Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 AM EST Thu Nov 23 2023

Valid 12Z Thu Nov 23 2023 - 12Z Sun Nov 26 2023

...Northern and Central Rockies into the Central Plains...

Days 1-3...

An impressive winter storm will bring rounds of heavy snow for the
Thanksgiving Holiday and post-holiday travel weekend across
portions of the Northern and Central Rockies, and extending into
the Central Plains.

This event appears to be two-phased, with the first area of heavy
snow developing this morning across ID/MT/WY and then dropping
southward while expanding in coverage and becoming more intense.
The primary driver for this area of snow will be an amplifying
mid-level shortwave digging out of British Columbia this morning
and then amplifying into a closed low across the Great Basin
Friday morning. This will produce increasing downstream ascent
through height falls and divergence into the Rockies and central
High Plains, with moist isentropic ascent also becoming more
robust between 295-305K. At the same time, a progressive zonally
oriented jet streak will pivot over the N Plains into the
Northeast coincident with secondary jet development occurring
upstream /southwest/ of the deepening trough axis. This will
favorably result in a modestly coupled jet structure with the most
intense diffluence directly overlapping the best isentropic lift
and mid-level divergence. Additionally, a cold front dropping
southward will produce low-level convergence and fgen, with
post-frontal flow driving local upslope ascent into the terrain of
WY/CO. Mid-level RH increases dramatically, and although PW
anomalies according to the NAEFS ensemble tables are modest, the
moistening column and deepening DGZ suggest heavy snow will spread
from southern MT through the High Plains of CO on D1, with lighter
snow likely moving across the Great Basin and into UT/western CO.
Snowfall rates may reach 1"/hr at times, and WPC probabilities for
more than 6 inches are above 80% across much of the WY terrain and
into the High Plains, with locally 1-2 feet likely in the higher
elevations, especially of the Wind River range.

D2 will be a transition day of heavy snow as the forcing for
ascent gets split. The northern stream jet streak races away to
Canada while the trailing jet streak rotating around the base of
the trough lags over the Southwest. This will result in a rapid
weakening of upper diffluence Friday into Saturday, coincident
with a weakening of mid-level ascent due to the opening of the
primary closed low. Moderate snow will likely persist across the
Central Rockies and into the Central Plains as isentropic ascent
and fgen overlap, but snowfall on D2 is likely to be less intense
and less widespread than D1. The exception may be across the San
Juans where more favorable upper diffluence closer to the southern
jet streak occurs in tandem with impressive upslope and moisture
advection into the terrain, and in this range WPC probabilities
for more than 6 inches are above 90%, and locally 2 feet of snow
is likely, with more than 1 foot expected in much of the CO
Rockies. From the High Plains of WY through the Pine Ridge and
down into far NE KS, WPC probabilities for more than 6 inches are
30-60%, and locally more than 8 inches is possible.

The second portion of this event begins in earnest late D2 into D3
from the Southern Rockies through the southern High Plains and
into the Central Plains. The opening of the mid-level closed low
sheds a potent vorticity lobe eastward which will drive height
falls and PVA across NM/KS and the TX/OK Panhandles, while the
secondary jet streak intensifies placing favorable LFQ diffluence
aloft. Although the overall amplification of the mid-level pattern
weakens from D1 to D3, this secondary deepening of the 500mb
shortwave will allow the 700mb trough to remain closed, driving
downstream deformation from NM into KS, overlapping with some
mid-level fgen driven by both the diving cold front and the
ageostrophic response of the upper jet position. With moisture
increasing (NAEFS PW anomalies reaching +2 sigma in the TX
panhandle), this could result in renewed expansion and
intensification of precipitation, with heavy snowfall the likely
p-type. This setup may also support some banded structures lifting
SW to NE, and there is a clear signal for CSI in northern
OK/southern KS Saturday night reflected by folding of theta-e
surfaces in the presence of a saturated DGZ with SREF 50mb depth
probabilities of 30-50%. There remains uncertainty in placement of
these bands, but at least some heavy snow is becoming more likely
for this area on D3. WPC probabilities for more than 4 inches have
increased and are now as high as 40-60%, highest in south-central
Kansas, coincident with the greatest WSSI-P probabilities for
moderate impacts due to snow amounts, and after coordination with
WFOs ICT and DDC, winter storm watches are being issued for this
area. Large spread in the WSE plumes further suggests the
uncertainty this far out, but there is increasing potential for
impactful snowfall even through Saturday.


***Key Messages for Thanksgiving Snowstorm***

--Heavy snow today and Friday
Snow will expand today over Wyoming and progress southward into
Colorado through Friday. There is a high (>70%) chance of at least
6 inches of snowfall through Saturday over parts of Wyoming and
Colorado, with locally more than 12 inches possible. Lighter but
still impactful snow has a 50-70% chance of exceeding 4 inches in
the central High Plains.

--Bands of heavy snow developing Saturday
There is increasing confidence that moderate snow will spread
across southern and central Kansas Saturday. Embedded bands of
heavier snow with rates exceeding 1"/hr may lead to snowfall
accumulations above 4 inches.

--Hazardous travel due to snow, wind, and icy roads
Snowfall rates of 1"/hr combined with gusty winds will cause
hazardous travel due to snow covered/slippery roads and reduced
visibility. Additionally, rapid temperature drops behind an arctic
front may cause flash freezes Thursday evening into Friday morning.

--Much colder temperatures for the Rockies and Plains
An arctic front will push southward today through Saturday,
bringing much colder temperatures to the Rockies and Plains. There
is a 70-90% chance for high temperatures to remain below freezing
Friday and Saturday.


Weiss