FOUS11 KWBC 130703
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EDT Thu Mar 13 2025
Valid 12Z Thu Mar 13 2025 - 12Z Sun Mar 16 2025
...The West... Days 1-3...
...Extremely active weather pattern will spread heavy snow across
most of the West today through Friday, with a second wave
approaching for the weekend. This lead system will likely become a
significant winter storm in the Northern Plains Saturday. Key
Messages are in effect, and linked below...
The period begins with a short wavelength but extremely amplified
trough aligned on the immediate Pacific coast Thursday morning.
This trough will continue to amplify as it moves across the Great
Basin Thursday evening and then into the Rockies on Friday. During
this evolution, a potent shortwave and accompanying vorticity
maxima will swing through the base of this trough, This feature
will outrun the northern leg of this trough, resulting in a closed
mid-level low pivoting into the Southern Rockies Friday morning and
then into the Southern Plains. This will be an extremely impressive
feature, as NAEFS ensemble tables indicate 500-700mb heights fall
below the minimum within the CFSR database by Friday morning across
the Central and Southern Rockies, with a surrounding large area of
height anomalies below the 1st percentile across most of the Western U.S.
This mid-level height evolution is a clear signal for a powerful
and widespread system to bring heavy precipitation across an
expansive area. While height falls will drive ascent across a vast
region, this will combine with upper diffluence within the LFQ of a
subtropical jet streak which will wrap around the base of this
trough, producing enhanced deep layer lift, especially into CA and
the Four Corners. Additionally, periods of upslope flow on W/SW 700
mb winds will enhance ascent across much of the terrain, first
across the Sierra and Cascades, and then spreading east across the
Great Basin and into the Rockies Friday. Additionally, W/SW mid-
level flow will isentropically ascend the region, leading to
additionally enhanced lift across a large portion of the region.
In the presence of this pronounced ascent, moisture will steadily
increase as an atmospheric river (AR) pivots onshore downstream of
the primary trough axis. IVT progs from both the GEFS and ECENS
indicate a high probabilities (>90%) for a plume of 250 kg/m/s
IVT, although probabilities for 500 kg/m/s IVT are tepid (< 30%).
Still, this pronounced moisture plume will surge PWs to above the
90th climatological percentile from CA through the Southern
Rockies, with more normal PWs farther north. The overlap of the
aforementioned ascent into this moistening column will result in
widespread precipitation, with snow falling in the terrain.
While confidence is high in widespread precipitation, there is
still uncertainty into the snow levels. The steep lapse rates
beneath the core of the deepening upper trough suggest heavy
precipitation rates will drag down snow levels, indicating lower
NBM percentiles are likely more realistic. Using the NBM 25th
percentile as proxy, snow levels could fall to 2000-3000 ft during
periods of heavy snow across much of the West, with heights as low
as 1000-1500 ft across the Cascades. This indicates that travel
across many passes will be greatly impacted, especially where
snowfall rates reach 1-2"/hr, potentially as high as 3"/hr across
the Sierra, Transverse Ranges of CA, and parts of the Great Basin
terrain as reflected by the WPC prototype snowband tool.
Not only will rates be impressive, they will combine with strong
winds to produce substantial impacts, and the WSSI-P is forecasting
a high probability (>90%) for at least moderate impacts for much of
the terrain, especially from CA through the Four Corners. In these
areas, WPC probabilities forecast a high chance (>90%) for at least
8 inches D1 in the Sierra, Transverse/Peninsular Ranges, the
Wasatch, Mogollon Rim, San Juans, Uintas, and Wind River Mountains.
By D2 lingering snowfall may accumulate above 8 inches once again,
but primarily across the CO Rockies. Locally 1-3 feet of event
total snowfall is probable in these ranges.
As this first impulse departs into the Central Plains D2, /00Z-12Z
Saturday/, another wave immediately on its heels will approach the
Pacific Coast and spread renewed height falls onshore as far east
as the Central Rockies. This second wave appears to be less intense
as reflected by weaker height and PW anomalies, and is progged to
be shifted north of the initial impulse. However, renewed periods
of heavy snowfall are likely from the Sierra and northern CA
terrain north and east into the Cascades and then spread east D3
into the Rockies, generally from CO northward as a warm front
pivots through the area. Snow levels with this second wave are
expected to be higher, rising to 4000-5000 ft in CA and 2500-3500
ft elsewhere, but still low enough to be impactful to many passes
as Pacific air is slow to flood eastward.
With this second wave, WPC probabilities D2 re-focus in the
Sierra, the Shasta/Trinity region of northern CA, and the OR
Cascades where they are above 70% for 8+ inches. On D3 these
probabilities shift northeast, with 70%+ probabilities extending
across the WA/OR Cascades, the Olympics, and into the Salmon River range.
...Central and Northern Plains... Days 2-3...
The southern stream closed low emerging from the Four Corners and
into the Southern Plains Friday morning will become a formidable
winter storm across the Plains Friday into Saturday. As this closed
low emerges into the Plains early Friday, it will continue to
deepen, featuring 500-700mb height anomalies by 00Z Saturday that
are below the minimum percentile within the CFSR climatology
between 850-500mb across a large portion of the Plains. This is an exceptionally deep system, and the combination of the intense
height falls with the LFQ of a subtropical jet streak rotating
through the trough will result in rapid lee-cyclogenesis beginning
as early as Friday morning.
This surface low will deepen rapidly, approaching March record
pressure levels across the Plains as it tracks from eastern CO
Friday morning to eastern SD Friday evening, and then the U.P. of
MI Saturday aftn. While there is some uncertainty in the exact
placement and track of this low, the overall spread is relatively
small for a D3 system, leading to high confidence in the evolution.
With this low deepening rapidly, significant moisture will be drawn
northward from the Gulf on impressive isentropic ascent, and the
resultant theta-e ridge is progged to lift into a robust TROWAL
which will pivot across WI and into MN/ND/SD Saturday morning. As
this occurs, the combination of increasing ascent, aided by a
pivoting deformation axis W/NW of the low center, and cooling of
the column through both dynamic affects and ageostrophic flow/cold
air drainage from the NNW, will result in a p-type transition from
rain to mixed to snow.
There remains uncertainty into how long this transition will take
to occur, but the intense omega into the column Saturday morning
suggests the guidance may be under-doing the speed at which this
occurs, indicating a faster transition from rain to snow, with less
freezing rain/sleet in between. This is more typical of the
conceptual model for this type of system as well, so while mixed
precip is likely at times, the more impactful wintry precipitation
type should be snow, which will likely come down heavy at times. At
the same time, very strong winds, which are progged by model
soundings to exceed 50 kts in the 800-900mb layer, while help to
fracture dendrites leading to low SLRs which will be blow around
considerably. So, while total snowfall may be somewhat modest
except beneath the pivot point of this deformation axis, impacts
will be substantial with near blizzard conditions possible from far
northeast Nebraska through northern Minnesota. In this area, 48-hr
WPC probabilities from 12Z Fri to 12Z Sunday reach as high as
50-70% for 4+ inches, and isolated totals above 6 inches are
possible (10-30%). Additionally, some light freezing rain has a
30-50% chance of accumulating 0.1" or more in a small portion of NW MN.
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
$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)
Web-based telnet client
IRC
Email & news access