Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 261918
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
318 PM EDT Sat May 26 2018

.SYNOPSIS...
High pressure will remain over the western Atlantic through the
weekend. A cold front will cross the Mid- Atlantic late Sunday,
then stall to the south until Wednesday of next week. The front
will likely return north as a warm front Wednesday into
Thursday. The remnants of Alberto may pass to the west of the
region late next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Showers and a couple of thunderstorms are sliding eastward
across the western and northern half of our CWA. The main
threats since around 1230 pm have been very heavy rainfall. A
few thunderstorms over Augusta, Rockingham, Rappahannock, and
Fauquier Counties in Virginia produced rain rates as little as
1 inch per hour to as much as 4 inches per hour. A few Flood
Warnings and one Flash Flood Warning has been issued so far in
terms of this afternoon convection. Additional showers and
thunderstorms are expected to develop farther to the east and
south the remainder of this afternoon into this evening. This
activity will be slow-moving and is expected to produce very
heavy rainfall. A Flash Flood Watch remains in effect for parts
of the Potomac Highlands and Shenandoah Valley through 8 pm this
evening.

Storms should gradually weaken this evening with the loss of
heating, but hi-res guidance suggests storms will linger over
southern Maryland where the surface trough and residual outflow
from daytime convection combine to result in persistent showers
and heavy thunderstorms. Heavy rain potential is definitely
there but will have to investigate this further later this
afternoon and evening in order to decide if we are going to
issue any additional Flash Flood Watches farther to the east.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/...
Another round of showers and heavy thunderstorms is likely on
Sunday as a cold front drops southward out of Pennsylvania,
running into the humid airmass over the Mid-Atlantic. The
presence of the front and some increase in shear may lend to a
little more storm organization, but the details will rely
heavily on prior day`s convection. Training storms seem more
likely on Sunday on a more widespread scale, though, given storm
motions largely parallel to the low-level boundary.

Convection may linger well into Sunday evening as the front will
likely be slow to clear the area. The high pressure to the north
forcing it through is forecast to only be around 1020 mb, which
usually means it gets hung up over southern parts of the area,
which could lead to another round of showers and thunderstorms
Monday afternoon and evening across central VA to southern MD.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
To start out the long term period on Tuesday, a front will be
located off to our south. A stray shower or two can`t be ruled out
(especially over southern portions of the forecast area), but it
appears as though most locations should stay dry.

The main feature of interest throughout the long term period will be
the remnants of Tropical Storm Alberto. The official NHC forecast
has Alberto making landfall late Monday Night along the
Gulf Coast. After landfall, Alberto will weaken, but it`s remnants
will still have a large influence on our weather through the week.

Also of interest are a trough currently located over the western
CONUS and a stronger trough currently located in the Gulf of Alaska.
Over the next couple days, the western CONUS trough will become
largely cutoff from the polar jet and progress slowly eastward.
Meanwhile, the stronger trough over the Gulf of Alaska will dive
into the Pacific Northwest by Tuesday.

Alberto is expected to track north on Tuesday into the lower
Mississippi/Tennessee Valleys beneath an anomalously strong upper
ridge. As Alberto progresses further north, the aforementioned
western CONUS trough will be kicked out onto the Upper Plains by the
stronger trough moving into the Pacific Northwest. That will
place Alberto within the steering influence of the ejecting trough,
causing it to initially continue north, before turning more to
the northeast as the weakening trough tracks to it`s north. These
complex interactions between the two troughs and remnants of Alberto
suggest that there is a fair amount of uncertainty as to where the
remnants will track. However, most model guidance (deterministic and
ensemble) moves the system from the Tennessee Valley into the Ohio
Valley by Thursday.

