Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181407

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1007 AM EDT Sun Sep 18 2016

A weakening cold front will approach from the northwest today
before stalling over the region tonight through Monday night.
This front will dissipate on Tuesday. Another cold front will move
into the region and stall to our south later this week.


Weakening frontal system continues its gradual push southeastward
towards the region as Tropical Depression Julia spins off of the
North Carolina coastline. To start the day today, we`re looking at
a low cloud deck from about the DC/Baltimore metro areas southward
into central Virginia. This should gradually lift and dissipate
over the next couple of hours. Otherwise, a few isolated to
scattered showers west of the metro areas will be about it
precipitation wise through the morning. As the front approaches,
expecting more widespread showers and a few thunderstorms to move
in from the southwest, mainly along and west of the Blue Ridge
mountains, and especially near the Allegheny front. Could see some
scattered activity make it into the I-95 corridor, but most places
likely to remain dry there through the day. Precipitable water
values through today remain about 1.75 inches, so locally heavy
rainfall possible. High temperatures will range from the upper 70s
west to upper 80s south and east.


Front will make slow progress eastward through the region tonight,
before washing out over the area on Monday. As it moves eastward,
will see a moisture surge from Julia get picked up along the
front, with precipitable water values surging to over 2 inches
from the metros eastward late tonight and on Monday. Best signal
in terms of moisture convergence, deformation, and omega fields,
for widespread showers and embedded thunderstorms shifts from
western areas this afternoon/evening to the metro areas tonight to
metros and east on Monday. Jet dynamics also become more favorable
on Monday as region is projected to lie in the right entrance
region of the upper jet.

All of this means that showers/embedded thunder move eastward
through the region tonight, and remain/intensify late tonight and
Monday from the I-95 corridor eastward. Thus expecting two main
foci for heaviest rainfall amounts, first in western areas today
and this evening, and a second late tonight and tomorrow from
DC/Baltimore south and east. Total rainfall area-wide generally
0.5-1.5 inches, locally higher in any areas that see repeated
convective activity. Greatest threat for that will be in the 2
areas mentioned above. Because of dry antecedent conditions, flash
flood guidance values are quite high, from about 2-3" in 1 hour to
3-5" in 6 hours, so flash flood threat remains low, however can`t
rule out something isolated.

Shower chances will trend downward Monday night into Tuesday,
although with frontal boundary washing out and Julia`s main
circulation still progged to remain south of the region, its
possible some showers linger into Tuesday across southern/eastern


Surface high pressure builds east across the northern Mid-Atlantic
Wednesday, lingering off New England Thursday into Friday. Wednesday
is definitely dry with a north wind. An onshore flow starts Thursday
and continues into the weekend. Low pressure may develop again along
the Carolina coast in the Friday/Saturday timeframe which may allow
some rain into the CWA. Cold front from a digging trough over
eastern Canada looks to approach the area over the weekend.

Dry and sunny conditions Wednesday through Friday would allow for
above normal temperatures, namely low to mid 80s maxima.


Areas of low ceilings at IAD/DCA/BWI/MTN will gradually lift and
dissipate over the next couple of hours, with VFR developing
at all TAF sites for a period this afternoon. Sub VFR conditions
then possible again later today and tonight with showers and
t-storms moving in from west/southwest. Conditions should improve
gradually by Monday night with VFR returning Tuesday.

VFR conds prevail Wednesday through Friday as high pressure builds
east across the northern Mid-Atlantic (north wind Wednesday),
stalling off New England (onshore flow Thursday/Friday).


Southerly winds from 5-15 knots expected through today. Tonight, low
level jet may re- intensify so may need to add new SCA for late
today/tonight. Showers/t-storms also will be an issue later on
today/tonight into Monday. Sub-SCA more probable Monday/Tuesday.

High pressure builds east across the northern Mid-Atlantic Wednesday
with north winds before stalling off New England maintaining an
onshore flow Thursday/Friday.


South flow across the waters has maintained positive water levels
of 0.75 to 1.0 ft above astronomical prediction (normal). Minor
coastal flooding is expected to continue at the sensitive sites of
Annapolis, Straits Pt (St. Marys Co) and SW waterfront DC and
Alexandria at high tide through this evening. Coastal flood
advisories continue this time for Anne Arundel and St. Marys

DC/Alexandria have up to a 1.0 ft positive anomaly that may cause
minor coastal flooding through high tide tonight. Will consult
with MARFC before extending the headline beyond noon.

Low pressure is expected to persist along the Carolina coast through
at least Tuesday with north winds delayed until Monday night at the
earliest. Expect elevated water levels and potential continued minor
coastal flooding at least through Monday.


DC...Coastal Flood Advisory until noon EDT today for DCZ001.
MD...Coastal Flood Advisory until 11 PM EDT this evening for MDZ014.
     Coastal Flood Advisory until 8 PM EDT this evening for MDZ017.
VA...Coastal Flood Advisory until noon EDT today for VAZ054.


TIDES/COASTAL FLOODING...BAJ/LFR/MM is the U.S. government's official web portal to all federal, state and local government web resources and services.