Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Tue Feb 12 2019

Valid 00Z Wed Feb 13 2019 - 00Z Sat Feb 16 2019

...Northeast and Great Lakes...
Days 1-3...

Surface low moving through the Great Lakes this evening will
occlude to the east with secondary low development occurring off
the NJ coast tonight. This secondary low will lift northeast
across Boston and the coast of Maine through Wednesday night.
Low-level WAA will spread precipitation to the northeast, with a
burst of heavy snow likely into New England and upstate New York.
However, this warm advection will quickly push the warm nose above
0C all the way through central New England, producing a
precipitation type changeover from snow to sleet/freezing rain,
and eventually rain across southern New England.

The column will remain cold enough for all snow from the
Adirondacks and points east across northern New England. There
exists a good model consensus for more than 1 foot of snow across
northern and central Maine, and WPC probabilities show a high risk
for 12 inches there. Elsewhere in this region WPC probabilities
are high for 8 inches. Further south, WPC probabilities drop off,
becoming less than 10 percent for 4 inches along the MA/VT/NH
state border.

As the warm nose lifts northward, surface high pressure wedge will
hang on long enough to produce a period of sleet and freezing
rain. The best chance for significant accretion exists in the
terrain of the Catskills and Berkshires, where probabilities are
moderate for 0.25 inches. Elsewhere, a large area of 0.1 inches is
possible from northern PA into central New England and along the
coast of Maine, but outside of the terrain and south of
Massachusetts, the surface cold dome is forecast to eventually
erode as the wedge retreats and precipitation will transition to
all rain.

Behind this system, the environment is forecast to become
favorable for Lake Effect snows, which may be significant at
times. The upper low closes off and lifts to the northeast,
leaving CAA in its wake with unidirectional W/NW winds across the
Great Lakes. Although lapse rates are not extreme, a high
inversion over 700mb and MUCape approaching 500 J/kg within the
favorable shear profile will support a brief but intense period of
lake effect snow, especially downwind of Lake Ontario where WPC
probabilities are moderate for 6 inches Thursday, with higher
amounts likely in the terrain of the Tug Hill Plateau. Lesser
amounts are possible downwind of the other lakes.

The next CO low crosses the Plains Wednesday night into Thursday
with another northeast track into the Great Lakes Thursday and
toward Lake Huron Thursday night. A surface ridge over the Gulf
coast keeps Gulf moisture suppressed/out of the system until
Thursday, so little precip is expected until Thursday night over
the Great Lakes where there are moderate probabilities for two
inches over the northern Great Lakes.


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

A very positively tilted trough off WA will slowly drift south
through tonight with the focus of ample Pacific Moisture south of
the trough shifting to OR. Snow levels of 1-4kft are expected
through Wednesday over the Cascades of northern Oregon and
Southern Washington.  Day 1 WPC probabilities are moderate to high
for 18 inches over high terrain of far northern CA and the OR
Cascades, as well as with spillover moisture into the ranges of
Idaho. Lower, but still significant amounts, are likely into the
mountains of NW WY, MT, and UT, with 6-12 inches possible.

Late on Wednesday and through the remainder of the forecast
period, the trough over the Pac NW splits with northern stream
energy shifting east over the northern tier while low pressure
closes off the OR/CA border. The Pacific Jet will dig southward as
another low descends the persistent ridge into AK and off the BC
coast, reaching the Pac NW coast Thursday night. This southward
shift will focus the highest moisture into California, where
modeled PWAT values exceed all time winter values and rise above
+5 standard deviations above the mean. This 2-day AR IVT is
progged to reach 750 kg/m/s, indicative of the tremendous
potential for snowfall as the moisture feed spills across the
entirety of the west. Day 2/3 snowfall may exceed 5 feet in the
Sierra of California, with 1-3 ft in the terrain of northern
California, Idaho, NW WY, UT, and CO. However, with the source
being more of tropical origin, snow levels rise, remaining above
5000 feet south of 40N latitude and around 7000ft over the Sierra
Nevada before falling late on Day 3. This will confine the
heaviest snow to the highest terrain, and WPC probabilities are
moderate for 4 inches outside of these ranges.


Weiss