Very good AFD from LWX:
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
337 PM EST Sun Jan 5 2025
.SYNOPSIS...
A strong low pressure system will slide across the region tonight
into Monday, producing widespread wintry precipitation across the
region. Below normal temperatures continue through the coming week
as strong arctic high pressure builds towards the region. Another
area of low pressure may impact the region next weekend.
&&
.NEAR TERM /THROUGH MONDAY NIGHT/...
A potent storm system will approach the region from the west
over the next 24 hours. As of late this afternoon, a closed
upper low is present on water vapor imagery over KS/OK/MO, with
an associated surface low located just downstream over Arkansas.
This system will gradually progress eastward toward the Mid-
Mississippi Valley this evening, through the Ohio Valley
tonight into Monday, and then overhead Monday night.
Downstream of the upper low, water vapor imagery shows a fetch
of moisture being drawn northward out of the Gulf of Mexico. A
broad zone of warm advection/frontogenetic forcing in the
850-700 hPa layer extends eastward downstream of the low,
producing precipitation all the way into SW VA and NC. As this
zone of low-level warm advection/frontogenetic forcing spreads
further north and east, precipitation will break out across the
area this evening into the overnight hours. Snow is ongoing
across southwestern Virginia at the moment. Onset times of
snow are expected to be around 6-9 PM in the Alleghenies,
central Shenandoah Valley, and central Virginia, 9PM-midnight in
the northern Shenandoah Valley, DC/Baltimore Metros, and
southern Maryland, and around midnight in far northeast
Maryland.
The snow should pick up in intensity fairly quickly, and
continue steadily through around mid-morning tomorrow. As the
zone of strongest warm advection/frontogenetic forcing
translates across the region from west to east late tonight
into early tomorrow morning, there will be a window of 3-5
hours in any given location where snowfall rates may reach 1-2
inches per hour. This zone of heavier snowfall rates will reach
the Alleghenies shortly after midnight, the I-81 corridor by
around 3 AM, and then the I-95 corridor around 4-5 AM. This
period of heavier snow will overlap directly with the morning
commute hours in DC and Baltimore, before dropping southeastward
across southern Maryland during the late morning hours, and
then exiting off to the east during the afternoon. There will be
a bit of a lull in the precipitation during the late
morning/early afternoon hours, when just intermittent light
snow showers are expected. It`s also possible that some
locations from the I-66 corridor southward could lose ice
crystals for a brief time as a warm nose works in aloft. This
could potentially result in a little patchy freezing drizzle
during that lull, primarily from central Virginia to southern
Maryland. As we move into the late afternoon and evening hours,
a second round of steadier snowfall is expected across the area
during the late afternoon and early evening hours as the
system`s upper level low passes overhead. Precipitation will
then exit the area to the east by around midnight. The exception
will be along the Allegheny Front, where light snow showers may
linger in upslope flow through the remainder of the night.
Over the past 12 hours, model guidance has come into better
agreement with respect to both QPF amounts, and precipitation
types. The vast majority of the forecast area is now expected to
see all snow. The exception will be across central Virginia and
the central Shenandoah Valley, especially from I-64 southward,
where sleet may mix in. A broad 6-12 inches of snow is expected
across nearly all of the forecast area, with localized amounts
in excess of 12 inches possible where heavier banding lingers
longer. There are two potential exceptions to this, each of
which has a bit of forecast uncertainty associated with it. The
first is across central Virginia and the central Shenandoah
Valley (especially to the south of Charlottesville, Waynesboro,
and Staunton), where sleet may cut back on totals. The second
potential exception is across far northern Maryland from near
Hagerstown eastward to Cecil County, where QPF amounts may be a
bit lower. Depending on which model source verifies, amounts
there may be a bit above or below 6 inches.
The bulk of the snow should fall during the first round of
snowfall tonight through the mid-morning hours tomorrow. Snow
character will be an average snowfall (not overly wet and heavy,
or light and fluffy). The second round of snowfall tomorrow
evening should feature an additional 1-3 inches of snow across
the area. This snow should be very light and fluffy in nature.
With less mixing expected compared to earlier forecasts, much
less in the way of icing is expected, with ice accumulations
(generally around a tenth of an inch or less) confined mainly
to central Virginia, although a light glaze may also be possible
across the Central Shenandoah Valley or southern Maryland. For
more details on the storm, see www.weather.gov/lwx/winter.
