Morning discussion from LWX
KEY MESSAGE 2...Near-record cold this weekend, along with increasing
potential for a significant winter storm.
As mentioned above, a potent cold front pushes through the
region late Thursday into Friday, followed by a reinforcing
Arctic front late Friday into Saturday. This will lead to
increasingly cold conditions each day from Thursday through the
weekend. Well below average temperatures are forecast,
especially by Sunday. In addition, some upslope snow showers are
possible over the Alleghenies, with low but non-zero chances of
light precipitation further east on Friday. Extremely cold
temperatures are becoming likely Saturday night into next week,
with lows in the single digits and wind chills at or below zero.
Near-record low temperatures are possible during this time.
The upper-level pattern becomes quite active in both the northern
and southern branches of the jet stream heading into this
weekend. Models continue to be in generally good agreement
regarding the development of a low pressure system that tracks
across the southern U.S. A broad area of baroclinic/frontogenetic
forcing will develop along the interaction of an Arctic high to
the north and the deepening low. Ensemble guidance continues to
show an amplified southern stream influence - something we have
seen very little of so far this winter (typical of La Nina).
The surface low may not escape so easily out to sea given the
negative NAO block. The nearby steady source of very cold air
and the amplified southern stream occurring in tandem are what
raise some red flags for significant wintry precipitation
potential this weekend. Ample cold air will remain aloft and at
the surface with strong high pressure to the north and the
strong baroclinic zone to the south. As a result, the 01Z NBM
48-hour probability for at least 6 inches of snow (ending 12Z
Mon Jan 26) are at 40-70pct from NW to SE, and around 20-45pct
for 1 foot of snow from NW to SE. There are some noteworthy
probabilities for even higher amounts, as well, but uncertainty
still remains in regards to the exact track of the system which
could lead to a sharp QPF gradient on the north/northwest side
of the system possibly somewhere over our area.
The latest numerical model guidance continues to show
consistency in regards to the potential of this significant
weekend winter storm across the area. Trends for precipitation
onset time favor Saturday (most likely in the afternoon), with
the heaviest precipitation over the area then occurring through
Sunday afternoon. The last four runs of the ECMWF-AIFS
(deterministic) have shown a southward tick to the axis of
heaviest QPF, likely due in large part to a slower ejection of
shortwave energy out of the Southwest CONUS causing a slightly
flatter and more suppressed storm track. However, the 00Z
operational ECMWF (deterministic) as well as several members of
the EPS and EPS-AIFS (among a handful of members from other
ensembling systems) are also slower and more amplified with a
trailing northern stream wave dipping into the Rockies late in
the weekend; these two slower/deeper waves then phase as they
exit the Rockies and head for the Mississippi Valley. This
scenario results in a broad, flat area of low pressure the gets
caught underneath Arctic high pressure building over the
Northeast CONUS Saturday night, and drifts over the baroclinic
zone draped from the SW Atlantic into the Gulf. This low then
eventually gets absorbed into the deeper phasing trough
approaching from the west. This lifts the low pressure northward
up the coast causing precipitation to linger into Monday. It
should be noted that this is a relatively new shift, not yet
reflected in 01Z NBM progs that were used as a baseline to the
overnight forecast. Have not deviated from NBM and will instead
wait to see if there is some consistency in this outcome, or if
a more suppressed solution becomes more likely.
Depending on where the frontal zone is and where the area of low
pressure tracks, there could be significant wintry precipitation
this weekend over a large part of the South to Mid-Atlantic CONUS.
As noted, the potential is still there for that strong Arctic
high pressure to suppress the low pressure system to our south.
However, the strengthening upstream blocking pattern could
essentially put a floor on how far south the low will track.
Accumulating snow looks almost certain this weekend, with an
increasing likelihood of a warning-level event for much of the
region. But how exactly all of the moving pieces interact will
determine the difference between a moderately impactful snowfall
versus something much more significant.
It is important to monitor this threat closely this week as the
different parts of the system become better sampled by surface and
upper air observations. Stay tuned to weather.gov/lwx/winter
for the latest updates.
