207
FXUS61 KLWX 300759
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
359 AM EDT Mon Sep 30 2024

.SYNOPSIS...
An upper low will slowly track toward the area through Tuesday
before exiting offshore. Daily rain shower chances accompany
this system with some flooding potential from central Virginia
westward. A cold front moves through Wednesday morning before
high pressure returns during the middle to latter portions of
the work week. Another cold front pushes through late Friday with
high pressure settling over the northeastern U.S. this weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Currently, an expansive upper low is centered over eastern
Kentucky and southern West Virginia. This particular system
contains the remnants of what was Hurricane Helene a few days
ago. Some residual tropical moisture has lingered with current
precipitable water values around 1.50 to 1.75 inches. This is
easily nearing 2 standard deviations above average. Aiding in
these enhanced moisture levels is the semi-persistent easterly
onshore flow. The combination of favorable dynamical lift from
the slow moving trough and tropical moisture will aid in
additional bouts of heavy rainfall today.

As of 315 AM EDT, a light to moderate band of rain showers
continues to fall over the central Virginia Blue Ridge down into
Albemarle and Orange counties. The activity has decreased in
intensity over the last few hours with the more focused action
near the Richmond metro area. However, given earlier heavy
rainfall, flooding does remain a concern, particularly over
Nelson and Albemarle counties. These locations have seen earlier
flash flooding reports along with rises on a number of the
stream gauges. The current Flood Watch is in effect until 7 AM
this morning.

Elsewhere, a mixture of drizzle to light shower activity
continues to spread northward into northern Virginia. The weak
nature of these echoes have largely kept 3-hour totals at 0.05
inches or below. This is all underneath a very thick stratus
deck with current cloud bases around 300 to 500 feet. These
should stick around much of the morning given the prolonged
onshore flow.

For the remainder of the day, high-resolution models favor
another round of locally heavy rainfall across central Virginia
westward across the I-81 corridor into the Potomac Highlands.
Recent solutions bring another 2 to 4 inches to this area of
interest. This may require another Flood Watch given the
increasing threat for some flash flooding. Outside of this area,
amounts should be much lighter (generally 0.10 to 0.50 inches).
Area-wide high temperatures are likely confined to the 60s
although a few low 70s are possible along and east of I-95.

A bulk of the heaviest rainfall should wind down after midnight.
Residual showers will persist through the overnight period which
will add to the storm totals. Skies remain overcast again given
the persistent east to northeasterly flow. Low temperatures will
not fall a whole lot with readings in the upper 50s to low 60s.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
The upper low is expected to move directly over the local area
on Tuesday. Its overall appearance looks more strung out as the
system does begin to shear out some. As a result, the forcing
seems more disorganized and ragged in nature. On the other hand,
the air mass in place will have changed little with precipitable
water values still in the 1.50 to 1.75 inch range. Some locally
heavy rainfall signature could easily materialize out of this
setup. Will need to continue monitoring subsequent model
guidance packages to discern this particular threat. A cool
northeasterly wind off the Atlantic maintains plenty of cloud
cover on Tuesday. Forecast high temperatures likely do not
escape the 60s which is around 4 to 8 degrees below average.

Heading into Tuesday night, a bulk of the shower chances focus
west of the Blue Ridge Mountains. This is mainly driven by an
approaching cold front. A fairly uniform air mass across the
Mid-Atlantic keeps lows in the upper 50s to low 60s.

Some light shower activity may accompany the cold front on
Wednesday morning. However, east of the Blue Ridge, moisture
scours out which should keep those locations mainly dry. Behind
the cold front, there will finally be a low-level wind shift
that ends the longstanding onshore flow. The net north-
northwesterly flow should even bring some late day sunshine to
the region. Forecast highs range from the upper 60s to mid 70s,
locally into the low/mid 60s across the mountains. Skies stay
fairly clear into the night with lows ranging from the mid/upper
40s along the Alleghenies to 50s elsewhere.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
The longwave pattern across the continent features general
ridging across the Desert Southwest and southeastern U.S. Off to
the north, a very broad trough extends across a good chunk of
the northern tier of the country. While the core of lower
heights are focused across Nunavut in far north-central Canada,
multiple dips in the jet stream will allow for some notable
height falls.

