Morning discussion from LWX... note the bold and underlined...
DISCUSSION...
KEY MESSAGE 1...Confidence of a significant winter storm across
the region remains high Saturday into Sunday, but confidence in
exact amounts of snow versus ice is lower especially south and
east.
Ingredients are coming together to produce a significant winter
storm across the region this weekend. Based on the latest
guidance, wintry precipitation (snow) may arrive as early as
Saturday morning for west-central VA before spreading east and
northeast slowly through the day. Snow then continues Saturday
night into Sunday morning, then could mix with sleet and
freezing rain for some areas Sunday into Sunday night. While
deterministic guidance continue to show some run-to-run
variability, the overall trend has been bringing the higher
QPF amounts toward central Virginia, with a potential for mix
wintry precipitation Sunday afternoon and night. In a multi-
day sense, global ensemble probabilities remain very high for
seeing 6+ inches of snow. This comes with the presumed snow-
to-liquid ratio of 10:1 which this system will not be given the
Arctic nature of the air mass. Ratios could be considerably
higher yielding a fluffier snow. With the parent features still
across the more barren high latitudes, models may take another
day or two to resolve such features with better precision (note:
recon flights are expected to take place near the Southwest
CONUS later today - that`s where models have been most sensitive
to changes, so hopefully this additional data will help hone in
on details). The details will be important in determining any
precipitation type transitions. However, given the very cold air
locked in at and near the surface with strong Arctic high
pressure anchored firmly to our north, plain rain is highly
unlikely anywhere in the region. It will therefore come down to
snow versus ice, both of which could be significant and highly
impactful across a widespread area.
On the synoptic pattern side of things, many ingredients are in
place to produce the mentioned winter storm. On Saturday, Arctic
high pressure (1045-1050 mb) across the Midwest eventually
reaches New England by the evening as a still robust 1040 mb
anticyclone. This yields the familiar cold air damming wedge
setup which stretches down into the southeastern U.S.
Looking aloft, a split jet structure is evident with deep cyclonic
flow in the northern stream and a progressive feature ejecting out
of the Four Corners (this is the feature that introduces the
most uncertainty/model variability). The downstream confluent
flow will aid in ample forcing in the form of overrunning across
vast portions of the central/eastern U.S. While the main
surface low sweeps across the Gulf Coast states, a broad area of
isentropically driven precipitation spreads over the frigid air
mass to the north. Based on the latest guidance and trends,
precipitation type issues could arise Sunday/Sunday night. An
energy transfer eventually unfolds Sunday afternoon/evening
which focuses to deepening low pressure off the Outer Banks
of NC. Eventually, this features pulls away from the coast late
Sunday night which gradually brings the brunt of the
precipitation to an end, though some light precipitation
(perhaps freezing drizzle east/upslope snow showers west) could
linger into Monday morning.
With the system not arriving until Saturday evening sometime, now is
the time to prepare for this winter storm. Check back at the office
website (weather.gov/lwx) as well as the winter page as the system
moves into the Day 1-3 period (weather.gov/lwx/winter).
