Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Tue Feb 19 2019

Valid 00Z Wed Feb 20 2019 - 00Z Sat Feb 23 2019

...Central Plains into the Great Lakes...
Day 1...

An amplified trough ejecting from the Four Corners region will
lift northeast and phase with northern stream shortwave energy
across the Dakotas Wednesday afternoon. This will tilt the
mid-level trough negatively while it closes off, enhancing
diffluence across the central and northern Plains. At the same
time, a potent 140+ kt jet streak will rotate around the base of
this trough causing upper ventilation and surface pressure falls
as a low pressure develops near IA. This surface feature will then
lift northeast through Wednesday night, becoming positioned
northeast of Lake Huron on Thursday.

Synoptic ascent through height falls and within the front
diffluent portion of the jet streak will spawn precipitation as a
large plume of 1000-500mb RH lifts northward. This will be
enhanced by WAA on southerly 850-700mb flow, as well as enhanced
theta-e advection into the TROWAL which will rotate north of the
surface feature within the deformation axis towards the mid-level
center across ND. A period of moderate to heavy snowfall is likely
from extreme E NE northeast through IA, MN, and WI. The heaviest
snowfall is likely across portion of IA where intense snowfall
rates may reach 1"/hr, and WPC probabilities are moderate for 8
inches. Lower accumulations are likely surrounding the jackpot
region, with a high risk for 4 inches of snowfall from northeast
NE into northern WI. Dry air rapidly advects northward behind this
feature, so the temporal duration of heavy snowfall is relatively
short, limiting any higher amounts across the area, and bringing
an end to the snowfall everywhere except far northern MN and the
U.P. of MI by Day 2.

South of the heavy snow, a swath of freezing rain is likely in the
transition zone where surface temperatures remain despite the WAA
pushing a warm nose above 0C. Although the duration of intensity
of freezing rain is likely to be modest, a stripe of moderate
probabilities for 0.1 inches of accretion exists from Missouri
into Ohio.

...Mid-Atlantic through the Northeast...
Days 1-2...

Surface low moving through the OH VLY into the Great Lakes will be
accompanied by deep and moist warm advection on southerly winds
pushing plumes of high RH from the Mid-Atlantic into the
Northeast. As this occurs, a cold high pressure will be anchored
over New England before only slowly retreating to the east,
maintaining a wedge of cold high pressure down the coast east of
the Appalachians. Robust 290-300K isentropic lift will produce
widespread and heavy precipitation from North Carolina all the way
into Maine, with synoptic ascent aided by diffluence within a jet
streak to the north.

As precipitation falls, it will first occur into a column cold
enough for snow throughout, and a burst of heavy WAA snow is
likely from WV and points northeast into PA. This WAA thump of
snow will be accompanied by a brief but intense period of
750-600mb frontogenesis which is collocated with the saturated DGZ
in an environment with negative theta-e lapse rates. This suggests
intense snowfall rates which may exceed 1"/hr at times during the
morning across the Mid-Atlantic. The duration of intense snow in
the region will be limited as the warm nose will lift northward
turning precipitation from snow to sleet/freezing rain and
eventually rain by the evening. The guidance may be too quick to
warm the column due to reinforcement of the wedge by falling
precipitation, but eventually precipitation will changeover,
leading to a prolonged period of freezing rain across the terrain
of WV/VA and into PA. The heaviest snowfall in the Mid-Atlantic is
likely from the Panhandle of WV across MD and into Southern PA,
where WPC probabilities are high for 4 inches. The heaviest
freezing rain is likely in the terrain mentioned above, where a
risk exists for 0.5" of accretion even after the snowfall.

Further north, the frontogenetical forcing weakens, and lift
becomes more synoptically driven within the jet streak to the
north, and the warm nose will lift northward in tandem with a dry
slot such that precipitation amounts will be less into New York
and New England. There will still be an initial wave of snowfall
from NJ into New England, but amounts are expected to be less than
4 inches outside of the terrain of NH and most of Maine. Freezing
rain is likely in the transition zone again as the warm nose lifts
north but surface temperatures stay cold in response to the wedge
of high pressure, and a large area of moderate probabilities for
0.1" of accretion exists from PA into southern New England and
upstate New York.


...Pacific Northwest...
Day 1...

Potent vorticity impulse and associated intense Pacific Jet streak
will dive down the Pacific Northwest coast into the longwave
trough across the western CONUS. Increasing column moisture and
robust lift both due to the jet aloft and a surface cold front
moving onshore will produce heavy snow above 1000-2000 ft in the
Cascades, and spreading across into the mountains of ID, northern
CA, and NV. WPC probabilities are high for 8 inches in the Oregon
Cascades, with lesser amounts likely elsewhere above 1000 feet.
Snow levels will fall through day 1 into day 2, but should remain
high enough until the precip shuts off to spare the cities of
Seattle and Portland snow accumulations this time around.


...Southwest and Southern Rockies...
Days 2-3...

The Pacific Jet and shortwave from Day 1 will drop further
southward and may briefly close off over southern CA before
opening and drifting eastward through Friday. The slow motion of
this feature will allow prolonged and focused forcing in an
increasingly moist environment with lowering snow levels into the
Southwest. Strong upper diffluence and persistent upslope flow
into the Mogollon Rim, Wasatch Range, and San Juans, will produce
widespread heavy snow across this area with snow levels falling to
2000-3000 feet. The long duration of this event will produce the
potential for exceedingly heavy snowfall along the favored upslope
region of the Mogollon Rim as well as the San Juans, where 1-3
feet of snow is likely and WPC probabilities are high for 18
inches over the 2 day period. With snow levels falling to 2-3 kft,
several inches of accumulation is also likely down into the
valleys and canyons, and WPC probabilities are above 50 percent
for 4 inches across nearly all of northern AZ, southern UT, SW CO,
and into western NM. By the end of day 3, Friday night, the best
forcing should be ejecting eastward into the high plains bringing
a slow end to the heavy snow across this area.


Weiss