Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Sun Feb 17 2019

Valid 00Z Mon Feb 18 2019 - 00Z Thu Feb 21 2019


..Lower Great Lakes and PA to the Northeast...

A mid to upper level trough is expected to move east and gradually
deamplify as it moves east across New York/PA and New England. Low
to mid level theta-e advection and frontogenesis supports a stripe
of light to moderate snows extending from the lower Great Lakes to
New York and southern New England on Day 1.  A second low pressure
center develops south of New England near the 40n 70w benchmark,
resulting in an enhanced frontogenesis and locally higher amounts
in southeast New England.

A wintry mix is expected further south with light icing possible
in northeast Ohio and much of PA and northwest NJ, southeast NY
and southern CT.  The warm advection turn snow over to mixed
sleet/freezing rain from Long Island NY to southern CT, RI, and
adjacent southern coastal MA.

...Southwest to the Southern Rockies...

Tonight into early Monday, shortwave energy moving into the base
of a broad upper trough centered over the southwestern U.S. is
expected to support significant snowfall totals from higher
elevations of the mountains of Southern California, where amounts
have increased today in the proximity of the 30 mb jet crossing
the ranges this evening, downstream into the mountains of AZ
northeast into southern UT and southwest CO.  

Bolstered by low to mid level frontogenesis favorable difluence
aloft, some of the heaviest two day totals are forecast to focus
along the Mogollon Rim into the White Mountains of central
Arizona, as well as the San Juan and Sangre de Cristo mountains in
Colorado and New Mexico.  On day 2, the upper trough crosses AZ
into NM, with persistent upper divergence in the ranges of NM
favoring continued snows the first half of the period before
ending as the upper trough moves out on to the Plains. 2 day
totals of 1-2 feet are expected.

Additional light accumulations are expected to continue across
portions of the central and southern Rockies into late Tuesday,
diminishing by early Wednesday as the upper trough lifts northeast
into the Plains.

...Central Plains into the Lower Missouri and Mid-Upper
Mississippi Valleys and adjacent western Great Lakes...

Probabilities for significant snows increase across the region
late Tuesday through  Wednesday as the upper trough ejects east
northeast from the Rockies into the central Plains.  As the 700 mb
low treks east across CO, downstream southerly flow provides
warm/moist advection and convergence on the high plains of Co to
adjacent KS/NE Mon night to Tue. As the low moves northeast on
Tue, the swath of warm/moisture advection
moves across the rest of KS/NE into IA Tue night and then northern
IL and WI on Wed.
Several inches of snow are expected from eastern NE across
northwest MO, IA, and then southwest WI.  The models still differ
as to how far west the snow axis extends in NE/SD/MN.

Probabilities indicate a Moderate to High Risk for accumulations
of 4-inches or more on day 3 across Iowa and adjacent MO/NE/MN/WI.

...Southern and Central Appalachians Day 3...

As as strong area of high pressure shifts east into the Northeast,
models show a well-defined cold air damming signature developing
over the East.  Strong low to mid level theta-e advection and
convergence ahead of the trough moving into the central U.S. will
support precipitation spreading north over this shallow dome of
cold air, producing mixed precipitation with the potential for
several inches of snow followed by sleet and then significant ice
accumulations across the region late Tuesday night into 
Wednesday.  WPC Day 3 probabilities indicate a Moderate Risk for
snow accumulations of 4 or more inches in eastern WV/western VA
and ice accumulations of 0.25 inch or more extending from western
North Carolina into western Virginia to eastern West Virginia.

...Pacific Northwest...
After a respite, the next upper level jet maxima dives southeast
from the northeast Pacific into OR around 12z Wed according to the
ECMWF, reaching northern CA thereafter.  This places the Wa/Or
Cascades into the favored left exit region of the upper jet.  The
models show 700 mb convergence maxima, aiding upslope flow in
producing a sustained period of snow.  The longer duration lift
could lead to locally heavy snow in the southern Wa Cascades, OR
Cascades, and then the Blue Mountains. 

Petersen