rex Block starts to break down, the next Pacific trough
will move into the West. Strong southerly flow will raise snow
levels to around 6000ft as precip moves inland Saturday
afternoon/night. By the end of the period, weakening mid-level
shortwave will carry light snow into the northern Great Basin in
ID to western WY and northern UT. Day 3 PWPF for >6" are 40-80%
for the Sierra Nevada, Shasta/Siskiyou, higher OR Cascades, and
central ID ranges.


For Days 1-3, the probability of significant icing is less than 10
percent.


Fracasso