Regardless of the exact details of the track, it appears as though
we will remain to the east of the track. Southerly flow to the east
of the system will transport a very tropical airmass into the
region. The deterministic GFS and Euro, as well as their respective
ensembles suggest that precipitable water values will approach or
exceed two inches by late Wednesday into Thursday. Recent model runs
have the best forcing for ascent associated with Alberto`s remnant
circulation located off to the north of our area. However, this far
out there is a fair amount of track uncertainty. At the very least,
showers and thunderstorms will be possible on both Wednesday and
Thursday. Given the near record precipitable water values, any
storms that do form could produce heavy rainfall. In the wake of
Alberto`s remnants, model guidance pushes a cold front through the
region. There are considerable differences between the models
with regard to the timing and sensible impacts of this frontal
passage.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
VFR conditions with a broken mid level deck this afternoon. One
exception will be brief MVFR conditions due to very heavy
rainfall from showers and thunderstorms through 22z this evening
near the Metropolitan terminals. IFR vsby possible in the
heaviest thunderstorms. Winds generally SW 5-10 kts with a few
gusts around 15 kts. A few storms may also be accompanied by
isolated strong wind gusts, but more widespread severe
convection seems less likely.

Clouds should preclude dense fog formation tonight, though fog
still hinted at strongly in guidance wherever clouds break.

Similar conditions expected Sunday as a cold front drops south
out of PA. Front likely stalls near CHO to southern MD Monday
with more convection possible in this corridor. North of the
boundary, winds will become northerly generally AOB 10 kts.

The weak front will slip south through the area on Tuesday and
remain nearby on Wednesday. While shower and thunderstorm
coverage may be limited, they will be possible each day.

&&

.MARINE...
SCA the remainder of the day for middle/lower MD Chesapeake and
lower tidal Potomac waters on marginal SW gradient. Thunderstorms
likely to continue late this afternoon into this evening and a
few could produce SMW (34+ kt) gusts as well as heavy rain and
frequent lightning. Guidance indicates convection could linger
over lower MD Chesapeake through much of the overnight. Gradient
becomes stronger overnight but low- level inversion brings
mixing into question. SCA conditions possible especially over
open waters overnight into early Sun AM but confidence too low
for a headline at this point.

As a cold front approaches the waters from the north Sunday, a
renewed round of showers and thunderstorms is likely. The front
will probably get hung up near southern MD through Monday with
at least scattered thunderstorms possible again Monday aft/eve.

Light winds are expected Tuesday as the weak front sags
southward through the area, although there could be a few
showers and thunderstorms. Southeast winds may near SCA
thresholds on Wednesday as the front lifts back to the north and
high pressure moves off the New England coast. Additional
thunderstorms will be possible.

&&

.HYDROLOGY...
A Flash Flood Watch remains in effect through 8 pm this evening for
portions of the central Shenandoah Valley in Virginia and
adjacent portions of eastern West Virginia. Slow storm motion
combined with very heavy rainfall (PWATs 1.75+ inches) may
result in flash flooding, especially given recent heavy rains.

The threat is more isolated (though certainly non-zero) further
east this afternoon and evening. Hi-res guidance indicates
persistent convection mainly S of DC overnight, so this may be
another area to watch. Some HREF members also hint at NE MD late
this afternoon.

More widespread training thunderstorms with heavy rain are
possible Sunday as a cold front drops south out of Pennsylvania.
Additional watches may be needed to account for this threat
Sunday into Sunday night.

&&

.TIDES/COASTAL FLOODING...
Subsequent high tides at DC SW waterfront through the weekend
should fall just short of minor flood as well.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...Flash Flood Watch until 8 PM EDT this evening for VAZ026-027-
     029-030-507.
WV...Flash Flood Watch until 8 PM EDT this evening for WVZ055-505-
     506.
MARINE...Small Craft Advisory until 8 PM EDT this evening for
     ANZ532>534-537-540>543.

&&

$$
SYNOPSIS...KLW
NEAR TERM...KLW
SHORT TERM...KLW
LONG TERM...KJP
AVIATION...ADS/KLW
MARINE...ADS/KLW
HYDROLOGY...ADS/KLW
TIDES/COASTAL FLOODING...ADS/KLW



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