901
FXUS61 KLWX 170028
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
828 PM EDT Mon Sep 16 2024

.SYNOPSIS...
Blocking high pressure will remain south off the New England coast
while low pressure pushes into the Carolinas. Rain chances will
increase from southeast to northwest late tonight into Tuesday
as the low pressure system drifts across North and South
Carolina. Another low pressure system may develop off the coast
of North Carolina and Virginia Wednesday into Thursday providing
additional rain chances. A backdoor cold front will push
through Friday into Saturday with wedging high pressure
returning Sunday into early next week.

&&

.NEAR TERM /THROUGH TUESDAY/...
A few sprinkles were attempting to develop south of US-50/I-66
and east of I-95 early this evening as low pressure moves inland
from the Carolina Coast. The low pressure system is currently
located just inland of the NC/SC border (in the vicinity of
Wilmington, NC and North Myrtle Beach, SC) and is expected to
move further inland tonight. Up to 15+ inches of rain has
already been observed in these locations (as of 8pm) with the
northern extent of the rain shield pushing into southern VA.

Most activity outside of the Carolina Coast is scattered/light
in nature. This lighter activity will gradually lift northward
into far south-central portions of the VA Piedmont overnight.

PWAT per the 00Z IAD was 1.12 inches (compared to 0.53 inches
on the 12Z RAOB). A similar value of 1.29 inches was found on
the 00Z RNK sounding with a bit more moisture noted in the mid
to upper levels of the atmosphere since the influence of the
blocking high to the north is farther away.

Moisture will continue to build across the region overnight,
especially in areas along and south of I-66/US-50. These
locations could see a few spotty showers or areas of patchy
drizzle/mist. It`s really not until Tuesday morning into
Tuesday afternoon when more widespread rain chances move into
the region, however. Most of us will see a continuation of
cloudy skies with perhaps a little drizzle/spotty shower
activity making it as far north as central Virginia. Areas of
drizzle are likely north of I-66/US-50 by late tonight into
Tuesday morning as steadier light to moderate showers push into
central VA and the central Shenandaoh Valley. PWATS will likely
increase into the 1.0-1.5" range this evening before hopping up
into 1.25-1.75" late tonight into Tuesday. No flooding is
expected given antecedent drought concerns. Rain amounts through
Tuesday will range between a tenth to a half an inch areawide.
Heaviest totals of a half to one inch are possible across the
central Virginia Blue Ridge Mountains due in part to an easterly
upslope enhancement.

Outside of the rain will be the wind. Gusts up to 30 mph are
possible along the higher ridges of the WV/VA Highlands and
central Virginia Blue Ridge. Elsewhere, gusts of 15 to 25 mph
are possible, especially in areas along the waters.

Lows tonight will be in the low to mid 60s with a few low 50s
in the western mountains. Slightly cooler conditions are
expected Tuesday with highs in the mid to upper 60s and low 70s
west of the Blue Ridge, and mid 70s further east.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
On and off rain chances look to continue through midweek as low
pressure slowly moves north through the Carolinas into the
central Appalachians region. Model guidance continues to shift
the system further south and west of the region Tuesday into
Wednesday with uncertainty remaining in regards to the placement
of the low along the coast Thursday into late week. This is due
largely in part to a lack of a steering mechanism aloft,
blocking high pressure to the north, and lack of a front at the
surface. Most of the modeling has the low basically spinning
over the Carolinas into the central Appalachians before
meandering back toward the coast Thursday. The bulk of any
substantial rainfall looks to fall Tuesday into Wednesday.
Flooding does not appear to be a concern given the antecedent
drought concerns. With the track of the system further south,
rainfall amounts have come down substantially over the last 24-36
hours. Most locations will see a 0.25" to 0.75" of rain with 1"
to 2" rain totals across the central VA Piedmont and central
Shenandoah Valley. Some upslope enhancement is possible on the
eastern facing slopes of the central Virginia Blue Ridge where
localized 2" rain totals are possible. Some instability and bulk
shear around 25 knots associated with the system as it pivots
into and through the Carolinas could lead to a few spotty
thunderstorms Tuesday night through Wednesday afternoon, though
confidence is very low.

