Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 201859

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
259 PM EDT FRI MAY 20 2016

High pressure will retreat to the northeast this evening. 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.

A ridge of high pressure remains from the eastern Great Lakes to
Cape Hatteras this afternoon. As a result...the bulk of the
moisture has been delayed. Cirrus now streaming across the area,
and there have been a few cumulus clouds that have been able to

Denser moisture will overspread the area tonight as low pressure
tracks from the southeastern states toward the Appalachians. Based
on meso-model trends, this translates to increasing clouds for
the evening hours. Am shifting the onset of precipitation to late
evening in the far southwestern section of the forecast area, and
during the overnight hours for the bulk of the area. We still will
have a ribbon of strong theta-e advection advancing eastward, so
I`m fairly confident that rain will begin before dawn for most of
the area.


A sharpening trough axis in the Great Lakes/Ohio Valley will
assist in transferring energy to a coastal low off the Carolinas
early Saturday. This coastal low will then head northeast up the
coast during the day. The area will have ample moisture in place,
with a conveyor coming from the Atlantic ahead of the low.
Therefore, the morning to midday hours likely to see the heaviest
rain, as the low level jet interacts with an airmass with
precipitable water above an inch. Due to the synoptic pattern,
there will be a wedge of cool air in place; am therefore going
with a forecast of steady rain. Amounts of an inch to an inch and
a half should be quite common in central Virginia and southern
Maryland by midday Saturday.

Could see some upslope enhancement along/east the spine of the
Blue Ridge, especially in central Virginia. The lowest flood
guidance values extend west of there, so after collaboration a
slight risk of excessive rainfall was painted by WPC in these

As the low tracks north of the area by mid-late afternoon, we will
experience a greater influence from the trough axis. There will be
multiple shortwaves embedded within the flow that will cross the
area. This will last for the rest of the weekend. Therefore,
needs to continue at least a chance of showers (with mostly cloudy
to cloudy skies). Timing these vorts will ultimately determine
when the best chance of rain will be. While skill not present to
do that specifically, it would seem as though Saturday evening and
Sunday afternoon would have the best chance at additional showers.

This pattern will keep the daytime hours quite cool. It may be a
struggle to see 60 degrees Saturday. Since Sunday looking to be
cloudier/rainier, high temperatures for Sunday have been lowered.
Not much change for low temperatures, which should be kept near or
above 50 degrees due to high dewpoints.


An upper level low near the Delmarva Peninsula will gradually
pull away from the coast and move toward the northeast Monday
and Monday night. There is a chance for a couple of showers or a
thunderstorm Monday with a slight chance expected in the eastern
half of our region Monday night. Temperatures will be
slightly below normal.

High pressure will build into the region Tuesday through Wednesday.
Dry conditions and warmer temperatures expected each period.

By Thursday, a trough of low pressure will develop and bring the
next chance of showers and thunderstorms. Temperatures will be
above normal.

A cold front should slowly work its way southward across the region
Friday. The threat for showers and thunderstorms will exist along
and ahead of the front with temperatures remaining above normal and
increasing humidity.

VFR conditions this afternoon, and it should continue as such into
this evening. Rain spreads from southwest to northeast tonight. It
likely won`t affect the terminals until the overnight hours. Since
winds ahead of the rain should be easterly, anticipate a rather
sudden drop from VFR to IFR. Am not certain when this will happen,
but it should be complete by mid-morning Saturday. If may very
well be ongoing during the morning push. IFR conditions may
continue for a good portion of Saturday. The best to hope for will
be MVFR.

The pattern will remain wet through the rest of the weekend, but
not a washout. In general, majority of the time should be MVFR.
Periods of IFR certainly possible, especially during the overnight
and morning hours. Precipitation will become more showery, which
does mean local/brief restrictions possible.

Mvfr conditions possible with showers and thunderstorms around
Monday. Winds northwest around 5 knots Monday.
Vfr conditions possible at MRB, CHO, and IAD terminals Monday
night. Mvfr conditions possible elsewhere to the east. Winds
becoming light and variable Monday night.

Light and variable winds ongoing, and that should be the case for
much of the evening. low pressure will be approaching tonight, and
passing east of the waters tomorrow. Will have an increased flow
ahead of this low. Small Craft Advisories have already been
in place for the lower waters overnight and all waters Saturday.
Momentum transfer looks reasonable during the evening hours on the
Bay/mouth of the Potomac; have extended Advisory for then.
Shortwave energy will be swinging across the waters Saturday night
through Sunday, but will ample cloudcover, mixing will be poor.


MARINE...Small Craft Advisory from 8 AM Saturday to midnight EDT
     Saturday night for ANZ530>533-538>541.
     Small Craft Advisory from 4 AM Saturday to midnight EDT
     Saturday night for ANZ534-537-542-543.
     Small Craft Advisory from 8 AM to 7 PM EDT Saturday for ANZ535-


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