Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Sat Feb 11 2023

Valid 12Z Sat Feb 11 2023 - 12Z Tue Feb 14 2023

...Southern Appalachians...
Days 1-2...

Deep closed low (500mb height anomaly around -2.5 sigma this
morning) moving along the northern Gulf coast will track through
the Carolinas on Sunday and out into the Atlantic by Monday
morning. A weak area of low pressure along a stationary front
draped across northern FL will start to deepen as height falls
increase today, spurring cyclogenesis over Georgia and eventual
triple-point development by early Sunday. Much of the Southeast
will be in between a strong northern jet off the Northeast and the
subtropical jet across the northern Gulf, supporting a broad area
of lift through divergence aloft. Plenty of moisture from the
Gulf/western Caribbean/western Atlantic will feed into the system
via the WCB and wrap into a developing TROWAL as the occluded
cyclone continues to mature. Overall environment is mild with
temperatures in the 40s/50s this early morning over NC/SC with
sub-32F Tds over central NC northward. Though surface high
pressure will bridge the low from the west/northwest/north to the
northeast, flow is off the Atlantic with no cold air source. Thus,
any wintry precipitation will be confined to the mountains in the
central but especially southern Appalachians.

Approaching cold-core low will bring in cooler temperatures behind
the main surface low, and dynamical cooling will help to drop snow
levels through the terrain, but still remain above about
2500-3000ft for much of the event. With such a marginal
environment, SLRs will be rather low (<10:1) for many areas
outside the highest terrain. Strong dynamics of the system support
potential for 1-2"/hr rates over the southern Appalachians per the
WPC Snowband Probability Tracker page, aided by upslope
enhancement/convergence. With plenty of QPF, owing to PW anomalies
around +1 sigma to the mountains, areas that can support snow may
see significant accumulations. WPC probabilities for at least 4
inches of snow are >50% over higher elevations above ~3500ft or
so. In addition, some more sheltered areas that do eventually fall
below freezing tomorrow may see a period of icing as the warm nose
aloft moves in from the east/southeast. Guidance has been quite
varied in amounts, but have remained on the conservative side
given no cold air source. WPC probabilities for at least 0.10"
icing are moderate (>40%) from far northwestern NC into
southwestern VA and along the WV/VA border.


...Sierra and Four Corners region...
Days 1-3...

Closed low moving southward along and then off the CA coast will
maintain some light precip over the Sierra D1 where a few inches
are likely. This upper low will take a long/wide turn across NW
Mexico early Monday and start to tap the eastern North Pacific for
additional moisture into the Southwest D2. Snow will start to
accumulate in the Mogollon Rim and White Mountains overnight
Sunday into Monday. On D3, upper jet will amplify and shorten its
wavelength with increased divergence aloft over the Southwest,
promoting broad lift as the upper low progresses through AZ into
NM. Moisture will be only slightly above normal for mid-February
but the combination of approaching height falls and upslope
enhancement will still yield several inches of snow D3 over the
Mogollon Rim/White Mountains and then into the Sangre de Cristos
as the 700mb low moves through. WPC probabilities for at least 4
inches are above 50% in the mountainous areas generally above
7000-9000ft.


...Pacific Northwest...
Days 2-3...

Progressive yet amplified wave train of trough/ridge couplets will
move across the northern Pacific D2, pushing ridging into the Gulf
of Alaska D3 with downstream troughing digging rather sharply into
the Pac NW. This will take a strong cold front through the area
with much colder temperatures and dropping snow levels all the way
to some valley floors/near sea level behind the front at the
tail-end of the precipitation. With a source region for moisture
to the northwest, IVT should remain low. However, strong incoming
jet streak >150kts will support broad lift in the left exit region
along and behind the front. Initial snow levels around 5000-6000ft
will crash behind the front as QPF continues via onshore flow,
aided by upslope enhancement into the Cascades. WPC probabilities
for at least 8 inches of snow are >50% above about 3000ft which
will affect several passes. Additional snow is forecast for the
central ID ranges as enough moisture advects eastward.


Fracasso