Area Forecast Discussion
Issued by NWS Sterling, VA

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412
FXUS61 KLWX 301901
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
301 PM EDT Thu Mar 30 2017

.SYNOPSIS...
High pressure will move offshore tonight. Low pressure will
pass through the area Friday. High pressure will gradually
return Saturday and Sunday before another low pressure impacts
the area Monday and Tuesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Latest surface analysis shows high pressure near the Jersey
shore, wedging southward along the east side of the
Appalachians. A warm front snakes its way northwest from near
Savannah, Georgia to a low pressure centered near St. Louis,
Missouri, with a cold front extending southward to Lousiana and
the western Gulf of Mexico. Another center of high pressure is
located over the southern Rockies. Aloft, a ridge is located
directly over our area, with a closed low is centered over
western Missouri, and another ridge over the southern Rockies.

Tonight, the surface high will slide east off the coast and the
surface low will also move east, reaching Indiana by dawn. The
warm front to our southwest will slowly push northward, reaching
southwestern Virginia and North Carolina by dawn. Our area will
remain stable through the night, but elevated convection may
move overhead, resulting in some showers during the evening. A
stray thunderstorm can`t be ruled out. Steadier rain will
develop overnight as warm advection/isentropic lift increases.
Temperatures will drop into the 40s in most of the region with a
southeasterly upslope flow.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Surface low pressure in the midwest will slide into western
Pennsylvania during the day Friday, with a secondary low
developing over western North Carolina and heading northeast
across our area. The two surface lows will consolidate near New
Jersey Friday evening. Aloft, the shortwave to the south of the
approaching closed low will develop a negative tilt as it slides
eastward towards us Friday, with the closed low itself then
crossing over the region overnight Friday. Both the surface and
upper level systems will move eastward away from the region
during the day Saturday.

The negatively tilted shortwave will promote signifcant rainfall
and perhaps a little embedded convection Friday morning. At the
very least, some periods of moderate to occasionally heavy rain
look likely given the ample moisture flow from the south and
good forcing, and soundings also suggest some instability above
the inversion. While the area has been dry, soils are
significantly more moist than they were a month ago. Thus I am a
little concerned about potential for isolated flooding, or at
least significant ponding of water on roadways. Given that right
now threat seems isolated, will not yet introduce any enhanced
wording for it just yet. Steadier rain looks likely to taper to
a more showery pattern by afternoon as the warm front tries to
lift northwest into the DC metro ahead of the approaching
secondary low. Some modest instability looks likely to develop
just ahead of the cold front which will be trailing from the
secondary low, and with some decent wind aloft, some gusty if
not severe storms are certainly possible. SPC marginal risk area
looks reasonable for areas near and south of the metro. Highs
will be dependent on just how far north the warm front gets,
with 60s likely south of the front and perhaps even 70s in the
warmest spots south of the front, while 50s will hold north of
it.

Showers will diminish Friday night with the surface low passing
east, but may linger as the upper low crosses the area
overnight. Temperatures will fall back into the 40s on the
northwesterly flow. The system will head off the coast and leave
a gusty but drying northwest wind for Saturday. Winds may gust
to 25-30 mph, but temperatures should be able to rebound to the
50s and low 60s. Some upslope showers will linger along the
Allgheny Front, but east of the mountains it should be dry.

All areas turn out dry for Saturday night as high pressure
pushes in from the west and the upslope flow weakens. Temps may
try to drop into the 30s in the cooler spots but the northwest
flow should stay up and prevent strong radiational cooling.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
High pressure will settle into the region Sunday through early
Monday. Dry and seasonable temperatures expected throughout the
period.

The next storm system will move into the region midday Monday
through Tuesday, bringing more rain showers to the region once
again.

Brief high pressure will build into the region Wednesday and
Wednesday night before the next storm system approaches
Thursday.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
Conditions will deteriorate through tonight as low pressure
approaches from the west. Showers will become more common with
steady rain, reduced cigs and vis expected late tonight. IFR
likely all terminals Friday morning before conditions relax a
bit in the afternoon as rain diminishes to showers, though a
thunderstorm will be possible through the day, with best odds in
the afternoon. Strong winds just off the deck may also result in
some wind shear on Friday. Conditions improve slowly back to
VFR Friday night into Saturday, though a gusty NW wind will
remain. Winds will diminish Saturday night.

Vfr conditions expected Sunday through early Monday. Winds
light and variable Sunday through early Monday. Mvfr to ifr
conditions midday Monday and Monday night. Winds southeast 5 to
10 knots midday Monday and Monday afternoon becoming northeast 5
to 10 knots Monday night.

&&

.MARINE...
Approaching low pressure will bring southerly winds across the
waters, with gusts increasing to SCA (20-30 knots) as we head
through tonight and Friday. A gusty thunderstorm may cross the
waters on Friday, most likely later in the day. Showers diminish
and winds shift NW behind the front Friday night, with SCA gusts
remaining possible behind the system Saturday.

No marine hazards expected Sunday through Monday night. Winds
northwest 5 to 10 knots Sunday through Monday. Winds becoming
northeast 5 to 10 knots Monday night.

&&

.TIDES/COASTAL FLOODING...
An onshore flow will increase tonight into Friday as high
pressure continues to move offshore. Low pressure will pass
through the area Friday night. The onshore may increase ahead of
this system.

Elevated water levels are expected tonight through Friday night
and minor tidal flooding is likely near times of high tide.
Coastal flood advisories are likely.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 PM this evening to midnight EDT
     Friday night for ANZ531>534-537-539>541-543.
     Small Craft Advisory from noon to 6 PM EDT Friday for ANZ536-
     538.
     Small Craft Advisory from 6 AM Friday to midnight EDT Friday
     night for ANZ530-542.

&&

$$
SYNOPSIS...RCM
NEAR TERM...RCM
SHORT TERM...RCM
LONG TERM...KLW
AVIATION...RCM/KLW
MARINE...RCM/KLW
TIDES/COASTAL FLOODING...LWX



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