Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Mon Jan 16 2023

Valid 00Z Tue Jan 17 2023 - 00Z Fri Jan 20 2023

...Maine...
Day 1...

Complex low pressure evolution will continue tonight into Tuesday
as a vertically stacked system crawls northward through Nova
Scotia, continuing to spread moisture westward into Maine. While
much of this moisture is being expanded westward through WAA, the
occluded low shifting northward after 00Z tonight results in
rapidly increasing CAA on N/NW winds in its wake. This will erode
the warm nose through Tuesday morning, resulting in a p-type
transition from freezing rain/sleet to sleet/snow, and likely
finishing as all snow across Maine. The guidance does still differ
in the timing of this transition, but most of the guidance
indicates several hours of snow before the column dries on
Tuesday. An elevated DGZ and modest ascent will likely limit
snowfall accumulations, and WPC probabilities for more than 4
inches of snow are generally just around 5%, but widespread 2+
inches is likely across northern ME, especially in the higher
terrain.


...Pacific Northwest...
Days 1-3...

A shortwave moving onshore OR tonight will advect inland into D2
followed by high amplitude but short duration ridging on
Wednesday. A more significant shortwave trough will pivot onshore
Thursday morning associated with a modest AR pivoting into the
region noted by IVT of 200-300 kg/m/s according to NAEFS ensemble
tables. Lift associated with this second shortwave will be
enhanced by the weak LFQ of an approaching Pacific jet streak as
well, helping to wring out the moisture inherent to the AR as
heavy snow across the terrain. The heaviest snow this period
should be late D2 into D3 associated with the WAA/AR, which will
drive snow levels to above 3000 ft before collapsing again the
latter half of Thursday behind the accompanying cold front. The
overall forcing is transient so snowfall should overall be modest,
but WPC probabilities for more than 6 inches are above 70% in the
Olympics and northern WA Cascades, and 30-50% across the southern
WA Cascades and into OR. This could result in at least modest
impacts at the important Cascades passes as well.


...California through the Four Corners...
Days 1-3...

The exceptionally active period of winter weather that has been
impacting the region most of January will begin to wane this
period, but before things quiet down two more waves of forcing and
moisture will produce periods of heavy snow across the region.

Tonight through Wednesday morning, a positively tilted shortwave
just off the CA coast will shift inland and begin to amplify,
becoming negatively tilted as it dives across the Great Basin and
then closes off over the Four Corners Wednesday morning. This
amplifying trough will be accompanied by a 150kt subtropical jet
streak pivoting south of the trough, providing additional ascent
through LFQ diffluence into the Great Basin and Four Corners.
Moist advection within this Pacific jet will drive a modest AR
noted by IVT reaching +2 standard deviations above the climo mean
according to the NAEFS ensemble tables into the Four Corners, with
the overlap of this moisture and ascent driving heavy snowfall
into the terrain, generally above 4000-6000 ft. While modest
accumulations exceeding 6 inches are likely across the Great Basin
and into the Wasatch according to the WPC probabilities, the
heaviest snowfall is likely D1 in the Mogollon Rim, the White
Mountains of AZ, the San Juans, and into other parts of the CO
Rockies where more efficient upslope flow will drive more intense
ascent for heavier snowfall. In these areas, WPC probabilities for
8 inches or more are generally 60-80%, with locally 1-2 feet
likely in the higher terrain.

By D2, brief shortwave eastern Pacific ridging will develop and
spread eastward, generally shutting off precipitation until
another trough digs out of the northern Pacific and into northern
CA during Wednesday night into Thursday /late D2 into D3/. Height
falls and PVA associated with this impulse will drive ascent,
aided by modest WAA within confluent flow south of the trough
axis, and at least subtle LFQ diffluence as a jet streak pivots
onshore. This will result in additional moisture and heavy
snowfall D3, primarily above 3000 ft and from the
Shasta/Trinity/Siskiyou region into the Central Sierra. WPC
probabilities for more than 6 inches are above 70% for these areas
late D2 into D3.


...Eastern Colorado/Central Plains through the Upper Midwest...
Days 2-3...

A shortwave trough moving onshore CA D1 will amplify as it digs
southeast, closing off near the Four Corners Wednesday morning and
then shifting eastward into the MS VLY Thursday while continuing
to deepen. Downstream of the accompanying longwave trough axis, a
zonally oriented northern stream jet streak will become
increasingly coupled with a poleward arcing subtropical jet
streak, with this overlap of robust synoptic ascent driving
surface cyclogenesis in the lee of the Rockies Wednesday morning.
This surface low is then progged to deepen slowly as it shifts
northeast into the Great Lakes by the end of the forecast period.

Ahead of this surface low, ascent will become maximized through
downstream mid-level divergence and height falls, aided by
increasing warm and moist advection from the Gulf of Mexico
reflected by a surge of moist isentropic ascent at 295K with 4-5
g/kg mixing ratios, collocated with PW anomalies of +2 to +3
standard deviations according to the NAEFS ensemble tables lifting
into the system. This will manifest as at least a modest TROWAL
wrapping cyclonically around the low into the Central Plains, with
a weak TROWAL signature continuing into the Upper Midwest as the
system begins to move more rapidly eastward D3. While overall this
low should remain relatively transient, there is increasing
potential for pivoting bands of heavy snow to develop NW of the
low across eastern CO and into parts of NE where the deformation
axis overlaps with this TROWAL, some omega is enhanced through
easterly/upslope flow, and modeled mid-level theta-e lapse rates
fall to < 0C/km suggesting the greatest risk for CSI. Although
regional soundings do not look ideal for excessive SLR, snowfall
rates of 1-2"/hr still seem likely across this area of the High
Plains in the most intense bands, which will quickly accumulate
within these bands, and the ECMWF EFI reaches as high as 0.95 with
a shift of tails of 2 late Wednesday into Thursday. Outside of the
heaviest snow axis, a longer duration of light to moderate snow is
also likely within a region of broad, but slightly less intense,
ascent stretching from CO through MI.

Some uncertainty continues into exactly where this low will track,
which will likely have significant impacts as to where the
heaviest snow will occur. This is primarily due to its affect on
the placement of both the northward extend of WAA surging a warm
nose above 0C, and where the dry slot, which is progged to be
impressive via all global guidance, will pivot. Just
north/northwest of this dry slot and warm nose will likely be the
heaviest snow axis, with a mix of sleet and freezing rain
occurring just to the south. The EC members are quite well
clustered and supported by recent GEFS members, and outside of the
12Z GFS, most models are well aligned with the surface low
placement by 00Z Thursday. However, model trends have been a bit
north the past few runs, so additional changes to the forecast are
likely, especially by D3. At this time however, WPC probabilities
for more than 6 inches are already above 80% from the NE High
Plains of CO through central NE, with locally more than 10 inches
likely in the most intense banding. For D3, WPC probabilities for
more than 6 inches are slightly lower due to the more progressive
nature of the system by then, but still are above 40% from NW Iowa
through the U.P. of MI, with a local maxima above 8 inches
possible in parts of WI.


Weiss