222
FXUS61 KLWX 200805
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
405 AM EDT Fri Sep 20 2024

.SYNOPSIS...
Low pressure will continue to drift off the Mid-Atlantic coast
through tonight with high pressure building north of the
region. A frontal system will move into the area Saturday
afternoon before stalling through Sunday. A strong front and
area of low pressure over the Great Lakes and Upper Midwest will
approach the area during the early or middle part of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Low pressure remains southeast of Cape Cod this morning with
high pressure building into the area from the north. With mainly
clear skies and light winds, fog is presenting the primary issue
of the morning. Satellite and web cams suggest thicker fog is
confined along the valleys, especially west of the Blue Ridge.
Some dense fog has been observed at various airports at times,
so will be monitoring for a potential Dense Fog Advisory should
it become more widespread.

After the fog dissipates, it will be a warm day with few to
scattered clouds. Highs in the lower to mid 80s are about 5 to 8
degrees above normal. Recent guidance has backed off a bit on
the shower chance this afternoon, mainly along the immediate
Allegheny crest where there will be some weak convergence.

Tonight remains dry. The best signal for fog is across southern
parts of the forecast area. Some guidance is also indicating
potential for low clouds to develop east of the Blue Ridge. Lows
will be in the upper 50s to mid 60s.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
This is my seventh night shift, and I feel like this amplified
upper level pattern has changed this weekend`s forecast a little
bit each day. For Saturday, the low remains off the coast and
high pressure remains to the north. However, now there appears
to be a bit more defined low and frontal pattern that approaches
from the Great Lakes combined with a shortwave in northwest flow
aloft. The warm sector will feature highs in the lower to mid
80s and dew points in the lower to mid 60s. This will aid in
developing some instability, so showers and thunderstorms should
develop with the approaching forcing. Most models suggest decent
coverage (40-60 percent) starting midday west of the Blue Ridge
and then spreading east during the afternoon and evening. While
instability will be somewhat modest, shear of 30-40 kt is
forecast by some guidance. This could result in some organized
storms that could produce hail and localized wind damage. Right
now the greatest chance for severe weather appears to be west of
I-95, where storms are most likely during peak heating (along
with slightly higher instability). While storms should weaken
some as they push east, there does appear to be enough forcing
for showers and isolated storms to last well into the night.

At some point, the frontal system overhead gets overtaken by the
surging high pressure to the northeast...what we`ve been
describing as a backdoor cold front. The conglomerate boundary
now appears to stall across the area Sunday, with cloudy, cool,
and drizzly conditions to its northeast, and more sun and
temperatures closer to 80 to it southwest. The wedge of high
pressure pushes further south Sunday night, so cloud cover may
overspread most of the area. Some drizzle is possible, and a few
showers may approach the Appalachians ahead of the next low
pressure system in the Great Lakes.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
High pressure will build over the region on Monday before
sliding offshore on Tuesday. There is a chance for a few showers
over the Appalachians on Monday as weak energy moves up and
over a ridge of high pressure. Temperatures will remain cool.

Once the high moves offshore Tuesday, a shortwave trough of low
pressure will move across the eastern Great Lakes and help to lessen
the strength of the high. This will enable or increase the chance
for showers and a thunderstorm Tuesday into Wednesday, not just in
the Appalachians but also east through the Shenandoah Valley and
perhaps the metro areas. Temperatures each day will remain cooler
than average.

A more significant cold front could approach late Wednesday and
bring an enhanced threat for showers or a thunderstorm into
Thursday. Timing and intensity of this stronger front is uncertain.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Fog will be the main issue for CHO and MRB through sunrise,
becoming dense at times. Expect improving conditions by 13-14Z.
IAD could have a brief period of BR, but not expecting
restrictions at the other terminals. For the remainder of the
day, expect VFR conditions and light NE winds becoming SE by
tonight. There`s a little more uncertainty in fog development
for tonight, but would most likely be at MRB at CHO again.
However, some guidance indicates low clouds could form east of
the Blue Ridge. Will see if refinement can be made in this for
the 12Z TAFs.

A front will approach Saturday with showers and thunderstorms
possible during the afternoon and evening. This boundary will
stall near the area through Sunday. Sub-VFR ceilings and
occasional light rain or drizzle may continue at times Sunday
and Sunday night, although confidence is low.

VFR conditions Monday through Tuesday night. Winds predominantly
southeast 5 to 10 knots gusts up to 15 knots each period.

&&

.MARINE...
Upper level low pressure will remain off the coast through the
weekend as high pressure builds north of the region. Light
northerly winds turn toward the east and southeast through
tonight and Saturday. A frontal system will approach Saturday,
with thunderstorms potentially reaching the waters during the
late afternoon or evening. The front will stall near the area
Sunday before high pressure surges from the northeast Sunday
night and turns winds to the northeast. There is potential for
marginal SCA conditions at times late Saturday through Sunday
night, although in general computer guidance has backed off on
stronger winds.

No marine hazards expected Monday through Tuesday night. Winds
southeast around 10 knots with higher gusts each afternoon.

&&

.TIDES/COASTAL FLOODING...
Anomalies are above one foot across much of the waters this
morning, which has been resulting in minor flooding in several
locations. Water levels are forecast to rise further into the
weekend as offshore low pressure traps water in the Chesapeake.
Combined with high astronomical tide levels, additional, and
more widespread, coastal flooding can be expected. Advisories
will likely need to be expanded to northern parts of the bay by
tonight. A Coastal Flood Watch remains in effect for Anne
Arundel County Saturday evening. However, there could be
additional bouts of moderate flooding thereafter, and DC SW
Waterfront may also near moderate flood stage. Unfortunately
this looks like a long duration coastal flood event as an
onshore wind component persists well into next week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 2 AM EDT Sunday for DCZ001.
MD...Coastal Flood Advisory until 6 PM EDT Saturday for MDZ014.
     Coastal Flood Watch from Saturday evening through late
     Saturday night for MDZ014.
     Coastal Flood Advisory until 8 AM EDT Sunday for MDZ016.
     Coastal Flood Advisory until 11 AM EDT Sunday for MDZ017-018.
VA...Coastal Flood Advisory until 2 AM EDT Sunday for VAZ054.
     Coastal Flood Advisory until 8 AM EDT Sunday for VAZ057.
WV...None.
MARINE...None.

&&

$$

SYNOPSIS...ADS
NEAR TERM...ADS
SHORT TERM...ADS
LONG TERM...KLW
AVIATION...ADS/KLW
MARINE...ADS/KLW
TIDES/COASTAL FLOODING...ADS