FOUS11 KWBC 270856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Mon Jan 27 2025

Valid 12Z Mon Jan 27 2025 - 12Z Thu Jan 30 2025

...Great Lakes, Northeast, Mid-Atlantic... Days 1-3...

An active period of winter weather is expected through mid-week as
progressive flow drives multiple impulses with rounds of snow from
the Great Lakes through the Northeast.

The period begins with a lead shortwave exiting New England into
the Atlantic, leaving sharp NW CAA in its wake. THe most impressive
CAA is likely to linger primarily across Lake Ontario and into
Upstate NY, at least early in the period before a subtle surge of
WAA occurs ahead of the next front. This will result in a band of
heavy LES waning right at the start of the period, with minimal
additional LES accumulation. 

However, the respite will be short as a potent shortwave,
basically demarcating the edge of the larger cyclonic gyre digging
south from Hudson Bay will drop south Monday night. This will 
drive a potent cold front southward, first into the U.P and L.P. of
MI late Monday evening, and then continue to race east through the
interior Northeast and New England Monday night into Tuesday 
morning. Impressive CAA along the front will drive an intense 
baroclinic boundary, leading to a stripe of impressive low-level 
fgen, especially in the 925-850mb layer. This will drive strong 
mesoscale lift into a saturating column reflected by high low-level
RH, and as lapse rates steepen, some weak to modest instability 
will develop. The high-res guidance has become more aggressive 
showing a band/corridor of convective snow showers and snow 
squalls, with HREF probabilities showing a 20-30% chance for 1"hr 
snowfall rates within these elements. The accompanying steep lapse 
rates will help mix down strong winds, encouraging the development 
of low visibility during heavy snow showers. The limiting factor to
true snow squalls may be a relative dearth of 0-2km CAPE, and it 
is beneficial that the most significant impacts will occur at 
night, but still, convective snow showers and isolated snow squalls
are expected Monday night into Tuesday morning from the U.P. of 
Michigan through much of the interior Northeast and New England, 
and potentially as far southeast as the Mid-Atlantic. 
Accumulations during this time will generally be minor, but some 
LES and some upslope flow with the front across the Adirondacks has
resulted in WPC probabilities reaching 70-90% for 4 inches on D1,

Some modest LES will follow in the wake of this front, but more
significant and widespread snow will occur D2 into D3, as yet
another fast moving impulse dives through the broad cyclonic flow
across the east. This third impulse will track along the residual
baroclinic gradient left by the potent cold front, and will be
overlapped by a strengthening jet streak downstream of the trough
axis to produce more robust deep layer ascent. This will support
modest cyclogenesis as a clipper-type low swings southeast from 
Minnesota Tuesday aftn, to Upstate New York Wednesday morning, and 
then across New England, exiting into Atlantic Canada by Wednesday 
evening. Brief but impressive WAA downstream of this low will 
result in expanding precipitation falling as moderate to heavy snow
across many areas. 

Then, in the wake of this system, additional LES will develop as
NW flow causes additional accumulations in the favored NW snow 
belts, and with strong winds, some of this could push as far SE as 
the coastal Mid-Atlantic states by the end of the forecast period.
WPC probabilities D2 are high (70-90%) for 4+ inches across the
U.P., the NW L.P. near Traverse City, and east of Lakes Erie and
Ontario, with a swath of moderate probabilities for 2+ inches
surrounding those areas for the more synoptically forced snowfall.
For D3, the greatest risk for 4+ inches of snow shifts into the
Adirondacks and Greens where WPC probabilities feature a high risk
(>70%) while additional modest LES occurs east of Lakes Erie and
Ontario. Locally 1-2 feet is possible in some areas through the period.

Finally, a second round of convective snow showers or snow squalls
is becoming more likely on Wednesday, this time a little south of 
the event Tuesday morning. These convective snow showers will occur
during a more favorable time of day to support better CAPE behind 
the secondary front, and the SnSq parameter is suggesting a risk 
across areas from eastern OH through southern Upstate NY and PA. 
This secondary set of squalls, if they occur, will again be 
accompanied by strong winds and heavy snow rates leading to the 
potential for significant travel impacts despite modest snowfall
accumulations. 

The two rounds of potential snow squalls have prompted the issuance
of Key Messages linked below.

...Southwest through the Four Corners... Days 1-3...

The anomalous upper low pivoting slowly across CA will begin to
slip east this week, bringing snowfall across the Southwest US
through Thursday. This cutoff begins the period quite amplified,
with 500-700mb heights as low as -2 sigma over CA according to the
NAEFS ensemble tables. Two distinct spokes of vorticity will dance
around the core of this upper low, helping to elongate it with 
time as it becomes stretched NE to SW into the Four Corners by 
Tuesday night. The interaction of these vorticity impulses 
rotating around the closed center will help keep the low amplified,
but also very slow moving, reaching NM/CO by the end of the 
forecast period.

During this synoptic evolution, the upper pattern will alter
considerably as well. Initially, a strong 250mb jet streak will be
arced downstream of the upper low, lifting northeast into the
Southern Plains. However, this feature will weaken D1 as the upper
low elongates, leaving less substantial diffluent-caused ascent
into D2. However, a secondary surge of jet level energy will occur
Wednesday into Thursday as the jet streak re-amplifies and arcs
meridionally from the Gulf of California into the Southern Plains,
placing intense LFQ diffluent ascent over the Four Corners. This
jet evolution, combined with the mid-level closed low, and at least
subtle low-level baroclinicity in the vicinity of a weakening front
will cause waves of low pressure to move eastward from CA to the
Four Corners D1-D2. This will cause periods of snowfall from the
Transverse and Peninsular Ranges of CA eastward through the
Mogollon Rim and Kaibab Plateau where WPC probabilities are
moderate (30-50%) D1 for 4 inches of snow, and high (>70%, but only
in higher elevations of AZ) on D2.

However, the most significant snowfall is likely to begin D3 as a
low pressure develops near the TX Big Bend and moisture advection
surges on isentropic ascent across TX. The accompanying theta-e
ridge is progged to lift cyclonically into a TROWAL, with the most
intense theta-e advection being directed into the DGZ over NM/CO.
There is still quite a bit of uncertainty regarding the placement
and evolution of this system, but snowfall chances are increasing,
especially in the higher terrain above 5000 ft in the Sangre de
Cristos and San Juans. Here, WPC probabilities D3 are as high as
70% for 6+ inches, and impactful snow is becoming more likely for
parts of this area Wednesday into Thursday.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" 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/La...

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