Gaithersburg, Maryland 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

135
FXUS61 KLWX 231939
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
239 PM EST Mon Dec 23 2024

.SYNOPSIS...
Dry and cold conditions continue through tonight as strong high
pressure sits nearby. Mixed precipitation chances increase across
portions of the area Tuesday as a weak wave of low pressure and
front pass through the region. Brief high pressure returns Wednesday
before another quick-hitting system passes through Wednesday night
into Thursday. Another wave of low pressure brings renewed chances
for precipitation this weekend.

&&

.NEAR TERM /THROUGH TUESDAY/...
Flow is largely zonal across the eastern CONUS with the exception of
a notable upper-level trough pivoting across the Upper Midwest/Great
Lakes. A cold front associated with this trough, as well as cold
high pressure anchored just off the New England Coast, will work in
tandem to bring a threat of light freezing rain/ice to parts of the
Mid-Atlantic for the Tuesday morning commute.

Clouds will increase from west to east through this evening, then
lower and thicken overnight as the cold front approaches from the
west. Meanwhile, the high pressure to the northeast will keep cold
air in place over the Mid-Atlantic at the surface. Temps will drop
into the 20s tonight into early Tuesday morning.

A swath of light precipitation along the cold front currently
approaching the OH Valley is on pace to move into the Appalachians
between 3 and 5 AM. Given some saturation between 5-10 kft AGL,
there could be some very light precip approaching western MD as
early as midnight or 1 AM, but a lack of lift in this layer during
this time as well as low-level dry air should keep much of anything
from reaching the ground and limit accumulations prior to the start
of the Winter Weather Advisory at 09Z/4 AM EST.

Further east, very low FROUDE numbers indicate a shallow inversion
height. This implies it will be very difficult for existing precip
to move east of the Allegheny Front. This results in a high
likelihood of a "shadow" area devoid of precipitation immediately in
the lee of the higher terrain and west of the I-81 corridor.
However, modest mid/upper diffluence in the right entrance region of
a jet streak, weak PVA ahead of the flattening trough passing just
to the north, and modest isentropic ascent ahead of the approaching
cold front should result in a band of light precipitation
redeveloping near the I-81 corridor near/north of I-66/US-50 between
roughly 5 and 6 AM. Depending on the strength of forcing along the
front and amount of available moisture, spotty light precipitation
may extend further south, but confidence remains low and the time
window remains short. Temperature profiles aloft support freezing
rain (ice) as the main precipitation type, with several thousand
feet of above freezing temperatures just above a shallow sub-
freezing layer right at the surface. One exception will be some
slightly deeper cold air supporting a brief period of snow at the
onset over the mountains before a change to freezing rain.

Areas along a line extending from the northwest suburbs of Baltimore
and Washington SW to the northern Shenandoah Valley in NW VA are
most likely to see precipitation developing between 5 and 7 AM,
right around the start of the Tuesday morning commute. Precip will
then most likely reach a line from NE MD southwest into the north-
central VA piedmont between 7 and 9 AM, including the immediate
DC/Baltimore areas. After this, precipitation begins to weaken as it
continues southeast due to the upper trough lifting away and the
surface cold front beginning to dissipate. By 11 AM or noon, most
precip should have either ended, pushed southeast of the area, or
changed to a very light plain rain as temps rise above freezing.

With very cold temperatures in the few days leading up to this,
surfaces like roads and sidewalks will be cold. Therefore, even
light precip should freeze/stick creating slick and icy conditions.
Even light amounts of ice can create very dangerous travel
conditions. For this reason, Winter Weather Advisories remain in
place where even just a trace (not explicitly measurable amount) of
ice is forecast.

Light precipitation, probably in the form of freezing drizzle, will
likely continue through the day Tuesday along and west of the
Allegheny Front. Therefore, advisories in those areas remain in
effect until early Tuesday evening.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
There are indications that shallow moisture and sub-freezing surface
temps will linger/overlap along and west of the Allegheny Front well
into Tuesday night. Given the shallow and fragile nature of this
setup, have opted to not extend the Winter Weather Advisory just
yet, but will monitor closely in subsequent cycles.

Dry conditions are forecast for much of the rest of the area through
Wednesday. Another weak wave approaches Wednesday night, then
crosses the area Thursday. Although lift will be modest and moisture
will not be abundant, a weak overrunning setup of warm air over cold
air could still result in at least spotty light precipitation across
northern/western MD into eastern WV and perhaps NW VA. Again, given
profiles of warm air aloft and near freezing surface temperatures,
some light wintry mix/freezing rain or ice is possible. Model
soundings indicate pockets of low/mid-level dry air, which may
ultimately limit the extent of any precip, hence the low
probabilities in the current forecast.

Dry and milder conditions are forecast from late Thursday morning
through Thursday afternoon as the disturbance pulls away.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Wedging high pressure will continue to remain over southeast Canada
Thursday night leading to the continuation of below average
temperatures across the region. The wedge tries to break Friday
as high pressure slowly retreats into eastern Canada. 12z
guidance shows a slower erosion of the wedge during the late
week period. Will continue to advertise this trend with high
temperatures Friday in the upper 30s and low to mid 40s and
lows in the mid to upper 20s and low 30s. Friday may bring some
more opportunity for a few breaks of sun depending on the
placement of the high to the north and orientation of the wind
off the water.

