Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EST Thu Dec 26 2019

Valid 12Z Thu Dec 26 2019 - 12Z Sun Dec 29 2019

...Southwest U.S....

Days 1-3...

The low level cyclonic circulation drifts southeast just off the
coast of southern CA today to the northern Sea of Cortez through
Thursday night before ejecting northeast across the south-central
Rockies Friday night. Persistent upper divergence and 700 mb
convergence will support heavy precipitation into the mountains of
southern CA through today. Heavy snow accumulations are expected
for the Transverse Ranges, southern Sierra Nevada and Peninsular
Ranges. Total accumulations of 2 ft are expected, with locally
higher amounts.

Snow develops in the ranges of central to northern AZ as the upper
trough drifts east tonight through Fri, ending as the upper trough
passes. The southwest flow ahead of the upper trough places
windward locations is sustained upslope flow late Thu night thru
Fri. Moderate probabilities for a foot or more of snow are over
the San Juans of CO/adjacent northern NM on day 2.  The event
winds down on day 3 as the upper trough progresses onto the Plains.

...Central and Northern Great Plains...

Days 2-3...

Lee side surface low pressure develops over northeastern NM late
Friday and moves northeast into western KS by 12z Sat and then
somewhere in the upper MS Valley 12z Sun.
The exact track/intensity of this low remains uncertain with the
NAM farther east than the ECMWF/UKMET/CMC. The preferred farther
western track would have upslope snow enhancement on the
central/northern high plains of CO/NE/WY/SD with plenty of Gulf
moisture available to wrap into the comma head.

The NAM axis of snow/freezing rain was east of the model majority.
The 00z GFS stood out with higher QPF than most of its ensembles
and 18z run across northeast CO to southwest NE and IA on day 3. 
The most likely axis of moderate to heavy snow is from western NE
across to northeast SD and northern MN.  A low confidence forecast
remains in place due to run to run changes and lack of agreement
among the NAM, GFS, ECMWF,UKMET, and Canadian global models.

A precip type transition zone would occur along the low track,
resulting in low to moderate probabilities for a quarter inch of
ice over northeast NE across southeast SD and into southern MN,
including adjacent portions of northwest IA and central to
northern WI.  

...Northeast...

Day 1...

A warm front lifts across northern NY and northern New England
today.  Initial sfc temps are below freezing with modest cold air
damming in place.  The high moving east should allow a southerly
component of flow and warming.  The warming leads to modest light
precip with freezing drizzle or light rain expected while temps
are still at or below freezing early today.
The SREF and high res windows indicate potential for a tenth of an
inch freezing rain accumulations in higher elevations of northeast
NY and VT, with less in the Champlain Valley.
A few pockets of residual measurable freezing drizzle are possible
in northern VT and NH into early day 2.

Petersen