FOUS11 KWBC 070825
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Thu Nov 7 2024

Valid 12Z Thu Nov 07 2024 - 12Z Sun Nov 10 2024

...Central/Southern Rockies and Adjacent High Plains...
Days 1-2...

...Two phase winter storm to bring heavy snow and major impacts to
the Southern Rockies...

The forecast begins with an impressive closed and cutoff low
sinking slowly southward across the Four Corners region. This low
is progged to continue to amplify today, reaching as low as -4
sigma with respect to 500mb heights across AZ, and then sinking as
far south as Sonora, Mexico before finally rounding through the
base of the longwave trough across the west and beginning to eject
northeast. This is a classic position for heavy precipitation into
the Southern Rockies as downstream height falls and robust
divergence combine with the LFQ of an intensifying subtropical jet
streak to produce large scale impressive ascent. This will impinge
into a moistening column as well as both the subtropical jet streak
and southerly 850-700mb flow surge moisture into the region
leading to PWs that are +1 to +2 sigma today into Friday. This
synoptic lift into the moistening column will combine with
persistent and intensifying moist isentropic lift, especially near
the 300K level where wind speeds will approach 50 kts, pushing
mixing ratios above 6g/kg Thursday aftn.

The result of this evolution will be an expanding precipitation
shield across NM/CO, with heavy snow the primary precipitation type
except across far eastern NM and far southeast CO. Evaluation of
forecast soundings during this time suggests impressive omega
crossing the DGZ, with a near isothermal layer just beneath it.
WHile the DGZ seems a bit elevated, the strong ascent and aggregate
maintenance supported by the sub-DGZ isothermal layer indicates
the likelihood for large dendrites and rapid accumulation.
Additionally, cross-sections indicate a threat for CSI on Thursday,
indicating the potential for convective snowfall rates as the
isentropic ascent maximizes, and this is reflected by the potential
for 1-2"/hr snowfall rates in the WPC prototype snowband tool.
Overall, D1 will feature widespread heavy snow across NM and CO,
and WPC probabilities for more than 8 inches are above 80% for the
Sangre de Cristos, Raton Mesa, and surrounding foothills/High
Plains including the I-25 corridor between Pueblo and Santa Fe.
Locally 2-3 feet is possible in the higher terrain leading to
substantial travel impacts. Farther north, WPC probabilities are
70-90% for 4+ inches across the Palmer Divide.

The second phase of this event will begin on Friday as the core of
the upper low begins to slowly weaken as it pivots, still slowly,
northeast into the High Plains of NM by Friday aftn. This will
help spawn surface cyclogenesis across northern Texas, with this
low shifting northward within the broad southerly flow into
Saturday. As this low slowly strengthens, a secondary surge of
moist advection will lift northward, spreading snowfall farther
north once again into CO and maybe even southern WY, while a
pivoting dry slow shuts off precipitation over NM. The
strengthening theta-e advection this time will likely result in an
impressive mid-level TROWAL pivoting NW into CO, with accompanying
upslope flow driving heavy snow rates into CO through D2. This will
lead to some elevated instability, especially on the periphery of
the dry slot, so once again snowfall rates could be intense,
especially where the DGZ deepens (SPC SREF probabilities for >50mb
of DGZ depth eclipse 70% in eastern CO), so another day of
impactful heavy snow is likely.

The setup also seems to support a pivoting band of heavy snow
somewhere across eastern/central CO Friday evening. Exactly where
this sets up is still uncertain, but the synoptic environment
appears to match the conceptual model for a pivoting band, and the
high- res models simulated reflectivity all feature something that
looks like this, but with different placement. Will need to
monitor this closely as this band could result in much heavier snow
totals and strong snowfall gradients, but at this time, WPC
probabilities for more than 8 inches are again high (>70%) across
the Raton Mesa and Sangre de Cristos, leading to event-total
snowfall of 3-4 feet in the higher terrain. As snow pivots
farther northward D2, WPC probabilities also indicate a 70% chance
for more than 6 inches of additional snowfall across the Palmer
Divide. ALthough the heaviest snow will likely again be in the
higher terrain, where this band pivots, snow fall rates should
overcome any terrain features leading to local enhancements in
snowfall even in lower elevations.

By late D2 and then into D3, the low occludes to the east, shunting
the best moisture fetch into the Plains and bringing a slow wane to
the heavy snow. There is potential that some dynamic cooling could
still produce pockets of heavy snow, and guidance still features a
lot of longitudinal spread in the axis of heaviest precipitation,
but in general snow should come to an end, finally, on D3.

The probability of significant ice across the CONUS is less than
10 percent.


Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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