As the slow moving upper low from earlier in the week tracks
further offshore, the southeastern U.S. ridge briefly pulses
northward on Thursday. Eventually the influence of the upstream
longwave trough begins to near the region late in the work week.
In response, surface high pressure initially in place will
weaken as a cold front tracks through late Friday. Unlike the
mid-week cold front, this system should usher in a much cooler
and drier air mass. Recent guidance shows 24-hour 1000-500 mb
thickness changes on the order of 6 to 9 dm. Ultimately this
lowers high temperatures into the upper 60s to low 70s which is
right around early October climatology. This helps usher low
temperatures into the 40s on Saturday and Sunday nights. At this
point, temperatures should not be cold enough to support any
frost concerns.

High pressure holds strong through the weekend before the next
system tracks toward the Ohio Valley early next week. Until
then, expect a dry forecast through the period.

&&

.AVIATION /07Z MONDAY THROUGH FRIDAY/...
Persistent northeasterly onshore flow has maintained an LIFR to
IFR cloud deck over the area terminals. Although some subtle
improvements are possible to MVFR across the Baltimore
terminals, most locations are socked in the IFR ceilings through
the day. The best chance for heavier showers will be across KCHO
where flash flooding also remains a concern.

Tuesday should offer minimal improvements as lengthy
restrictions persist. Some breaks are again possible at the
Baltimore terminals, but confidence is low at this time. Any
true improvements do not arrive until Wednesday afternoon as a
cold front tracks through. This finally brings a wind shift to
north-northwesterly.

VFR conditions are expected Thursday and Friday underneath
mostly sunny skies. Winds are fairly light on Thursday which may
be accompanied by a series of wind shifts as high pressure
pushes through. By Friday, a more pronounced southerly wind is
expected ahead of the next frontal system.

&&

.MARINE...
A breezy northeasterly wind will maintain near 20 knot gusts
through tonight across the southern half of the Chesapeake Bay
as well as the lower tidal Potomac. As a result, Small Craft
Advisories are in place for these aras. Little change is
expected in the surface pattern which may warrant additional
marine products. Northeasterlies may gust to around 20 knots at
times through Tuesday evening before gradients begin to weaken.
A cold front pushes through on Wednesday afternoon although
winds are not very strong with this system.

High pressure returns into Thursday and Friday. A shift to
southerlies is expected the second half of Friday as the next
cold front approaches from the west.

&&

.HYDROLOGY...
A Flood Watch remains in place until 7 AM this morning.
Decreasing rainfall rates may allow for an earlier expiration
though. However, another round of moderate to heavy rain showers
are noted in the high-resolution guidance. The main signal seems
to focus over central Virginia westward across I-81 into the
Potomac Highlands. Additional Flood Watches may be needed from
late this morning through the evening hours. Given previous
bouts of heavy rainfall and swollen rivers/streams, flash flood
guidance values may lower further.

&&

.TIDES/COASTAL FLOODING...
Onshore flow will persist through mid-week, keeping water
levels elevated. In addition, astronomical tides will be getting
higher each day through Oct 02 when the Moon will be in its new
phase. So, prolonged period of minor coastal flooding is
expected. A cold front does push through the area on Wednesday
afternoon which should help lower water levels some in its wake.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 8 AM EDT Wednesday for DCZ001.
MD...Coastal Flood Advisory until 8 AM EDT Wednesday for MDZ014.
     Coastal Flood Advisory until 6 AM EDT Wednesday for MDZ016-018.
     Coastal Flood Advisory until 5 AM EDT Wednesday for MDZ017.
VA...Coastal Flood Advisory until 6 AM EDT Wednesday for VAZ057.
     Flood Watch until 7 AM EDT this morning for VAZ025>027-029-030-
     036>040-050-051-503-504-507-508.
WV...Flood Watch until 7 AM EDT this morning for WVZ055-501-502-505-
     506.
MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ532>534-537-
     540-541-543.

&&

$$

SYNOPSIS...BRO
NEAR TERM...BRO
SHORT TERM...BRO
LONG TERM...BRO
AVIATION...BRO
MARINE...BRO
HYDROLOGY...BRO
TIDES/COASTAL FLOODING...BRO