Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Wed Mar 24 2021

Valid 12Z Wed Mar 24 2021 - 12Z Sat Mar 27 2021


...Southwest and Southern Great Basin to the Southern Rockies...
Days 1-3 Wed-Fri...

Deepening moisture and large-scale ascent near the track of a
low-to-mid level cyclone and accompanying front moving east across
Arizona into New Mexico is expected to support some several
additional inches of snow across the ranges of central to northern
New Mexico and southwestern Colorado.  Meanwhile farther east,
post-frontal upslope flow will help produce heavier amounts along
the Sangre de Cristo and Sandia Manzano mountains.  Probabilities
remain high for snowfall accumulations of 4 inches or more and low
for 8 inches for the central and southern Sangre de Cristo
mountains and for areas further east along the Raton Mesa. The
event winds down as the 700 mb low shears and lifts northeast onto
the southern Plains this evening.

On Thu., the next 700 mb shortwave trough amplifies as it moves
south across Nevada into Utah. This will bring high elevation snow
back into the ranges of Nevada, Utah and northern Arizona, with
potential for several inches of snow late Thursday into Friday.  A
greater threat for heavy accumulations is farther east, where a
period of southwesterly inflow ahead of the approaching trough is
expected to support heavy snows occurring across the San Juan
Mountains. The probability of 8 inches is moderate 12z Thu to 12z
Fri in the CO portion of the San Juans.

...Pacific Northwest to the Northern Rockies and Northern Great
Basin...
Days 1-3 Wed-Fri...
A shortwave trough diving south into the northwestern U.S. is
expected to bring mountain snows across the WA/OR cascades, OR
Blue Mountains, and ranges of central to northern ID and northwest
MT. The highest probabilities for 8 inches of snow are across
the WA Cascades, where favored windward terrain should see amounts
of a foot in several locations.  On Thu., the upper trough drifts
east across MT, southeast ID and WY. The best overlap of 700 mb
high relative humidity and pockets of ascent is across southeast
ID into western WY and adjacent southwest MT, where several inches
of snow is expected across favored terrain. Probabilities for 8
inches of snow are low. On Fri, the 700 mb trough moves east from
WY onto the Plains.  Several inches of snow are possible in the
Bighorns and Laramie Range where confluent moist northwest flow
turns upslope. Elsewhere in eastern WY, accumulations are forecast
to be light.

...Northern Maine...
Day 3/Fri...
The models show potential for snow across northern Maine late Fri
through Fri night as a wave of low pressure moves east across
Maine, bringing widespread precipitation.  Current forecasts show
it being too warm for snow at the onset.  When the low pressure
passes, cold advection commences, with rain changing to snow
across northern Maine.  The longer duration snow appears to be
near the northern most portion of Maine near the New Brunswick and
Quebec border, where there is potential for several inches of
snow. There is still latitudinal differences in the low track and
thus how far south the band of snow gets. The PWPF shows a 40-50
percent chance of 4 inches of snow along Maine's northern border
with Canada.

...Northern Minnesota...
Day 1/Wed...
An 850 mb low is forecast to track northeast from southeast MN to
Lake Superior.  West of the low, rain currently in place across
northern MN will change to snow within the stronger lift aligned
with the mid level frontal band.
The precipitation winds down tonight as the low moves across the
border up into Canada. The WPC PWPF has a 40-50 percent
probability of  snow accumulations of 4 inches or more across
parts of the northeast MN.

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Petersen