Beyond Friday, the weather pattern looks to become much more active
into the final few days of 2024. It`s a battle between incoming
troughing over the central U.S and a building ridge in the western
U.S. Several pieces of shortwave energy will push eastward from
the western U.S. between these two upper-level features leading
to increased precipitation chances for the upcoming weekend.

High pressure retreats further north late Friday as a wave of low
pressure and series of fronts lift north and east from the southern
Plains into the Mid-Mississippi River Valley. This wave of low
pressure and its associated front look to impact the area
Saturday bringing widespread light precipitation chances to the
region. With the aforementioned strong high pressure still
locked in to our north Saturday, cold temperatures are expected.
Highs are currently forecast in the mid to upper 40s Saturday
afternoon as the high continues to retreat. However, we could
see some cold air being locked in during the morning hours on
Saturday. If any precipitation were to begin during that
timeframe, it could be wintry in nature, especially west of the
Blue Ridge. As it stands now, most locations will see light rain
Saturday morning into Saturday afternoon.

By Sunday, the front/upper-level trough axis look to cross the
region bringing more widespread precipitation. 12z guidance
continues to show better agreement on precip timing during this
timeframe. The GFS is slightly slower with a front passing
through Saturday night into Sunday compared to the ECMWF/GEM
counterparts. What we do know is that a warmup should ensue in
response to this system as winds turn southerly ahead of it.
Rain is expected for much of the area Sunday, but the question
will be: how much (most guidance at less than 1")? This level of
detail will come in future iterations of the forecast, but for
now it is just something to watch moving forward. Temperatures
will trend at or above normal during this period.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
VFR conditions are forecast through this evening with light S/SE
flow. Winds likely go light and variable or calm for a time a few
hours on either side of daybreak as a weak cold front drops in and a
high pressure wedge persists at the surface. Despite the approach of
precipitation, overall weak forcing and low-level dry air leading
into FROPA may keep conditions VFR. Terminals most likely to see
restrictions would be BWI/MTN. Any precip that does fall will most
likely be in the form of freezing rain, which would create ice on
any surfaces rather quickly given the recent cold snap.

Winds will turn NW Tuesday afternoon into Tuesday night, then become
NE Wednesday. Shallow moisture may become trapped beneath a
strengthening subsidence inversion resulting in MVFR CIGs Tuesday
night into Wednesday morning. These should largely scatter out
Wednesday afternoon into Wednesday night as mid/high level clouds
replace the lower clouds and low-level winds shift/dry air moves in.
Some patchy light freezing precip is possible mainly near MRB
Wednesday night/Thursday AM. VFR is expected Thursday with N/NW
flow.

Sub-VFR conditions look possible at times Friday due to a CAD
wedge driven by strong high pressure north of the region.
Generally low end VFR to MVFR ceilings can be expected with
limited precipitation chances during this time. Additional sub-
VFR conditions are possible over the weekend with renewed
precipitation chances as a series of fronts cross the area.

&&

.MARINE...
Southerly winds are expected through tonight ahead of an approaching
cold front. Some modest channeling is possible over the main channel
of the Chesapeake Bay, but given the marginal conditions and low-
level stable layer, have opted to hold off on any SCAs for the time
being. Winds shift to northwesterly behind the front Tuesday, become
northeasterly Wednesday, then northerly Thursday. Other than a brief
period of light mixed precipitation Tuesday morning into early
afternoon, conditions should be dry through Thursday. Winds are
expected to stay below SCA criteria, though a brief period of
N/NE gusts of 15-20 kts are possible early Wednesday morning.

Light onshore east to northeast flow will persist into Friday
as a strong CAD wedges sits over the region. The wedge will
gradually erode Saturday, although sub-SCA level winds will
continue. No marine hazards are expected during this time. Next
chance for potential SCAs comes with a cold front late Saturday
into Sunday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Winter Weather Advisory from 5 AM to 1 PM EST Tuesday for
     DCZ001.
MD...Winter Weather Advisory from 5 AM to 1 PM EST Tuesday for
     MDZ004>006-008-011-013-014-503>508.
     Winter Weather Advisory from 4 AM to 7 PM EST Tuesday for
     MDZ001-501.
     Winter Weather Advisory from 5 AM to 10 AM EST Tuesday for
     MDZ003.
VA...Winter Weather Advisory from 5 AM to 1 PM EST Tuesday for
     VAZ053-054-501-505-506-526-527.
     Winter Weather Advisory from 5 AM to 10 AM EST Tuesday for
     VAZ028-030-031.
WV...Winter Weather Advisory from 4 AM to 7 PM EST Tuesday for
     WVZ501-503-505.
     Winter Weather Advisory from 5 AM to 10 AM EST Tuesday for
     WVZ051>053.
MARINE...Winter Weather Advisory from 5 AM to 1 PM EST Tuesday for
     ANZ530.

&&

$$

SYNOPSIS...DHOF/EST
NEAR TERM...DHOF
SHORT TERM...DHOF
LONG TERM...EST
AVIATION...DHOF/EST
MARINE...DHOF/EST