Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
203 PM EST Sat Jan 20 2024

Valid 00Z Sun Jan 21 2024 - 00Z Wed Jan 24 2024

...The West...
Days 1-3...

Active pattern will continue in much of the West over the next few
days, with a strong ~180kt jet stretching into SoCal and
northwestern Mexico, leaving the region in a broad area of upper
divergence as mid-level impulses push into the region out of the
Pacific. A long stretch of moisture from north of Hawai'i will
sustain above normal precipitable water values for the entire West
into the Rockies, wavering between +1 and +2 sigma and with the
highest anomalies over the Sierra. However, with the flow overall
from the west, milder air will eventually overspread the region
compared to the past week, resulting in higher snow levels over
the region and a transition from freezing rain to plain rain in
sheltered interior valleys.

For the Pacific Northwest, the D1-D2 period will see a rather
steady influx of moisture into a warming column, especially along
and west of the Cascades, with snow levels rising from around
4000ft today to over 4500ft by late Sunday into Monday, with
variability through the Cascades into the eastern foothills. This
will lead to a chance of freezing rain in areas that remain below
freezing at the surface but beneath a warmer layer moving in from
the west. Areas through the Columbia Gorge east of Portland may
hold onto sub-freezing surface temperatures the longest, extending
northeastward into the Columbia Basin in southeastern WA.
Additional freezing rain is likely in many Cascade passes until
either milder air moves into lower passes or the column becomes
favorable for just a rain/snow delineation. WPC probabilities for
at least 0.10" ice along the OR/WA border are at least moderate
(40-70%) with local higher probabilities where over 0.25" icing is
possible (10-50% chance). In higher elevations, generally above
5000ft or so, a more or less steady snow is likely for the next
couple of days. There, WPC probabilities for at least 8 inches of
snow D1-2 are at least 50% over the higher WA/OR Cascades and into
the Blue Mountains. By D3, weak ridging may allow for a respite
ahead of another system.

To the south, the Sierra will be closer to the upper jet and
higher moisture flux, as evidenced by the leading edge of the 250
kg/m-s line into at least coastal CA into the Sierra foothills per
the GFS/ECMWF. Snow levels will be on the higher side, generally
above 7000ft, with 700mb temperatures around -5C to the north and
a few deg higher over the southern Sierra. D1-2 will see the
highest snowfall as several vort maxes zip through. By Tuesday,
the broad mid-level trough will start to buckle, allowing weak
ridging into the area as snowfall lessens. East of the Sierra,
generally light snow is forecast for much of the Intermountain
West with wavering snow levels around 6000ft or so. Upslope flow
on westerly to southwesterly flow aloft will favor the north-south
ranges, especially the Wasatch in UT, but also into the Mogollon
Rim and White Mountains in central AZ by days 2-3 as the troughing
axis shifts eastward. WPC probabilities for at least 8 inches of
snow through the period are at least 50% above about 7000ft.

Fracasso


...Southern Plains into the Ohio Valley..
Days 2-3...

Expanding cyclonic flow within a deepening trough across the
Intermountain West will pivot eastward Sunday, with an amplifying
shortwave digging through the Four Corners by Monday morning. This
trough will become negatively tilted as it shifts into the
Southern Plains Monday evening, leading to increasing downstream
divergence and height falls into the Ozarks and MS VLY.
Pinched/confluent flow downstream of this negatively
tilting/amplifying shortwave will cause an increase in the
700-500mb SW flow, while at the same time 850mb flow surges out of
the Gulf of Mexico and converges into the lower MS VLY. The
overlap of this flow will surge moisture into the area, and PW
anomalies according to NAEFS could rise to +2 to +3 sigma Monday
through Monday night. At the same time this moisture increases,
deep layer ascent will maximize as a poleward arcing jet streak
amplifies downstream of the primary trough axis, leading to
impressive RRQ diffluence overlapping the greatest mid-level
height falls/PVA. The intensifying deep layer ascent into the
moistening column will drive an expanding shield of precipitation
from SSW to NNE, and a prolonged precipitation event is likely to
begin Sunday over TX and then lift northeast into the
Mid-Mississippi Valley by Tuesday morning.

Although the accompanying WAA is expected to be strong, cold air
will be entrenched at the surface during precipitation onset,
suggesting a period of freezing rain is likely. The warm nose is
progged to be quite warm, +5C to +8C, but a sharp inversion will
keep surface wet-bulb temps well below 0C, at least for several
hours. The surface high slowly retreating to the east indicates
that eventually the WAA will cause a p-type change from freezing
rain to rain, but not until a significant accretion of ice occurs
in some areas. There remains some uncertainty as the duration of
the freezing rain due to the known bias of models under-estimating
the strength of cold air trapping, but the strong WAA will allow a
changeover during Monday, likely precluding widespread damaging
ice. Still, WPC probabilities for more than 0.25" are above 30%
from generally south-central OK near the Red River Valley
northeast through the Ozarks and approaching the St. Louis metro
area. The highest probabilities are in the higher terrain of the
Ozarks, including the Ouachita Mountains.

Farther to the northeast, the duration of heavy freezing rain is
expected to be less both due to lower anomalous moisture and
stronger ascent during the event to erode the cold surface layer
more rapidly. A stripe of significant freezing rain is still
probable however, reflected by WPC probabilities for 0.1" of ice
or more reaching 20-50% on D3 from near St. Louis northeast
through lower Michigan.

Weiss