Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Sat Oct 24 2020

Valid 00Z Sun Oct 25 2020 - 00Z Wed Oct 28 2020

...Western and Central U.S. Winter Storm...
Days 1-3...


A shortwave trough diving trough the broad cyclonic flow across
the CONUS will dive southeastward from Canada tonight and Sunday,
before gradually amplifying and closing off across the Desert
Southwest on Tuesday. This feature will be accompanied by Pacific
moisture embedded within a potent jet streak, which will couple
and combine with the subtropical jet streak by early next week
across the Southern Plains. A surface low is likely to drop
southeast beneath this feature coincident with the mid-level
trough. Although initially guidance is agreeable, by D3 /Tuesday/
there are significant differences in both the position of the
upper trough and the low-level thermal response. The GFS is a
fast/progressive outlier kicking the closed low across the
Southern tier, while the ECMWF/NAM suggest a slower, more likely,
solution. However, the ECMWF may be overdoing the warmth in the
low levels as a strong cold front digs across the Southern Plains,
with most of the WAA confined to 700mb and above, the NAM and the
GEFS mean thermals were used heavily to develop the snow and ice
forecast.

As the low drops southward through Monday, it will be accompanied
by heavy snowfall. Ascent will be provided through robust synoptic
forcing within the RRQ of the upper jet, height falls, as well as
upslope enhancement on east/northeast flow behind the cold front
digging across the Plains. Additionally, mid-level fgen may be
ageostrophically enhanced by the upper jet streak to provide
intense mesoscale lift, and the guidance has come up considerably
with its snowfall across the High Plains of WY/NE/KS through D2.
Confidence is modest in these higher snowfalls due to minimal CSI
potential, but an extremely deep DGZ with cold temperatures could
overcome the lack of convective potential to drive heavy snow
rates and robust accumulations. WPC probabilities have increased,
and now show a high risk for 6 inches along the Cheyenne Ridge and
into central Nebraska on D1, with moderate probabilities for 6
inches extending southward into NW Kansas on D2. This includes the
I-25 corridor of Colorado which includes the Denver Metro area.
Further west into the Colorado Rockies, Sangre De Cristos, San
Juans, WPC probabilities are high for more than 12 inches of snow
in the highest terrain, with upwards of 2 feet possible in
isolated locations.

As the system drops southward further on D3, the forcing for
ascent weakens over the Southwest and the temperature profile
becomes more marginal. WPC probabilities for snow are still high
across the terrain of AZ/NM including as far west as the Mogollon
Rim, but for generally 4 inches or more.

More significantly, on D3, an overrunning/mixed precipitation
event begins to materialize across the High Plains of New Mexico,
and more impressively into north Texas and Oklahoma. While some
light freezing drizzle/freezing rain is possible across OK/KS on
D2, accretion is expected to be light as the duration of precip is
modest through a short period of WAA, and some of that will be
lost to drying the mid-layers of the column. However, by D3 the
300K isentropic ascent begins to intensify, especially across
TX/OK, with surface wet-bulb temperatures below freezing. Guidance
is still very spread as to where the heaviest freezing rain will
occur, and whether there will be mixing with snow/sleet at times.
However, the signal for a potentially significant freezing rain
event has increased, and thus WPC probabilities have followed.
There is now a moderate risk for 0.1" of ZR on D3, with low-end
probabilities for 0.25" now existing across parts of TX and OK.


Weiss