Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Tue Jan 01 2019

Valid 12Z Tue Jan 01 2019 - 12Z Fri Jan 04 2019


...Texas, Oklahoma, Western Arkansas, and Missouri Days 2-3...

The upper low is forecast to eject from NM into the TX Panhandle
on day 2 and then over the southern Plains to possibly Arkansas
Day 3. The juxtaposition of southwesterly mid-upper level flow
over a shallow arctic airmass over central/northern Texas and
eastern Oklahoma on day 2 leads to light to moderate freezing rain
in these areas. A low risk for a quarter inch reflects higher
amounts shown in the NAM and NAM Conus Nest solutions from 00z.

On day 3, Thu, the 0z NAM and 12z ECMWF intensity the low so QPF
amounts in the cold sector pick up, presenting both the
possibility of heavy snow in the western semicircle of the low
level circulation and freezing rain in the eastern semicircle,
with heavier freezing rain amounts focused from northeast OK
across southwest MO and southeast KS.  The NAM, ECMWF, and ECMWF
ensemble mean indicate potential for a quarter to half inch
freezing rain accumulations.

On the west side of the low level circulation in central to
eastern OK, the NAM/18z GFS/12z ECMWF show potential for 6-10
inches of snow. This cluster of models were in the middle of a
cluster flanked by the much slower 21z SREF Mean, which was given
the least weighting.  There are multiple ensemble members of the
ECMWF and GEFS forecast systems that are now supporting 4 and 8
inches across central and eastern Oklahoma, with a few members
extending over adjacent southeast Kansas and nearby southwest
Missouri, and the 00z ECMWF over western Oklahoma.  The
precipitation is well supported by well defined 300 mb divergence
maxima north of the circulation crossing OK in the 00z ECMWF.

...Ranges of southern CO across New Mexico and eastern AZ Days
1-2...

A closed low dropping south from the four comers across Arizona
with a jet max over New Mexico will lead to coupled bands of upper
level divergence and low level convergence. 
Where the flow turns upslope in windward facing terrain,
additional moderate to heavy
snow is expected from the San Juans shifting southward through
Wednesday across the Sangre De Cristos of NM and the Gila and
White Mountains of eastern Arizona.
On day 2, Wed, the 700 mb low starts to move east, so the peak
low-mid level convergence departs AZ and moves across NM on the
way to west TX.
Several more inches are possible in the ranges of southern NM on
Wed before the event ends.

...Pacific Northwest for Days 2/3...

The next snowfall event is expected once the 700 mb jet increases
to near 50 kt Wed afternoon/evening in Vancouver island and then
down into the Olympics by 06z Thu. 
This strong onshore flow continues into Thu., so heavy snow
appears likely
for the northern Washington Cascades, and the higher summits on
the Olympic Peninsula.
The 00z NAM/ECMWF/21z SREF Mean show potential for 1-2 feet of
snow in the northern WA Cascades. Secondary maxima are expected
downstream in the ranges of northern ID.

Amounts and probabilities drop rapidly heading snow into the OR
Cascades and ranges of southern ID as the airmass is drier and has
a shorter duration where moisture is deep enough to support
precipitation.

...Maine Day 1...

In Maine the initial primary surge of mid level warm/moist
advection moves off the coast with triple point cyclogenesis
steadily progress off the coast.  Mid level deformation and
frontogenesis on the northwest fringe of the low allows snow to
continue this morning in northwest Maine until the low moves far
enough off the coast to end the large scale lift and snow. The
models have typical agreement on the cyclone track and intensity.

Petersen