Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EST Mon Feb 11 2019

Valid 00Z Tue Feb 12 2019 - 00Z Fri Feb 15 2019


...Midwest and Great Lakes...
Days 1-3...

A mid-level trough will deepen and close off as it advects towards
the Great Lakes. This will allow a surface low to intensify as it
lifts into Michigan Tuesday. Accompanying this feature will be
significant height falls and robust mid/upr diffluence to provide
ascent, which will be aided by 700mb deformation in the vicinity
of the 700mb trough and increasing frontogenesis. A period of
intense snowfall is now more likely during Day 1, with guidance
centering the heaviest snowfall across Wisconsin. Robust ascent
below and into the DGZ suggests a period of heavy snowfall rates
of 1"/hr or more, and WPC probabilities are high for 8 inches
across central WI, with more than 12 inches possible in locations
which experienced banded snowfall. Surrounding this jackpot area,
WPC probabilities show a high risk for 4 inches of accumulation
from NE IA, far SE MN, and into the northern portions of lower
Michigan and into the U.P. as well as the system ejects to the
northeast during day 2.

Farther south and east, a wintry mix will occur as a warm nose
drives 850mb temperatures to above 0C, and light to moderate ice
accumulations are expected from along the IA/MO border across
northern IL to Lake Erie, with Day 1 WPC probabilities indicating
a Moderate Risk for ice accumulations of 0.25 inch or more across
southern Lower Michigan to the IN/OH borders.

Snows will continue across the upper Great Lakes into Wednesday as
the trough closes off into an upper low Tuesday night, before
shifting farther northeast from the Great Lakes. W/NW flow and
cold advection will likely produce a brief but intense period of
Lake Effect snow from the U.P. and western lower Michigan on D2,
to downwind of Lake Erie and Ontario on Day 3. The highest
snowfall totals are forecast east of Lake Ontario, where WPC
probabilities are moderate for 6 inches in the Tug Hill Plateau by
Thursday.


...Northern Mid-Atlantic and Northeast...
Days 1-2...

A surface low moving through the Great Lakes will lift a warm
front into the Mid-Atlantic and New England through tonight. As
this occurs, cold high pressure will only slowly retreat to the
northeast, creating a situation with pronounced
overrunning/isentropic lift and widespread precipitation.
Initially, precipitation falling into the cold high pressure will
occur as snow as the column is below freezing throughout. Strong
800-600mb frontogenesis will produce intense omega, which at the
leading edge will collocate with the DGZ. This suggests a quick
"front-end" thump of WAA snow from southern PA northeast into
upstate NY and New England. While this will only begin a period of
heavy snow across upstate New York into central/northern New
England, for the southern areas this will occur before the WAA
overcomes the cold column to change the precip over to sleet,
freezing rain, and then rain. The guidance has warmed today
pushing the warm nose well into central New England, and a
secondary surface low developing along an occlusion from the
primary low will lift across Long Island, Boston, and into far
eastern Maine. While heavy snow accumulation is a near certainty
from the Adirondacks and points east into Maine, accumulations are
less certain further south, especially into the urban areas of
Boston and New York. Despite a quick 6-8 hour front end snowfall,
WPC probabilities are low for 4 inches, but rise quickly just NW
of of the urban corridor. Further north, WPC probabilities climb
for heavy accumulations, rising to a high risk for 12 inches in
northern Maine before the system pulls away Wednesday night.

As precipitation falls into the cold high pressure initially, the
wedge will be reinforced and a period of heavy freezing rain is
expected from Pennsylvania into southern New England. There is
good model consensus that the highest accretions will occur across
central and southern Pennsylvania with lesser amounts to the
northeast where sleet will mix in more significantly. However,
have undercut the freezing rain forecasts from most of the
available guidance. A period of heavy precip rates seems likely
beneath robust isentropic lift/WAA, and without any notable dry
advection to cool the web-bulb temperatures, the self-limiting
process inherent to freezing rain will dominate, aided by runoff
due to the aforementioned heavy precip rates. Still expect a large
area of 0.25" or more across central PA, with isolated amounts up
to 0.5" possible in elevated sheltered valleys and the terrain.
Further northeast, WPC probabilities are 50-70% for 0.1" as far
north as southern New Hampshire.


...Western U.S....
Days 1-3...

A quasi-stationary mid-level low off British Columbia will drift
southeast and advect onshore Wednesday. South of this feature,
ample Pacific Moisture will push into the west both due to
prolonged SW 850-700mb fetch, as well as the approach of a strong
Pacific Jet. Snow levels will be modest on Day 1, 2-3kft, but will
focus into the Cascades of northern Oregon and Southern Washington
where WPC probabilities are high for 18 inches, and may approach
2-3 feet in the favored upslope regions near Mount Rainier. As
snow levels begin to fall, even the urban areas of Seattle and
Portland may receive some light accumulations before the best
moisture gets shunted southward during Day 2. With moisture
spillover occurring well into the Rockies, the prolonged period of
moist advection and robust ascent will produce heavy snow in the
mountains of MT/ID/WY on Day 1, sinking southward during day 2.

Wednesday into Thursday /Day 2 and 3/ the Pacific Jet will dig
southward along the Pacific Coast while a secondary upper low
moves eastward towards northern California. This will focus the
heaviest snow into the mountains of California, including the
Sierra Nevada where 48-hr snowfall may exceed 5 feet, with heavy
snow also likely in the Shasta/Trinity and Siskiyou Ranges.
Considerable lift within the exit region of the upper jet will tap
anomalously high moisture as far east as Colorado and south as
Arizona. However, with the source being more of tropical origin,
snow levels rise into Day 3 remaining above 5000 feet south of 40N
latitude. This will confine the heaviest snow to the highest
terrain outside of the California mountains, with WPC
probabilities showing a high risk for 8 inches in the San Juans,
Colorado Rockies, and Wasatch into Thursday.


Weiss