Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EST Thu Jan 17 2019

Valid 00Z Fri Jan 18 2019 - 00Z Mon Jan 21 2019


...South Dakota to the Mid-Upper MS Valleys, Midwest, and lower
Ohio Valley...
There will also be a 700 mb wave in the northern stream, moving
from the Dakotas toward Lake Michigan.  Upper divergence maxima
combines with low-mid level frontogenesis and 700 mb convergence
to trigger a band of snow along the path of the frontogenesis from
southern SD across Iowa, southern MN, and into the southern most
Great Lakes.  Lake enhancements may lead to locally high totals
off Lake Michigan in far southeast WI and northeast IL.

The models show cyclogenesis on Friday and Saturday, with a
surface low likely to track from Oklahoma into Kentucky/Tennessee
and continuing northeast from there.  These waves are expected to
eventually phase, but that should happen to the east of the
Mississippi River in the midwest. Until that happens, there may be
separation of the bands of precipitation and snow. A widespread
portion of the Midwest is likely to see several inches of snow.

The 00z ECMWF and its ensemble members show the potential for
greater snow in the deformation band on the NW and W periphery of
the surface low, while the 12z GFS/NAM/12z ECMWF are more
progressive and dry the column faster, resulting in lower snow
accumulations northwest of the low.  The deformation band, like
the low itself, should make steady eastward progress and thus the
favorable forcing is unlikely to be aligned for a long period of
time in any one location in the Mid South. North of the surface
low and south of the snow area in the northern Ohio Valley, there
is a chance of mixed freezing rain and sleet as well as you head
south across IN/OH towards the Ohio River.

...Upper Ohio Valley across the northern mid Atlantic, New York,
and New England...

There is a very strong signal for heavy snow from Ohio and
Pennsylvania into interior New York and central and northern New
England.  The snow maxima mostly follow the low-mid level
frontogenesis maxima and deformation zone, supported by a
developing coupled jet region with well defined upper divergence
maxima across the upper Ohio Valley and then progressing across
PA/NY/New England. Much of the wintry precipitation should be
driven by the mid level warm air advection. 

The WPC winter ensemble has a high probability (over 70 percent)
of 8+ inches of snow from northern Ohio into northern Pennsylvania
and interior New York and central to northern New England, along
with moderate probabilities of a foot of snow. 
Lake enhancements off Lakes Erie and Ontario lead to near
shoreline higher snow totals off Lakes Erie and Ontario.  The
models still have timing differences, with the faster cyclone
track of the UKMET and GFS resulting in lower snow amounts in
northern New York and northern VT/NH/ME.

On the southern periphery of the heavy snow area, some freezing
rain and sleet are expected. The probability of 0.25 inches of ice
accumulation is over 30 percent from southern Ohio, to the western
Maryland and southern Pennsylvania, northern NJ, and southern New
England. A strong surface high over the Northeast in advance of
the precipitation should help keep low level cold air in place, so
snow should transition to sleet before freezing rain, keeping
freezing rain amounts low.

...California, Pacific Northwest, Great Basin, Central to Northern
Rockies...

The biggest snow event is expected to be on day 1 as the upper
trough that caused the heavy snow in the CA Sierra Nevada
mountains moves further inland,
with the favored upper difluent flow with embedded upper
divergence maxima progress along with the deeper moisture across
the ranges of Utah into western CO. The longer duration lift and
orographic lift in windward flow are expected to produce sustained
ascent and the long duration results in local maxima of a foot to
18 inches in these ranges

The northern stream portion of the upper trough drifts northeast
across OR and then WA and ID, with higher snow totals in the WA
Cascades, where local maxima of a foot are expected.

On Day 2 Fri night-early Sat, with the upper level ridge building
in the northern Great Basin, snow is shunted north in the ranges
of ID where local upper divergence maxima persist, and over the WA
Cascades, as the SREF Mean 300 mb jet move from the eastern
Pacific onshore across WA state, with the SREF Mean maxima of
130-150 kt providing both moisture fluxes and a source for ascent.
 An additional 1 foot is possible

On Day 3, Sat night-early Sun, as the Pacific trough amplifies and
drives the jet south, height falls and ascent drop south out of
the WA Cascades and across the OR Cascades and into the ranges of
northwest CA.  As a coupled upper jet develops over ID and western
WY, upper divergence maxima promote additional snows in the ranges
of southern ID to northwest WY.  Locally heavy snow is possible in
windward terrain of the OR Cascades, with 8-12 inches possible.

The probability of one quarter inch of freezing rain is less than
10 percent on Day 1.

Petersen