With added cloud cover and increased rain chances, expect
temperatures to struggle. Lows Tuesday night will fall into the
upper 50s and low to mid 60s, with highs Wednesday in the upper
60s and low to mid 70s.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
A broad upper trough remains in the vicinity of the Mid-Atlantic and
Carolinas through the end of the work week. Eventually, heights
start to build over the Great Lakes which pushes this negative
height anomaly farther south into the weekend. At the same time,
a large positively-tilted trough works its way across far
eastern Canada before departing into the Atlantic. While the
core of height falls remains well to the north, there is enough
southward push to help draw a backdoor cold front through the
local area. Based on the latest guidance, this frontal system
slips through the region late Friday. In the wake, high pressure
returns and settles over the entire Eastern Seaboard over the
weekend. By early next week, one notable feature to pay
attention to is an inverted trough arcing along the Carolina to
Delmarva coast. This could re-introduce some shower chances by
Monday.

In terms of sensible weather, the presence of the upper low will
maintain scattered showers over the area on Thursday and Friday.
Unlike earlier in the week, there should be some breaks in the
clouds at times. Daily high temperatures stay fairly close to mid-
September normals, generally in the upper 70s to perhaps low 80s.
With winds eventually shifting to northerly, overnight lows are able
to cool back down into the mid 50s to low 60s. The pattern turns
drier over the weekend given the mentioned shift to high pressure.
Toward the conclusion of the weekend, highs drop back into the low
70s with 60s across the mountains. Winds will mainly be out of the
northeast with afternoon gusts to around 15 to 20 mph. These winds
could be slightly stronger depending on how tight the gradient is
with high pressure over New England and low pressure off the
Carolina coast.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
MVFR to IFR CIGs will likely return tonight as low pressure
pushes further into the Carolinas. Drizzle and rain shower
activity will move into terminals south of the corridor late
tonight before spreading north Tuesday morning into Tuesday
afternoon. IFR ceilings are likely during this time with the
bulk of the rain falling late Tuesday morning into Tuesday
night. The best chance of IFR CIGs is 12Z-18Z Tuesday, then
again Tuesday night (though some guidance has IFR CIGs
developing as early as 04Z-06Z from SE to NW).

Rain showers will likely continue on and off through Wednesday
as low pressure meanders nearby. Winds will remain E/NE 5-15 kts
through Wednesday. Gusts of 15-25 kts are possible mainly each
afternoon and evening, especially at terminals close to the
waters and along the ridges. With the residual low pressure
nearby, expect periods of sub-VFR conditions.

Underneath a persistent upper low, scattered showers may fire up
across the area on Thursday into Friday. However, there should be
plenty of dry time during the period. Clouds should be abundant
which may afford periods of sub-VFR conditions. Winds will generally
be north to northwesterly through Friday before shifting to
northeasterly on Saturday as a backdoor front exits to the south.

&&

.MARINE...
SCA conditions will continue to increase this evening into the
overnight hours especially over the open waters of the Chesapeake
Bay/tidal Potomac River. This is in association with a tightened
pressure gradient from blocking high pressure to the north and
low pressure moving inland across the coastal Carolinas. At
this time, the risk of gale force winds appears low, though some
gusts around 30 knots have been observed over the mid bay. As a
weakening area of low pressure tracks through the area, winds
will be out of the east on Wednesday while likely remaining at
SCA levels for at least portions of the waters.

Expect mainly northerly winds through Friday with wind gusts
most likely staying below Small Craft Advisory levels. A
backdoor cold front pushes through late Friday into Saturday
morning. In the wake, an increase in northeasterly winds may
lead to 20 to 25 knot gusts on Saturday.

&&

.TIDES/COASTAL FLOODING...
Onshore flow strengthens through Tuesday morning with minor
coastal flooding becoming increasingly likely. However, surge
guidance has been trending downward, perhaps due to the low
remaining farther south and winds a touch weaker and more from a
northeasterly direction. Advisories remain in effect but may
need adjustments. Moderate flooding is much less likely now.
However, between the full moon and continued onshore flow, tide
levels will likely remain elevated and near minor flood
thresholds much of the week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 10 PM EDT Tuesday for DCZ001.
MD...Small Craft Advisory until 6 AM EDT Wednesday for MDZ008.
     Coastal Flood Advisory from 2 AM to 10 PM EDT Tuesday for
     MDZ014.
     Coastal Flood Advisory from midnight tonight to 6 PM EDT
     Tuesday for MDZ016-018.
VA...Coastal Flood Advisory from midnight tonight to 6 PM EDT
     Tuesday for VAZ057.
WV...None.
MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ530>534-
     536>543.
     Small Craft Advisory until 6 PM EDT Tuesday for ANZ535.

&&

$$

SYNOPSIS...DHOF/EST
NEAR TERM...DHOF/EST
SHORT TERM...DHOF/EST
LONG TERM...BRO
AVIATION...BRO/DHOF/EST
MARINE...BRO/DHOF/EST
TIDES/COASTAL FLOODING...LWX