Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
247 AM EDT Wed Mar 13 2024

Valid 12Z Wed Mar 13 2024 - 12Z Sat Mar 16 2024

...Central Rockies, Southern Rockies, & Central High Plains...
Days 1-3...

***Significant and long lasting winter storm to produce heavy snow
 across much of the terrain of the Central and Southern Rockies,
 with impactful snowfall likely into the lower elevations of the
 High Plains as well.***

Two-phased major winter storm begins this afternoon as the mid-
level pattern begins to evolve into one supporting a long-duration
system across the Central and Southern Rockies. The event begins
as a wave of low pressure developing in the lee of the southern CO
Rockies in response to a lobe of vorticity swinging east out of
the Desert Southwest overlapped with a strengthening jet streak
arcing northeast into the Central Rockies. The overlap of these
two will cause rapid pressure falls in the lee of the terrain, and
this low is likely to deepen as it moves east into the Central
Plains by tonight before finally ejecting into the Upper Midwest
and Great Lakes on Thursday. The guidance has continued its trend
southward with this surface low, which will allow for more rapid
cold advection to sink across WY/CO, especially behind the
southward advancing surface cold front behind this leading low
pressure.

Moisture will become impressive through Thursday as both low and
mid level moisture advects into the Rockies. In the surface-850mb
level, southerly flow out of the Gulf of Mexico will draw PW
northward as the attendant theta-e ridge surges cyclonically into
a TROWAL around the deepening low. This causes an axis of elevated
moisture anomalies reaching +1 to +2 sigma according to NAEFS
surging through the MS River Valley and then arcing westward into
the Central Rockies and High Plains. At the same time, an
impressive deformation axis is likely to develop on the
intensifying NE winds behind the front and around the back side of
the low, which will overlap with robust fgen to enhance ascent
and cause dynamic cooling to help cause p-type transition from
rain to snow. While the column across the High Plains is marginal
for snow, this dynamic cooling combined with rapid CAA behind the
front should be sufficient for at least some snow in the lower
elevations D1. Still, however, the most significant snowfall and
snow rates, which will likely reach 2-3"/hr, are expected in the
Front Range where upslope flow maximizes into the moist column.

The guidance has continued to shift the lead impulse a little
faster resulting in slightly lighter moisture advecting into the
region with phase 1, and the regional soundings indicate a near
isothermal layer at temperatures slightly above the DGZ,
suggesting the potential for riming. This could cause SLRs to be
slightly lower than forecast in the NBM/deterministic models,
which has caused at least a small reduction in amounts with this
forecast update. However, impacts are still likely to be
significant as reflected by the WSSI-P showing higher than 50%
chance for major impacts in the Front Range/Sangre de Cristos, and
above a 50% chance for moderate impacts in the other Four-Corners
terrain.

WPC probabilities for more than 6 inches on D1 are above 80% from
the Big Horns south through the Wind Rivers and Laramies, and
then across much of the Front Range including the Palmer Divide,
south into the Sangre de Cristos. The highest snowfall is likely
in the Front Range with more than 2 feet possible.

As this surface low pulls away, the best low-level moisture will
be shunted to the east. However, it is during this time when the
500mb low deepens towards -3 sigma at 700-500mb according to NAEFS
over the Desert Southwest. This low will become extremely slow
moving as it gets trapped beneath the omega ridge to its north,
which will result in a long duration of ascent through downstream
divergence and waves of PVA. At the same time, this persistent SW
flow between 700-500mb will advect copious Pacific moisture
northeast through the Four Corners, leading to part 2 of this
system with snow quickly overspreading much of the terrain from
the Mogollon Rim northeast through the Wasatch and CO Rockies. At
the same time, the NE flow behind the southward sinking cold front
will persist robust upslope ascent into the Front Range and
eventually the Sangre de Cristos, resulting in additional heavy
snow in these areas as moisture increases aloft and some terrain-
induced fgen occurs. This suggests that D2 will have the most
widespread coverage of heavy snow, with rates of 1-2"/hr or more
likely in many areas, before the focus transitions primarily to
the Four Corners terrain with drier air and weaker forcing
shifting into CO.

During D2, the greatest WPC probabilities for more than 6 inches
of snow pivot south and west, but still peak along the Front
Range, Palmer Divide, Sangre de Cristos/Raton Mesa, and back into
the San Juans where they exceed 80%. Additional high probabilities
above 60% extend into the Wasatch, Uintas, and Mogollon Rim. The
heaviest accumulations D2 are again expected in the Front Range
where an additional 1-2 feet is possible, and impacts along the
I-25 urban corridor, especially across the Palmer Divides, will be
most substantial. By D3 the event winds down across the Front
Range, but high probabilities for more than 6 inches persist near
the Four Corners including again in the San Juans, Wasatch, and
portions of the Mogollon Rim. 2-day snowfall in these areas will
approach or exceed 2 feet.


The probability of receiving at least 0.1" ice is less than 10%
across the CONUS through Day 3.


Weiss


**Key Messages for March 13-16 Winter Storm have been started and
 can be found on our website at https://www.wpc.ncep.noaa.gov**