-- Discussion containing changed information from previous version are highlighted. --
171
FXUS61 KLWX 180756
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
356 AM EDT Sat May 18 2024

.SYNOPSIS...
-- Changed Discussion --
Low pressure will drift off the Carolina Coast through the weekend.
High pressure will build overhead Monday into Tuesday. A cold front
will approach from the Midwest Wednesday, then cross by Friday.
-- End Changed Discussion --

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Surface observations across the Mid-Atlantic region show light
east to southeasterly flow with early morning temperatures
mainly in the upper 50s to low 60s. To the south, a frontal zone
sits along the North Carolina/South Carolina border, while
arcing back into the Great Smoky Mountains. The 06Z RAP
objective analysis shows a well established 925-850 mb theta-e
boundary near the Virginia/North Carolina line. Southwesterly
isentropic lift over this boundary continues to drive a shield
of light to moderate rain showers over portions of the area.
Based on the latest radar trends, the more focused area of
locally heavy showers have been maintained over areas of Mineral
County back into Grant County in West Virginia. This is largely
due to a series of mid-level impulses as seen on multi-channel
water vapor imagery. The shield of more intense shower activity
is expected to push into the Flood Watch area which spans the
Central Virginia Piedmont back into the Shenandoah Valley and
Potomac Highlands. This watch remains in effect until 10 AM this
morning. Total rainfall amounts within this watch area could
push into the 1 to 3 inch range, locally up to 4 inches
depending on any convective elements.

Overall lightning activity has been non-existent at this point
owing to a very limited reservoir of instability. The latest
analysis shows under 100 J/kg of most-unstable CAPE, mainly
along and west of the Allegheny Front. As the mentioned impulses
aloft track eastward, expect a band of light to moderate
rainfall to overspread the remainder of the area today. Outside
of the watch area, total forecast amounts should range from 0.50
to 1.00 inches, locally down to a 0.10 to 0.25 inches for those
northeast of D.C. Any thunderstorm threat should be limited and
dependent on breaks in the clouds. By this evening, a bulk of
the remaining showers are likely to focus over southern Maryland
with perhaps a light mist or drizzle in the wake. Given the
continued onshore flow, low clouds and patchy fog linger into
the night.

Forecast temperatures remain fairly steady state thanks to the
persistent onshore flow, extensive cloud cover, and threat for
showers. The overall diurnal range should be on the low side as
the high/low temperature spread may be around 4 to 7 degrees.
Today`s highs are mainly confined to the low 60s although some
mid/upper 60s are possible toward the Shenandoah Valley back
into the Allegheny mountain valleys. This is where shower
activity comes to an end earlier. Nighttime low temperatures
stay in the 50s.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
-- Changed Discussion --
A thick area of low cloud cover is expected to start off the day
with winds gradually shifting from east to northeasterly. Aside
from patchy fog and areas of drizzle, it should be a dry start
to the day for most. There should also be more breaks in the
clouds across the area, with even some afternoon clearing for
the Shenandoah Valley and points westward. With the increase in
sunshine, this will also introduce a slight chance for showers
and thunderstorms. Any threat should wane by dusk given the loss
of heating. Much warmer weather is expected on Sunday as high
temperatures rise into the low/mid 70s, with mid/upper 60s
across the higher terrain. Overnight low temperatures drop into
the low/mid 50s.

Given low pressure exiting the Carolinas and high pressure off
the northeastern U.S. coast, the net gradient will favor further
easterly onshore flow on Monday. Despite this flow regime,
mostly sunny skies are expected as heights build aloft in
response to an eastern U.S. ridge. Forecast highs rise back into
the upper 70s to low 80s which is close to average for mid/late
May standards. Expect a shift to light southerly winds overnight
with mainly clear skies. Nighttime temperatures stay fairly
close to the past couple of nights.
-- End Changed Discussion --

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
-- Changed Discussion --
Ridging both at the surface and aloft is expected to persist through
Tuesday. This should lead to fair and dry weather with warming
temperatures compared to previous days.

