Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EST Wed Jan 09 2019

Valid 12Z Wed Jan 09 2019 - 12Z Sat Jan 12 2019

...Great Lakes and Northeast...

Days 1-2...

Low pressure shifts eastward over southern New England this
morning and turns up the Maine coast this afternoon. Gulf stream
sourced moisture wraps around this low with bands of locally heavy
snow expected across interior New England into this evening before
wrap around snow continues into Thursday. Low to moderate Day 1
WPC probabilities for eight inches are across interior New England.

Cold air advection develops over the eastern Great Lakes this
morning and moist west-northwest boundary layer flow supports snow
developing in the lee of Lake Ontario and windward terrain of the
western Adirondacks/ Tug Hill and Green Mountains. Once the great
lake moistened airmass turns upslope in WV, another area with
several inches of snow is depicted in the central Appalachians
centered over West Virginia. The high res windows show about half
an inch liquid equivalent in the favored terrain, which should
translate to about 6 to 8 inches of snow in the mountains of WV.

Post frontal lake effect snow is likely in a cold advection
pattern with low level convergence in the lee of lakes Superior
and northern Lake Michigan, followed by
Lakes Erie and Ontario. The persistence of high relative humidity
and leeshore convergence along with increasing instability
supports several inches of snow downwind of the Lakes. The highest
amounts are likely downwind of Lake Erie and Ontario both due to
long duration trajectories crossing Lake Huron/Georgian Bay that
moisten the airmass, as well as upslope enhancement just east of
these Lakes. Lake Effect snow bands diminish Thursday afternoon as
a surface ridge spreads east over the Great Lakes.


...Northern California/Pacific Northwest/Northern Rockies...
Day 1 and 3...

A slow moving deep low pressure remain offshore as it shifts north
of Vancounver Island into Friday. Deep layer south to southwest
flow advects moisture and warm air into California and the Pacific
Northwest, with moisture continuing over into the MT Rockies
today. Heavy snow is confined to the higher elevations of the
Sierra Nevada and Siskiyous/Shastas of northern CA, Oregon and
Washington Cascades. There is potential for 1-2 feet of snow at
the highest elevations. Lesser amounts are likely into the ranges
of the ID/MT Rockies as the forcing shifts into Canada.

The models soundings indicate a signal for snow in portions of
central to northeast WA and into eastern OR for snow to transition
to a period of freezing rain
in areas in and east of the Cascades. Cold surface temperatures
reinforced by precipitation result in isentropic lift. The
southerly component of flow results in warming and a changeover.
As the warm front lifts north tonight, the potential threat area
shrinks with time.

The building deep layer ridge downstream of the next system
minimizes precipitation potential Friday night to the CA coast.
High snow elevations should limit heavy snow to the highest peaks
on Day 3.


Southern Rockies to Mid-South...
Days 2-3...

A southern stream trough develops in the lee of the Rockies around
the TX Panhandle Friday and shifts east toward the
Mid-South/Ozarks by Friday night. Return low level flow advects
moisture into the region, but model QPF amounts are generally 1 to
2 two tenths of an inch, so snow amounts should be light.
Downstream over northern AR/southern MO, precip develops Fri, with
in situ cold air providing a threat for light snow, sleet and
freezing rain to develop on the southerly flow. A light glaze is
possible before the dominant precip type becomes snow north of the
low center. Moderate WPC probabilities for six inches by Saturday
morning exist mainly over southern MO.

This low is forecast to shift east across the Carolinas/affect the
Mid-Atlantic through Sunday/Day 4.



Jackson