Area Forecast Discussion
Issued by NWS Sterling, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KLWX 180055
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
855 PM EDT Fri Mar 17 2017
A warm front will move into the area late tonight
into early Saturday before a cold front passes through Saturday
afternoon. High pressure builds over the area early next week.
Another cold front will cross the area on Tuesday.
.NEAR TERM /UNTIL 7 AM SATURDAY MORNING/...
Warm frontal precip will continue to push across the region
this evening and into tonight, before moving east of the area by
morning. Despite the relatively warmer temperatures today, wet-
bulb cooling has allowed temperatures to drop into the 30s after
precipitation has started. This has led to rain changing to snow
across portions of the area, so far mainly from the Blue Ridge
west. Have had reports of up to about 2 inches as of 830pm in
the higher terrain of the Allegheny Highlands. As warmer air
moves in aloft, will see precipitation change over to rain,
however lingering cold in the low levels and at the surface
will lead to some areas of freezing rain and sleet. This is
already being observed west of the Allegheny Front and will push
eastward over the next couple of hours. Thus have issued a
Winter Weather Advisory for a wintry mix of snow and ice for
these locations. Elsewhere, very little impact is expected as
rain and some light snow move across. May see a dusting to a
couple of tenths of snow accumulate on grassy surfaces across
portions of the area north and west of the metros.
After main batch of precipitation ends, some lingering light
fog, drizzle, and low stratus is likely. Lows will be generally
in the 30s to around 40F.
.SHORT TERM /7 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
The warm frontal boundary will remain nearly stationary over
the Potomac Highlands into central Virginia and southern
Maryland Saturday. Closed upper-level low will track into
Pennsylvania and this will cause low pressure to develop along
the boundary Saturday afternoon before developing into a coastal
low Saturday night.
The dry slot should keep most areas dry Saturday morning...with
the exception of low clouds/fog and drizzle due to moisture
trapped underneath the subsidence inversion. However...as the
low pass through our area later Saturday afternoon and begins to
develop off the coast...a north to northwest flow will develop.
This may mix out any cool air that`s trapped and cause warmer
conditions relative to recent days. Max temps will range from
the 40s across northern Maryland to the lower and middle 50s
near Washington and Baltimore to the lower middle 60s in
central Virginia. The warmer air along with unseasonably low
500mb heights from the upper-level low will trigger a few
showers Saturday afternoon into Saturday evening. Some showers
may even contain small hail if enough instability develops.
The upper-level low will pass through our area Saturday
night...bringing the chance for a few rain and snow showers. The
best chance for snow will be across northeastern Maryland where
forcing will be a bit stronger northeast of the upper-level low.
Snow may coat the ground across these areas. An upslope flow
will cause accumulating snow along and west of the Allegheny
Front as well. Winter headlines may be needed for this area.
The upper-level low will move away from the area Sunday and a
northwest flow will bring drier air to the area along with
chilly conditions for this time of year. However...it will not
be nearly as cold as recent days. Highs will be in the 40s
across most areas to lower and middle 50s in central Virginia.
High pressure will build overhead Sunday night...bringing dry
and chilly conditions.
.LONG TERM /MONDAY THROUGH FRIDAY/...
High pressure over the region Monday will move east to allow for
a cold front to sag southward and bring a chance for rain
showers Monday night into Tuesday.
A second area of high pressure will build southward from the
Great Lakes Tuesday night through Thursday night. Drier air
and colder temperatures will ensue into the region.
High pressure will move to the coast to allow for a return flow
to usher in milder air Friday.
.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
A warm front will stall out over central Virginia tonight. Low
pressure will track along the boundary Saturday before moving
off to the east Saturday night. A period of rain and snow is
expected this evening and tonight ahead of the warm front. The
highest chances for snow will be at MRB/MTN/BWI. Snow may coat
the ground near KMRB...but paved surfaces will likely remain
wet due to the warm conditions today. Cigs/Vsbys will
deteriorate this evening and MVFR conditions expected at all TAF
sites late tonight through most of Saturday morning, with
potential for IFR. Greatest risk of IFR at MRB/BWI/MTN Saturday
morning, where it is included in the forecast.
Cigs/vsbys should improve some by Saturday afternoon...but MVFR
conditions will persist through much of the day, with IFR even
into the afternoon at MRB/BWI/MTN. A few showers are possible
Saturday afternoon into Saturday evening. Snow showers are
possible Saturday night...especially across the northern
VFR conditions along with north to northwest winds will return
for Sunday and Sunday night. Vfr conditions are also expected
for Monday. A cold front may impact the terminals Monday night
into Tuesday. High pressure will return for the middle portion
of next week.
A warm front will stall out near the southern waters tonight
into Saturday. A Small Craft Advisory is in effect for middle
portions of the Bay and the lower Tidal Potomac tonight into
Saturday morning where the gradient will strengthen. Low
pressure will pass by to the south Saturday before developing
into a coastal low Saturday night. Winds may approach SCA
criteria Saturday afternoon...but confidence is not high enough
for a headline since cool air trapped underneath the inversion
may limit mixing.
A Small Craft Advisory is in effect for the waters Saturday
night and will likely be needed Sunday through Sunday night.
No marine hazards Monday through Tuesday night.
MD...Winter Weather Advisory until 6 AM EDT Saturday for MDZ501.
VA...Winter Weather Advisory until 6 AM EDT Saturday for VAZ503-504.
WV...Winter Weather Advisory until 6 AM EDT Saturday for WVZ501-503-
MARINE...Small Craft Advisory from 11 PM Saturday to 6 AM EDT Sunday
Small Craft Advisory until 8 AM EDT Saturday for ANZ532>534-