Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 240124
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
924 PM EDT Mon Apr 23 2018

.SYNOPSIS...
A low pressure system will bring unsettled weather to the
region tonight through Wednesday. A brief ridge of high
pressure will build in Thursday, ahead of the next low pressure
system that may affect the region Friday into Saturday.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
A closed low will slowly move eastward from the Tennessee Valley
tonight. South to southeast flow will be on the increase
through the night along with increasing moisture and warm air
advection.

Clouds will continue to thicken and lower tonight. The
rainfall, however, will be slow to arrive. Incorporating a time-
lagged ensemble approach continues to delay the arrival. Not
only are dewpoint depressions still 20 degrees or greater, but
there is a significant dry layer noted from the 00Z IAD RAOB
between 700-500 mb to overcome.

Overnight tonight rain probabilities will continue to increase
from southwest to northeast. Rain should be falling by sunrise
from about Winchester-Fredericksburg south/west. Further
northeast, locations will likely remain dry through the night.
Do have a buffering band of chance PoPs that includes
Hagerstown/Martinsburg-metro DC-southern Maryland. Lows tonight
will settle mainly in the mid/upper 40s to around 50F.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/...
The area of low pressure will continue its slow eastward trek
through the day Tuesday, reaching western North Carolina by late
in the day. A track northeastward to the Virginia Tidewater
region is then forecast for Tuesday night, and into the
Philadelphia/New York City area by late Wednesday.

Periods of rain are expected during the day Tuesday and into
Tuesday night, tapering off to showers on Wednesday. While
widespread hydrological concerns are not currently anticipated,
with precipitable water values topping off around 1-1.25", there
will likely be some areas of locally heavy rain. Total rainfall
amounts are expected to range from 1-2" over a 24-36 hour
period. With low level southeast flow into the higher elevations
of the Blue Ridge and nearby highlands, some localized amounts
of up to 3" are possible. 6-hour flash flood guidance ranges
from generally 1-3", so any flash flood threat is quite low,
however if some locations do manage to see 3", there may be some
minor hydrologic issues. WPC has outlined the region in a
Marginal Risk for flooding, and this continues to look
reasonable.

High temperatures on Tuesday will range from the 50s to near
60F, with lows Tuesday night in the upper 40s and low 50s. With
precipitation of a more showery nature on Wednesday,
temperatures will likely rise a few more degrees, with highs in
the 60s area-wide.

Drying will take place Wednesday night as the system pulls
northeastward. Lows in the 40s to around 50F.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Following the passage of the low pressure system from Tuesday and
Wednesday, a weak high pressure moves into the region, bringing a
period of clear weather on Thursday. With little evidence of cold
advection from this system, temperatures can be expected to remain
around average, with values in the upper 60`s possible.

On Friday, the forecast is a lot less certain at this time. Since
this time yesterday, there has been a bit of a shift in model
guidance, leaning towards the possibility of a weak coastal low
impacting our area, bringing a chance for some light rain. However,
the GFS is the only model really showing this at this point. Another
point to note is that only a few of the GEFS ensemble members show
any rain from a system on Friday. The European and Canadian both try
to develop a low across the southeast late Thursday/early Friday,
but neither bring this up into our area. For now, keeping the chance
for precipitation on Friday for the entire area due to the high
degree of uncertainty at this point in the forecast. Temperatures on
Friday look to remain near to slightly below average, and could vary
greatly depending on the evolution of this potential system.

On Saturday a weak cold front, associated with a low pressure
system to our north, will pass through the area sometime during the
morning. This front doesn`t look to bring a large chance for
precipitation, but a few showers can not be ruled out.
Temperatures don`t look to be affected to much either, and look
to remain pretty consistent with averages for this time of
year.

High pressure builds in on Sunday, which should keep things dry,
with temperatures slightly warmer than on Saturday.

&&

.AVIATION /02Z TUESDAY THROUGH SATURDAY/...
VFR conditions expected through tonight with a gradually
thickening and lowering cloud deck. MVFR conditions then become
possible at CHO by 1100 UTC Tuesday morning as rain and lower
ceilings approach from the southwest. MVFR is then expected to
spread across the remainder of the region during the morning and
into the early afternoon Tuesday, with IFR becoming likely by
later in the day as well. IFR in rain and low ceilings will
likely continue Tuesday night. Clouds and showers will persist
for much of the day on Wednesday with potential for sub- VFR
conditions continuing.

Expect VFR conditions for much of Thursday, and early Friday, as we
enter a relatively calm period as high pressure builds into the
area. Latest guidance still suggests a chance for some impact from a
weak coastal low on Friday, bringing about the chance for some low
clouds in the area. Sub-VFR conditions are possible during this
timeframe.

&&

.MARINE...
Winds have eased under Small Craft Advisory conditions. Do not
believe gusty winds will return until Tuesday, as low pressure
approaches the waters. Have therefore cancelled the Advisory for
tonight, and implemented a staggered return on Tuesday.

The strongest period of winds is expected Tuesday afternoon and
evening. Have reoriented the SCA to have a staggered end as
well, first on the mid-upper Tidal Potomac Tuesday evening,
then for the rest of the waters predawn Wednesday. Another
period of SCA- criteria winds will likely develop Wednesday
afternoon and Wednesday night as the system departs.

An exiting low pressure system early on Thursday could bring gusty
winds to the area, so some SCA criteria winds are possible during
this time. Friday is highly variable dependent on the development of
the potential coastal low on Friday.

&&

.HYDROLOGY...
Widespread rainfall amounts from 1 to 2 inches are forecast from
tonight through Wednesday, with locally higher amounts likely
along the central Blue Ridge and adjacent foothills to the east.
Due to relatively dry antecedent conditions and extended time
period over which the rain falls, widespread hydrological
concerns are not anticipated. However, with increasing southeast
flow, localized amounts of up to 3" are possible, again mainly
near the higher elevations of the Blue Ridge and near-by
highlands which could lead to some minor hydrology issues.

&&

.TIDES/COASTAL FLOODING...
As low pressure approaches from the southeastern U.S through
Tuesday night, a persistent onshore flow will develop. This
will yield increasing tidal anomalies, and the possibility of
coastal flooding toward Tuesday and Wednesday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 11 AM Tuesday to 4 AM EDT Wednesday
     for ANZ530-531-538>540-542.
     Small Craft Advisory from 7 AM Tuesday to 4 AM EDT Wednesday
     for ANZ532>534-537-541-543.
     Small Craft Advisory from 11 AM to 10 PM EDT Tuesday for
     ANZ535-536.

&&

$$
SYNOPSIS...MM
NEAR TERM...MM/HTS
SHORT TERM...MM
LONG TERM...CJL
AVIATION...MM/HTS/CJL
MARINE...HTS/MM/CJL
HYDROLOGY...MM/HTS
TIDES/COASTAL FLOODING...MM



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