Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 200759
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
359 AM EDT FRI MAY 20 2016
High pressure will remain over the area through this morning
before retreating to the northeast this afternoon. Low pressure
will track east across North Carolina tonight before turning up
the Mid-Atlantic coast Saturday. Upper level low pressure will
persist along the Mid-Atlantic coast Saturday night through
Monday night. High pressure returns Tuesday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The morning is starting with mainly clear skies save some cirrus
approaching from the SW as surface high pressure is located over
the eastern Great Lakes. Some patchy fog has developed in the
valleys and other typical low lying areas. Will continue to
monitor this, although most guidance has densest/most widespread
fog south of the area.
On the whole, a quiet day is expected as the high moves toward
the northern Mid-Atlantic coast. Mid and high level clouds will be
increasing by afternoon ahead of a low pressure system in the TN
Valley. Slowed down the timing of rain chances, as even in the far
SW CWA, any chance should hold off until after mid-afternoon.
There should be enough sun for most of the area to push into the
70s, although clouds may restrict temperatures to the south.
.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY NIGHT/...
As mid/upper level trough pushes east tonight into Saturday,
energy will transfer to a coastal low which will lift NE up the
Mid- Atlantic seaboard. A SE low level jet will inject a tongue of
higher PWs tonight into early Saturday. Still looking at a swath
of moderate to locally heavy rain to push across the area,
although it may not make it to the metro areas until around
daybreak Saturday. Hydro concerns are minimal -- see section
below. There may be some breaks, or at least lightening, of the
rain Saturday afternoon, but wrap-around moisture will at least
make for some off and on rain or drizzle. Temperatures will
struggle to rise much, possibly failing to reach 60F in many
Saturday night into Sunday will continue to be unsettled with the
upper level system closing off over the area. There may be an
embedded disturbance which enhances shower chances Saturday night.
On Sunday, the showers may take on more of a diurnal flavor with
the upper low directly overhead. Have not added thunder at this
point as instability doesn`t look deep. Knocked a degree or two
off highs for Sunday (mainly upper 60s now) as cloud cover may win
out at times, especially by afternoon. The low will inch eastward
Sunday night, so have a gradual decreasing of shower chances.
Minimum temperatures don`t change much through the period...upper
40s to lower 50s.
.LONG TERM /MONDAY THROUGH THURSDAY/...
Deep upper level low pressure will remain over the area Monday.
It will only very slowly shift northeastward away from our area
Monday night and Tuesday. Upper level ridging will gradually
become dominant Wednesday and Thursday.
At the surface, this means low pressure will remain to the east on
Monday but cold air aloft will result in showers and perhaps even
a thunderstorm. As the system shfits northeast Tuesday, warming
temperatures aloft combined with convergence in the wake of the
system and building high pressure will result in a dry day.
Wednesday looks like it will remain mostly dry. By Thursday, a
front will approach from the west, heights may drop a bit, and we
may end up getting some instability showers or thunderstorms
especially over the mountains.
Monday will remain cooler than normal with the upper low overhead.
By Tuesday, temperatures will return to normal, while Wednesday
and Thursday should exceed normal by several degrees.
.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Mainly clear skies and light winds with high pressure promoting
fog develop early this morning, but not at the TAF sites as of
this writing. Will be paying special attention to MRB, IAD, and
VFR with light winds today as the high moves east. Clouds
increase and lower tonight in advance of low pressure system.
Expect a swath of moderate to perhaps heavy rain late tonight into
Saturday morning. Would expect a period of IFR conditions. There
may be some improvement Saturday afternoon, but showers and low
cigs will remain in the area through Saturday night and possibly
Sunday with N to NE flow. Any showers should taper off by Sunday
Sub-VFR conditions remain possible Monday with the upper low
nearby and showers around. VFR should dominate Tuesday with dry
Light E to SE flow today as high pressure moves offshore. Rain
overspreads the area late tonight into Saturday which will inhibit
mixing, but as low pressure moves offshore, gradient will be
strong enough for SCA conditions. Have hoisted the SCA starting
late tonight for the southern waters, then for all waters during
the day Saturday. This may need to be extended for portions of the
waters Saturday night. N to NW flow now looks to stay 15 kt or
less for Sunday and beyond.
Period of moderate to heavy rain has the potential to bring 1-2
inches of rainfall to the area, with the highest amounts focused
on the southern CWA. This will not be a convective (1hr FFG) type
of event, and would probably need at least 1.5 inches in 3 hours
to begin hydro concerns. Certainly not out of the question and
will need to be monitored, but the rain may be just spread out
enough to preclude any major issues. Ensemble mean of MMEFS keeps
all river points below flood stage, so small streams and urban
areas will be the places to watch.
MARINE...Small Craft Advisory from 8 AM to 6 PM EDT Saturday for
Small Craft Advisory from 4 AM to 6 PM EDT Saturday for ANZ534-