FOUS11 KWBC 050844
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 AM EST Sun Jan 5 2025

Valid 12Z Sun Jan 05 2025 - 12Z Wed Jan 08 2025

...Central Plains through the Mid-Atlantic... Days 1-2...

...Major winter storm to impact areas from the Central Plains and
Ohio Valley today through the Mid-Atlantic on Monday...

The first major storm of 2025 is currently organizing over the
Central Plains this morning as an amplifying mid- level trough
ejects from the Central Rockies and should close off over Kansas
this morning. Latest radar, surface observations, and lightning
networks indicate moderate to heavy snow and mixed precipitation
with convective elements are already occurring over parts of KS
and MO. This aforementioned closed low will then continue to deepen
as it pivots eastward, reflected by NAEFS 500mb-700mb height
anomalies falling to as low as the bottom 2.5th percentile of the
CFSR climatology. This closed feature will continue to deepen
gradually as it moves into the OH/TN Valleys Monday, before slowly
opening into a sharp trough across the Mid- Atlantic Monday night.
Through this evolution, a strengthening jet streak will pivot
around the base of the trough, reaching as high as 150kts, and
overlapping effectively with the greatest height falls/PVA to cause
intense deep layer ascent. Beneath this synoptic lift, a surface
cyclone currently strengthening in the southern High Plains will
then move nearly due east with subtle deepening until it exits into
the Atlantic on Tuesday.

As this surface low deepens and move east, impressive moisture will
be drawn northward from the western Gulf of Mexico within a
corridor of intensifying isentropic ascent, especially within the
290-295K surfaces. Here, mixing ratios climb to 4-6 g/kg,
reflective of the impressive moist driven by the warm advection,
further evidenced by PW anomalies surging to above the 97th
percentile according to NAEFS. The accompanying theta-e ridge is
then progged to lift cyclonically into a TROWAL surrounding this
deepening low, and pivot back into KS/MO later today. This is
likely to cause an expanding area of precipitation, with the
accompanying elevated instability driving more intense
precipitation rates through D1. The pattern conceptually matches
one that will support both an intense laterally-translating band of
heavy precipitation east and downstream of the surface low, with a
secondary, likely more intense, pivoting deformation band to the
NW of the low. Locations that receive both of these bands, and stay
all snow, will receive the heaviest accumulations.

As far as snowfall amounts, they are likely to be quite impressive,
especially across the Central Plains and into the Ohio Valley.
Across the Plains, the lead WAA band will support snowfall rates of
1-2"/hr, and these types of bands can cause long duration of snow
in narrow corridors. While the guidance has come into much better
agreement with the placement of this band, there is still some
uncertainty as to the exact location, especially due to latitudinal
variations in the warm nose placement which will wreak havoc on
amounts due to mixing with sleet and freezing rain, as well as much
lower SLR (heavy-wet snow). As the low begins to push east, an
intense deformation axis will develop across central/eastern KS and
into NW MO. The guidance is quite aggressive with this axis, and it
appears probable that a pivoting band with convective snow rates of
2-3"/hr is likely as folded theta-e and -EPV overlap in cross-
sections suggesting upright (CI) convection potential. WPC's
snowband probability tracker using the 00z HREF confirms this
potential for 2-3"/hr snowfall rates between 12z this morning and
21z this afternoon across northern KS into MO. During this time
the column also cools significantly, DGZ depth increases, and
winds intensify, leading to blizzard and near blizzard conditions.
It is this region that will likely experience the most significant
snowfall, reflected by WPC probabilities that are high (70-80%)
for 12+ inches in N-Central KS. A broader swath of 8+ inches is
likely (70-90%) encompassing this region and spanning into northern MO.

Farther downstream, the system begins to move more quickly
across the OH VLY and into the Mid-Atlantic, which will limit the
duration of heavy snow moreso than in points west and reflected in
the lower probabilities for 8-12"+. Additionally, the weakening
upper low will cause the deformation axis to weaken resulting in
less intense snowfall rates. However, impressive rates that will
still exceed 1"/hr are expected within a WAA band from Illinois
through Ohio, and then continue across portions of the Mid-Atlantic
where some uncertainty still exists. Additionally, guidance has
become more excited about a secondary shortwave digging through the
opening longwave trough, leading to an inverted trough NW of the
departing low Monday, with secondary low development also possible
east of the DelMarVa. This will, at the very least, lengthen the
duration of light to moderate snow from OH to DE, with potentially
a secondary axis of heavy snow along this trough adding to snow
totals. At this time, despite some latitudinal uncertainty, WPC
probabilities are high (>70%) for 6+ inches of snow on D1 from
roughly Indianapolis to the higher terrain of western MD and WV.
Chances for 6+ inches are only slightly lower across the DMV,
Delmarva, and much of MD, with WPC probs at 50-70%. Locally as
much as 12" of snow is possible across the lower OH Valley and
central Appalachians, with lesser totals expected across the Mid-
Atlantic, although more uncertainty exists there.

Not to be outdone, this system will also pose a significant to
damaging ice threat within the warm nose south of the heaviest
snow. Intense WAA over-topping the cold surface layer accompanied
by persistent E/NE dry-bulb advection is a classic setup for
damaging freezing rain. Although there is some uncertainty into how
efficiently the ice can accrete at times due to what should be
intense precipitation rates, particularly from southern IL eastward
through southern IN and KY, the threat for damaging ice remains from
eastern KS through southern MO and into central KY. Here, WPC
probabilities for more than 0.25" of ice are high (70-80%) on D1,
with even some 20-40% chance for greater than 0.5" of ice in
central KY. This could result in downed limbs and power lines
leading to widespread long-lasting power outages and impossible
travel. It is worth noting that following the storm's exit on
Monday, temperatures will remain very cold across the Ohio Valley
and Central Plains with low temperatures dipping into the single
digits and below zero where snowpack exists. Farther east across
the Central Appalachians and surrounding foothills, WPC
probabilities for 0.25" of ice peak around 20-40%.

Finally, between the heavy snow and heavy icing, the forecast
profiles indicate a narrow stripe with cold layer depths greater
than 1000m, or above the 90th percentile for freezing rain in the
south-central US. This suggests sleet, especially in the presence
of a relatively weak warm nose aloft at around 750-800mb and the
aforementioned persistent dry advection at the surface. Heavy
accumulations of sleet are difficult to achieve, but in this setup
it is possible that 1-2" of sleet will accumulate causing
tremendous driving impacts. The greatest risk for the major sleet
accumulations are from near St. Louis southeast to southern IN,
but fluctuations in placement are still possible.

This system has prompted the issuance of key messages which are
linked at the bottom of the discussion.

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

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

... Continued ...

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