As the high at the surface moves offshore and ridging aloft breaks
down, heat and instability will build as a trough and cold front
slowly approach from the Midwest. The building instability offers
enough potential energy for strong to perhaps severe thunderstorms,
with the approaching front acting as a trigger to act on that energy.
Shear sufficient enough to organize storms resulting in a more
widespread threat remains in question, and may ultimately lag behind
the strongest instability. Predictability is low this far out, but
it`s a period to monitor with at least limited convective potential.

The cold front is expected to move offshore Thursday into Friday.
This will knock down humidity levels with slightly cooler
temperatures as winds turn northwesterly.
-- End Changed Discussion --

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
A lengthy period of restrictions are expected today given
showers and low ceilings. Any instance of thunderstorms should
be limited and highly dependent on breaks in the thick low
clouds. The highest confidence is around KCHO, mainly in the
mid-afternoon to early evening timeframe. Elsewhere, confidence
is too low to introduce into the TAF. Winds remain out of the
east to southeast throughout the day, accompanied by IFR
ceilings. Guidance continues to hint at LIFR ceilings by
tonight which is currently reflected in the KIAD, KCHO, and KMRB
TAFs. This seems reasonable given the degree of low-level
moisture, onshore flow, and saturated grounds.

Expect an eventual improvement to MVFR ceilings by midday
Sunday. This would include more breaks in the clouds as rainfall
ends the previous night. Winds become mainly northeasterly on
Sunday before shifting to east-southeasterly on Monday. VFR
conditions are likely on Monday with mostly sunny skies.

Mainly VFR Tue-Wed. SHRA/TSRA chc increases Wed. Winds S/SE 5-15
kts.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Some passing showers continue to push wind gusts up to around 15
knots. However, winds largely stay in the 5 to 10 knot range
outside of such brief bursts. Onshore flow persists throughout
the weekend with some uptick in wind fields by this afternoon
and evening. Near 20 knot gusts are possible over the lower
waters. Expect near Small Craft Advisory gusts across these
waters through tonight and into much of Sunday. As such,
advisories may be hoisted for this area. Onshore gradients
slowly weaken into Monday which brings gusts below advisory
thresholds.

Winds will turn south to southeasterly Tuesday into Wednesday. Given
generally weak gradients and warmer air over relatively cooler
water, most of the time should feature sub-SCA level winds, though
brief channeling effects from bay/river breezes could increase
gusts locally during the afternoon and evening hours. Thunderstorms
may also approach the waters by late Wednesday.
-- End Changed Discussion --

&&

.HYDROLOGY...
-- Changed Discussion --
A Flood Watch remains in effect until 10 AM this morning given
the threat for flash flooding. This currently covers the Central
Virginia Piedmont across much of the Shenandoah Valley into the
Potomac Highlands. Total forecast amounts within this watch area
are between 1 to 3 inches, locally up to 4 inches where
convective and/or terrain enhancements take place. Overall,
1-hour rainfall amounts have largely been capped around 1 inch.
However, 3-hour amounts continue to push to around 2 inches
which would support flash flooding where more vulnerable in
nature.
-- End Changed Discussion --

&&

.TIDES/COASTAL FLOODING...
-- Changed Discussion --
Persistent onshore flow (albeit light) is expected to persist into
early next week. Repeated rounds of flooding are possible,
especially along vulnerable shoreline. Given the light nature of the
flow, flooding should stay in the minor realm.
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
DC...Coastal Flood Advisory until 8 AM EDT this morning for DCZ001.
MD...Coastal Flood Advisory until 7 AM EDT this morning for MDZ014.
VA...Flood Watch until 10 AM EDT this morning for VAZ025>027-029-
     030-036>040-050-051-503-504-507-508.
     Coastal Flood Advisory until 7 AM EDT this morning for VAZ054.
WV...Flood Watch until 10 AM EDT this morning for WVZ055-501-502-
     505-506.
MARINE...None.
-- End Changed Discussion --

&&

$$

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