Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
906 AM EST Tue Feb 04 2020

Valid 12Z Tue Feb 04 2020 - 12Z Fri Feb 07 2020

...Southern Rockies, Southern Plains into the Ohio Valley...
Days 1-2...

Anomalously deep positively tilted upper trough will drive its
axis well south into Mexico. Within this feature, shortwaves will
rotate through the base to reinforce the trough, and spawn surface
cyclogenesis across Texas late tonight, lifting into the Ohio
Valley Wednesday night.

As the trough deepens, synoptic ascent will become robust as one
jet streak lifts off to the northeast, and a secondary jet streak
blossoms around the base of the trough. This will place the
Southern Plains in a region of intense jet-dynamics thanks to the
coupled jet structure to drive upper level diffluence. This
forcing combined with mid-level height falls and PVA will drive
deep layer ascent from New Mexico northeast towards Lake Michigan.
At the same time, a surface cold front dropping southeast will
begin to cool the column such that precipitation will change from
rain to snow (or mix), and frontogenesis will become maximized and
prolonged from the southern Panhandle of Texas through central OK
and into Missouri. This fgen may collocate ideally within the
saturated DGZ as well, and guidance is also indicating the
potential for some instability supportive of heavy snow rates.
Recent HREF probabilities have increased for 1"/hr snowfall in
TX/OK, which seems reasonable in the strongly forced and moist
environment. The heaviest snow is likely near the end of Day 1
into Day 2 /Tuesday night into Wednesday/ and WPC probabilities
are high for 6 inches in a narrow stripe across this region. WPC
super-ensemble plumes indicate a low chance for 10" of snow in
this area as well, although the short duration of intense forcing
should make this the exception rather than the rule.

Outside of this strongest band, and to the northeast where fgen
becomes weaker and the threat for convective snow decreases, there
is still likely to be a swath of moderate to heavy snowfall as
southerly low-level flow continues to advect moisture around the
developing surface low and into the cold air. WPC probabilities
for 4 inches are high for parts of central Missouri, and moderate
northeast into Illinois and Michigan.

Southeast of the heaviest snow, there will be a stripe of mixed
precipitation of sleet and freezing rain. This is due to a modest
LLJ transporting a warm nose above the cold surface high draining
down from the Dakotas. There remains some spread into how wide
this axis of mixed precipitation will be, and exactly where it
will setup due to differences in the low-level thermal structure
of the guidance. Additionally, instability advecting northward
could produce heavy rates which would inhibit accretions, so heavy
freezing rain is not anticipated. However, WPC probabilities
indicate a moderate risk for 0.1" of accretion from north-central
Texas through southern Missouri and the southern Ohio Valley, with
some low-end probabilities for 0.25".


...Mid-Atlantic and Northeast...
Days 2-3...

The same strong upper trough which will bring wintry weather to
the Southern Plains will deepen a surface low moving into the Ohio
Valley Wednesday, with secondary development possible along the
warm front over the Mid-Atlantic Thursday. This system will be
moving northeast into an air mass initially cold enough for snow
due to cold Canadian pressure centered over New England. As the
low moves northeast, WAA will rapidly displace the cold air, so
snow accumulations are likely to be limited across the
Mid-Atlantic and southern New England. Further north into the
terrain of Northern New England, WPC probabilities indicate a
moderate risk for 4 inches as WAA/isentropic ascent combine with
jet level diffluence to produce a period of moderate snow.

Further south from Southern New England into Pennsylvania and West
Virginia, the precipitation will transition to freezing rain for a
time before eventually becoming all rain later on Thursday.
Mid-level confluence and precipitation may reinforce the surface
wedge long enough that moderate accretions of freezing rain are
possible. The greatest risk for freezing rain appears to be within
the Allegheny Plateau of WV/MD and into the Laurel Highlands of PA
where WPC probabilities are 30-40% for 0.25" on day 2. On day 3,
WPC probabilities for 0.25" of freezing rain accretion are highest
in the Adirondacks of NY and White Mountains of NH, at 10-20%.


...Pacific Northwest to the central Rockies...
Days 1-3...

Strong ridge over the Pacific Ocean will drive jet and shortwave
energy anticyclonically around it into the PacNW beginning later
today, with moisture and energy rotating down into the Central
Rockies through the end of the period. This next atmospheric
river, with IVT progged at 500-600 kg/m/s will advect onshore
beginning tonight while a wave of low pressure moves onshore
British Columbia. This event will drive ample moisture into the
region, evident by PWAT anomalies exceed +2 standard deviations
above the climatological mean. However, this air mass will also be
warm, driving snow levels up to 7000ft in the Olympics by
Wednesday morning, and 2000-3000 ft in the Cascades. As the AR
energy continues to rotate onshore and to the southeast, snow
levels will remain elevated in the PacNW, and climb above 4000ft
as far east as Western UT/ID/MT. A continuation of these snow
levels is expected as well into day 3.

This anomalous moisture will be wrung out as heavy snow in the
terrain above these snow levels as synoptic ascent becomes
moderate to strong through periods of jet diffluence and height
falls/PVA as shortwave impulses rotate onshore. On Day 1, Only the
Washington Cascades are expected to receive heavy snow as WPC
probabilities are moderate for 8 inches. By day 2 /Wednesday/ high
probabilities for 8 inches extend through the Bitterroots,
Sawtooth, Northern Rockies, and ranges of NW Wyoming. Heavy snow
on day 3 is likely to be more widespread across all the
aforementioned ranges, while also extending down into the Wasatch
and Colorado Rockies. 3-day totals could exceed 3 feet in parts of
WY/MT/ID and eastern OR, with 5 feet or more possible in the
highest terrain of the Washington Cascades.


Weiss