Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Fri Dec 28 2018

Valid 12Z Fri Dec 28 2018 - 12Z Mon Dec 31 2018


...Southwest, Southern Rockies, and adjacent high Plains...
Day 1...

...Major Winter Storm will produce near blizzard conditions across
much of Central and Eastern New Mexico...

A mid-upper level trough will sink southward slowly south across
Arizona and adjacent northern Mexico into Sunday. As this occurs,
a strong upper jet of up to 130 kt will cross southeast NM and
place much of NM into the favorable left exit diffluent region,
which combined with strong moist advection and upslope flow in
windward terrain to  produce widespread moderate to heavy snow
from far southern Colorado through much of New Mexico. The
heaviest snow is likely across central and eastern NM which will
be in closer proximity to the upper jet, with enhanced low level
east flow and strong 700 mb convergence enhancing snow amounts in
the Sacramento mountains. Recent trends of radar indicate an
increase in snow coverage over central NM.

The heaviest snowfall is likely in the terrain of the Sacramento
Mountains, with up to an additional foot possible. Secondary
maxima are expected with several more inches in the southern
Sangre De Cristos, the Black Mountains, and the ranges on either
side of  Albuquerque. WPC probabilities are but lower in valleys
areas, including Albuquerque and Santa Fe, New Mexico due to
downsloping effects.

Further east on the high Plains across into the TX panhandle, the
upper jet over NM streams northeast across the region today. 
Moistening of the column and modest ascent leads to light snows
across the region, but lighter amounts are expected than over NM
due to lack of terrain enhancement plus lower QPF. WPC
probabilities remain low for 4 inches of snow. 

...Day 2...
As the 700 mb low shifts south into Mexico, bands of low-mid level
convergence gradually weaken and cover less territory, aided by
drying aloft as layer sfc-500 mb relative humidity drops below 70
percent. Only an inch or 2 of additional snow accumulations are
shown in most models/ensembles.

...Pacific Northwest/Northern Rockies Days 1-3/Northern Plains Day
3...
...Days 1-2...
Enhanced moisture streaming onshore atop a mid-level ridge
shifting southward will combine with mid level ascent to produce
snowfall into the mountains of WA and OR Fri through Fri night.
The persistent upslope flow coming into the WA Cascades leads to
potential for locally heavy amounts there, with several inches
expected.
As the next 700 mb wave moves onshore and inland across the
northwest, another round of moderate to heavy snow is expected
with a strong 700 mb jet crossing WA into ID and northwest MT on
Sat-Sat night.  Bands of convergence within this jet axis lead to
heavy snow in favored windward areas extending from the WA/OR
Cascades inland across the ranges of ID and northwest MT. The
heaviest snowfall is in the WA Cascades with the close proximity
to the moisture source and persistent jet. Probabilities are
moderate for 8 inches of snow.

...Day 3...
The 700 mb wave departs the northern Rockies and Great Basin and
progresses across the central Rockies and central to northern
Plains.  Bands of 700 mb convergence move east in tandem with the
trough and lead to a period of snow starting in western WY and
northern UT and progressing downstream across the ranges of
northern CO and eastern WY and then the northern and central
Plains. Several inches of snow are possible with higher amounts
focused in windward terrain in northern UT and WY and then in the
path of the wave in western NE and SD. Southwest flow ahead of the
wave results in warm advection and the possibility temps are too
warm for snow in southeast NE and southeast SD for most of the
period, with a brief period of snow possible at the end of the
period once the wave passes and cold advection cools the column
enough to allow a change over to snow.

...Northern New England...
Day 1...

Warm advection will produce precipitation across interior New
England today. Initially, the presence of cold high pressure will
keep the column cool enough for snow at the onset in the high
terrain of northern NH/western to northern ME. The ascent at the
onset of precipitation has the potential to produce a few inches
of snowfall from the White Mountains across the ranges of Maine to
northern Maine.

The strong warm advection will quickly cause a precip-type
transition to sleet/freezing rain and then rain as the column
warms above freezing from the Green Mountains of VT, the White
Mountains of northern NH, and  into the ranges of southwest ME.
Areal amounts of freezing rain accumulations will be on the order
of a tenth of an inch with isolated locations possibly getting
0.25 inches. 

...Western Great Lakes...
Day 1...

The highest WPC probabilities for 4 inches or more are along the
shore of Lake Superior in the central UP and western UP of
Michigan where lee shore convergence supports snow showers
developing after long cross lake Superior trajectories during
today. Additional snow showers develop in the lee of Lake Michigan
in northwest lower MI, with several inches likely in near shore
areas, according to the high res WRF ARW, ARW2, and NMMB. The
models indicate a low level ridge approaches from the upper MS
Valley, producing drying aloft that reduces snow shower
coverage/intensity for Sat.

The probability of a quarter inch of freezing rain is less than 10
percent days 2-3.

Petersen