Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Thu Mar 14 2024

Valid 12Z Thu Mar 14 2024 - 12Z Sun Mar 17 2024

...Central Rockies, Southern Rockies, and the Southwest...
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.***

A surface low moving into the Upper Midwest this morning will
continue to serve as the impetus for low-level moist advection
rotating cyclonically around it and then surging SW into the
Central Rockies and High Plains on impressive NE flow. A surge of
NE winds is likely both in response to the pinched pressure
gradient and behind the southward sinking surface cold front,
which will result in long lasting and intense upslope flow into the
Front Range and Sangre de Cristos today. Forcing for ascent via
this upslope is progged to become very intense today, with omega
increasing through the aftn as the flow pivots south through CO and
into northern NM. Forecast soundings indicate a nearly ideal
crossover of omega into the deepening DGZ as the column cools,
with some mid-level fgen also developing to enhance ascent. At the
same time, theta-e lapse rates approach 0 in cross sections
indicating the potential for CSI, which will likely produce intense
snowfall rates of 2"/hr or more as progged by both HREF and NBM
probabilities.

The heaviest snowfall D1 is expected along the Front
Range through the Sangre de Cristos, including the Palmer Divide
and Raton Mesa. Here, WPC probabilities are above 90% for 6 inches,
and above 80% for 12 inches in the higher terrain. Although SLRs
are likely to fall somewhat below Baxter climatological means, the
persistent moisture and ascent will allow for local amounts of 3-4
feet west of the urban corridor, with considerable snow load
impacts likely. In the I-25 corridor, the heaviest accumulations
are expected along the Palmer Divide due to its elevation, but WPC
probabilities for more than 6 inches are at least 30% along the
length of the corridor on D1 from Cheyenne to Pueblo.

While the heavy snow persists along the Front Range/Sangre de
Cristos, the second phase of this event driven by the evolving
synoptic pattern will develop in earnest. An anomalous closed mid-
level low dropping steadily south into the Desert Southwest and
southern Great Basin will continue to strengthen, reaching as low
as -3 to -3 sigma according to NAEFS within the 700-500mb level.
As this low gets trapped beneath the omega ridge to its north, it
will move almost imperceptibly as the flow becomes cutoff,
resulting in a long duration of downstream ascent through robust
mid-level divergence. At the same time, the 700-500mb SW flow will
become increasingly confluent downstream of this cutoff low,
surging moisture northeast into the Four Corners with PWs more than
+1 sigma according to NAEFS becoming widespread over the region. A
subtropical jet streak will also amplify downstream of this
trough, producing additional ascent through its LFQ, and adding
additional Pacific moisture to the column. While the moist flow out
of the SW will also produce WAA to raise snow levels from about
5000 ft to 7000 ft Fri/Sat, steep lapse rates beneath the cold core
aloft will allow for at least period of dynamic cooling, which
could permit some snowfall down into the valleys. However, most of
the heaviest snow, which will be impressive, is expected above
5000-6000 ft as reflected by WPC probabilities above 80% for more
than 6 inches from the Uintas and Wasatch into the San Juans, White
Mountains of AZ, along the Mogollon Rim, and into the Kaibab
Plateau. D1+2 snowfall amounts in the higher terrain could reach
2-3 feet in these areas as well.

By D3 /Saturday/ the closed low should begin to fill and eject
slowly eastward, bringing a reduction in precipitation coverage and
intensity, but at least pockets of heavy snow are likely in the
higher elevations, especially across northern NM and into southern
CO, primarily in the Sangre de Cristos and San Juans. WPC
probabilities D3 for more than 6 inches reach 50-70% in the higher
terrain of these ranges.


...New England...
Day 2...

A fast moving shortwave exiting the Great Lakes will swing across
New England Friday with modest height falls and PVA, resulting in a
shallow surface wave racing east to the Atlantic coast by Friday
evening. This feature has limited amplitude and is fast moving, but
is accompanied by modest WAA and a brief surge of moisture on 290K
isentropic ascent lifting into eastern New England. The column
ahead of this low is quite warm, so much of the precipitation
should fall as rain. However, in the higher terrain of NH and ME,
generally above 2500 ft, there is likely to be a brief period of
moderate to heavy snow, and WPC probabilities indicate a low risk
(10-20%) for more than 4 inches of snow across the White Mountains
and into northern ME.


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

Weiss


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