Area Forecast Discussion
Issued by NWS Sterling, VA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
000
FXUS61 KLWX 210848
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
448 AM EDT Mon Aug 21 2017

.SYNOPSIS...
High pressure will move offshore later today. A disturbance will
bring showers and thunderstorms to parts of our region this
afternoon and evening. High pressure will regain control overnight
tonight through Tuesday, before a strong cold front pushes across
our region and brings another threat for showers and thunderstorms
Tuesday night into Wednesday. The front could stall to our
southeast, while a second area of high pressure builds in from the
west later this week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Weak high pressure over the eastern one-third of our region will
allow for dry conditions through daybreak. We can`t rule out a brief
low-topped shower or just a few sprinkles in parts of northern
Virginia, near the District of Columbia, or parts of southern
Maryland shortly before or at sunrise. Some of the high resolution
models are showing this. Very little to no precipitation will occur
with these hints of a shower or sprinkles. Temperatures will bottom
out only in the upper 60s to lower 70s as dewpoint temperatures
linger in the middle to upper 60s.

During the course of today, a light and variable wind will become
more southerly at 5 to 10 mph. This will usher in additional low-
level moisture to help fuel some showers and thunderstorms this
afternoon and into this evening. As the high pressure overhead
inches offshore today, a disturbance currently over northwest Ohio
will make its way across and just north of the Mason-Dixon Line
later this afternoon and this evening. This disturbance, combined
with daytime heating, increased low-level moisture, indicated by
dewpoint temperatures in the lower to middle 70s, and some wind
shear should generate showers and thunderstorms this afternoon and
evening. The main threats will be damaging winds, some hail, and
very heavy rainfall due to precipitable water values between 1.80
and 2 inches. Most of this activity will be confined to an area from
US 15 to the Chesapeake Bay and south to the Fredericksburg. Timing
on this activity seems to be between 3 pm this afternoon and 9 pm
this evening. There could be an isolated shower or thunderstorm
develop over the mountains of western Maryland or northern
Shenandoah Valley as early as Noon. Also, some of this evening`s
activity could linger longer into the night, perhaps close to
midnight before weakening or dissipating. As rain activity lessen
overnight, some patchy fog could develop in areas where it has
rained and where dewpoint temperatures remain near 70 or in the
lower 70s. Tonight`s lows will be in the lower to middle 70s.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
A lull in the shower and thunderstorm activity could fulfill most of
Tuesday. High pressure will regain control and allow for dry
conditions. There is a chance for a shower or thunderstorm along the
Mason-Dixon region and northeast Maryland Tuesday afternoon and
evening as a weaker disturbance moves east across Pennsylvania.
Otherwise, most other places will be dry. Highs will reach the
middle 90s with dewpoint temperatures in the lower to middle 70s
once again. The timing on a strong cold front remains in question
late Tuesday into Tuesday night. We have a chance for showers and
thunderstorms across the northern and western half of our region
Tuesday night for now. Tuesday night`s lows will only bottom out in
the middle 70s.

The chance for a couple of showers linger Wednesday in the west,
central and southern areas. This is heavily weighted on when the
cold front can pass through the region. High pressure should
gradually build in from the west Wednesday night and bring cooler
and more comfortable conditions to the region. Highs Wednesday will
be middle 80s, cooler than previous days. Lows at night into the
middle to upper 60s.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
The extended forecast is blissfully quiet as high pressure builds
northeast from Canada. Not only will the chance for precip be nil,
but temperatures will once again be atypically mild. The only cause
for concern will come from the upper level trough axis dropping
across the northeastern US.  This feature may result in added cloud
cover. We shall see if subtleties from associated weak vorticity and
jet energy lead to a swing toward scattered showers in future model
runs.

&&

.AVIATION /09Z MONDAY THROUGH FRIDAY/...
VFR conditions through Wednesday night. Any heavy showers or
thunderstorms Monday afternoon and evening, as well as late Tuesday
and Tuesday night could reduce conditions to MVFR or IFR briefly.
Winds light and variable becoming south at 5 to 10 knots today.
Winds becoming southwest around 10 knots tonight through Tuesday,
then backing to the northwest at 10 knots Tuesday night and
Wednesday depending on frontal passage.

VFR conditions anticipated Thu-Fri under building high pressure.

&&

.MARINE...
No marine hazards expected through Tuesday morning. Have opted
to issue Small Craft Advisory for Tuesday afternoon-evening at
this time; gusts to 20-25 kt anticipated in mixing ahead of
cold front. Behind the cold front at the end of the week, expect
north winds. At this point, speeds look to be less than 15 kt.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from noon to 11 PM EDT Tuesday for
     ANZ530>543.

&&

$$
SYNOPSIS...KLW
NEAR TERM...KLW
SHORT TERM...KLW
LONG TERM...HTS
AVIATION...KLW/HTS
MARINE...KLW/HTS



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.