Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 PM EST Fri Feb 22 2019

Valid 00Z Sat Feb 23 2019 - 00Z Tue Feb 26 2019


...Southern Rockies across central Great Plains to the Upper MS
Valley/ Upper Great Lakes...

An amplified trough over AZ this afternoon ejects northeast across
the Great Plains to the upper Great Lakes through Saturday night.
A mid level low closes off over the central Plains Saturday aiding
rapid development of the surface low as it crosses the Great Lakes.

Day 1...
Convergence across the highlands of NM and CO leads to moderate
WPC Day 1 probabilities for eight inches across the southern
Rockies. Surface low development begins late tonight in the lee of
the NM Rockies over the TX Panhandle as the low shifts east before
taking a northeast turn. Convergent northeasterly flow north of
the low lasts for several hours allowing a stripe of 8 or more
inches (moderate probabilities) to develop from southeast CO and
into north-central KS.

The 12Z GFS/FV3 remain too far north (and FV3 too high magnitude)
for inclusion in WPC QPF with the Day 1 QPF preference for the 12Z
HREF mean/12Z ARW (which is NAM based) and some 00Z UKMET/ECMWF.
This results in a one inch QPF stripe from SW KS to southern IA in
the strong trowal band. This will also be a very windy system with
blizzard conditions possible.

Ahead of the low, increasing deep layer southerly flow transports
moisture up and over colder air near the surface from NE through
SD and into ND/across northern MN. The column remains cold enough
to support snow, with the confluent flow leading to multiple jet
maxima and differences where lift occurs within the broad
confluent zone. Day 1 WPC probabilities are moderate to high
confidence for an additional two inches, particularly across
northern MN. Farther south ahead of the low, light icing develops
in eastern NE to northern Iowa and adjacent southwest WI as
warming aloft brings temps above freezing while surface
temperatures are still below freezing for a multi-hour period.
Most areas are forecast to receive  measurable to a tenth of an
icing. Low to moderate probabilities of a quarter inch reflects
the higher 12Z NAM amounts in northeast IA to adjacent southwest
WI.

Day 2...
The track of the low and band of convergence along the track of
the 700 mb low combine with upper divergence maxima and mid-level
frontogenesis maxima to support increasing amounts in the stripe
of heavy snow from southeast NE across central IA, southeast MN,
and then northern WI and the western UP of MI. Given the strength
of the low a narrow band of 8 to 12 inches of snow is expected
with low Day 2 probabilities for 18 inches over the western UP.
The Day 2 QPF preference was for the 12Z ECMWF/NAM with some
UKMET/SREF and in house bias corrected blend. The 12Z NAM becomes
too fast during day 2, but with most associated QPF in the CONUS
in the first 12 hrs, it was included.

East of the low-mid level circulation, strong warm air advection
rides up and over retreating low level cold air from southeast NE
across central to northeast Iowa, northwest Illinois, southern to
northeast WI, and finally into northern lower MI and the eastern
UP of MI. In the strong mid-level warming, light snow transitions
to freezing rain. There was a signal for measurable to 0.10+ inch
ice amounts in the aforementioned locations, with low
probabilities for 0.25+ from central WI the eastern UP/far
northern lower MI.

After the low passes northeast from northern Lake Michigan into
Ontario, the synoptic snow decreases and what is normally strong
LES flow over Lake Superior develops. However, significant ice
coverage, particularly on the western arm of Lake Superior should
reduce the overall LES risk.


...Pacific Northwest into the Northern Rockies...
Days 1-3...
Low pressure shifting south down the BC coast continues to spread
onshore flow and low elevation and mountain snows to the Pac NW
through tonight.

An omega shape to the ridge into AK allows the trough to amplify
west off BC Saturday into Monday. This drives Pacific
moisture/atmospheric river into southern OR and far northern CA
Saturday through at least Tuesday with considerable snows for
terrain through this time. 72hr probabilities for 18 inches are
high over the OR Cascades and Salmon/Sawtooth mountains of ID and
Tetons of WY with three feet more likely. A baroclinic zone
develops in this plume with snow elevations increasing south
across OR. This may lead to low enough snow elevations to affect
the Portland metro area by Day 3. There are moderate probabilities
for two inches in Portland on Day 3.


...Central Appalachians to Northern Appalachians/New
York/England...
Days 1-2...
A strong surface high shifts east across the northern Mid-Atlantic
tonight with cold air damming setting up again east of the
Appalachians in the interior central Mid-Atlantic. Low Day 2 WPC
probabilities for a tenth inch of ice area west of the Shenandoah
Valley as return flow overruns the low level cold air. The
continuing warm advection over surface cold air favors a rapid
transition from snow to sleet and freezing rain, with light icing
in the Catskills/Adirondacks, and Berkshires of MA to the adjacent
Green Mountains of southern VT. Day 2 WPC probabilities for a
quarter inch of icing, across these higher areas.

Days 2-3...
The powerful low tracking over the Great Lakes drives a warm front
north across New England Sunday. Low pressure is expected to
develop along the front, enhancing low level convergence and lift.
Front end snow begins early Sunday over interior New England with
snow magnitude increasing north across interior Maine where ptype
remains snow. Moderate to high WPC probabilities of 8 inches of
snow span Days 2/3 over northern Maine.
The mid level warm air turns the snow to sleet and freezing rain
over VT and NH to adjacent southern Maine.
The continued forward progression of the 700 mb wave leads to
drying aloft toward Monday morning, so the precip
coverage/intensity over northern New York and New England should
start to taper Sunday night.

Jackson