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

Valid 00Z Thu Jan 10 2019 - 00Z Sun Jan 13 2019


...Southern Rockies to mid-lower MS Valley and lower Ohio Valley...

...Days 2-3...

A southern stream trough develops with the 500 mb wave crossing
CO/NM on Friday onto the central and southern Plains Fri
night-early Sat. Snow develops in the front range and Sangre De
Cristo Mountains and adjacent high plains of CO and NM as 700 mb
convergence is forecast in the region in concert with the
developing lee low. Several inches of snow are possible at the
higher elevations of the Sangre DeCristos.

The circulation causes low-mid level warm/moisture advection out
of OK across AR/MO Fri night-Sat, with snow and then mixed precip
types in northern AR and then southern MO.
On Sat the snow expands northeast across the rest of eastern MO
and southern IL/KY/southern IN.  The continuing southerly
component of flow east of the 700 mb trough produces a precip type
transition zone across southern MO to southern KY and northern TN.

The longer duration snow produces greater potential for heavy snow
across east central MO to southern IL, left of the 850 mb
circulation track forecast by the NAM/ECMWF.  On the north end, a
tight QPF gradient in southeast Ne, southern IA, and central IL
indicates uncertainty to snow amounts to be resolved. The 12z
NAM/ECMWF/09z Sref Mean and 12z UKMET cluster well with the
cyclone track.  Considerable QPF spread in the cold sector and
variable snow ratios provide the primary uncertainty. 

...Great Lakes and Northeast...

...Days 1...

Low pressure moving up the Maine coast this afternoon continues
into southeast Canada overnight.  The snow across lower elevations
of Maine tapers as the low departs.
Cold air advection 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,  Green/White
Mountains, continuing into the mountains of western ME.  Once the
great lake moistened airmass turns upslope in WV, another area
with continuing snow in the central Appalachians centered over
West Virginia that tapers on Thu as drier air aloft advects into
the region. Several more inches are possible in the mountains of
WV.

The persistence of high relative humidity and leeshore convergence
along with increasing instability supports several inches of snow
downwind of the lower 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, starting off Lakes
Superior and Michigan and then later Thu off lakes Erie and
Ontario.


...California/Pacific Northwest/Northern Rockies...

...Day 1 and 3...

A slow moving deep low pressure remain offshore as it moves north
off the British Columbia coast by tomorrow morning.  Deep layer
south to southwest flow advects moisture and warm air into the
Pacific Northwest, with moisture continuing over into the MT
Rockies today. Heavy snow is confined to the higher elevations of
the Siskiyous/Shastas and Oregon and Washington Cascades. There is
potential for a foot 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 pockets of freezing
drizzle tonight
primarily in areas along and east of the Cascades. 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 inland snow potential Friday night. The gradual approach
of the 700-500 mb trough on Sat leads to lowering heights and
temps, with snow possibly developing at higher elevations in the
mountains of southern CA on Sat.

The probability of a quarter inch of freezing rain is less than 10
percent day 1.

Petersen