Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 251108

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
708 AM EDT Thu Aug 25 2016

High pressure remains over the region today. A cold front will
cross the region on Friday. High pressure will return for the
weekend. Another front may slowly slide southward into the area
by the middle of next week.


High pressure is offshore this morning, extending to the southern
Appalachians. Decaying convection is moving into the upper Ohio
Valley. The remnants of the convection and associated vort max has
led to some timing changes in the forecast, albeit minor due to high
uncertainty. Some showers may reach the northern Potomac Highlands
shortly after daybreak, generally dissipating as they continue
eastward. Despite the diurnal minimum, some showers may survive in
an environment of theta-e advection/isentropic lift. Diurnal
convection along the terrain which was originally forecast, may be
minimized in the wake of the vort max, which will skim the northern
part of the area. It may be difficult for much to develop toward the
I-95 corridor due to lessening instability and the building upper
level ridge. Decaying convection originating from near Lake Erie may
make a run at the northern Potomac Highlands late this evening, but
should have a difficult time making it into our area. In summary,
convection may have more of a nowcasting component today, but
definitely lowering chances with SE extent across the area.

Otherwise, expect a good bit of cirrus originating from the
convection, at least for the first part of the day. This may take a
slight edge off temperatures, but locations from DC/CHO east have a
reasonable shot at 90F. Dew points will also be rising into the
lower 70s. For tonight, those higher dew points will prevent many
from falling below 70F.


Weakening cold front will be sliding south across the area on
Friday. There`s not a whole lot of airmass change with the front,
but its timing (of which there is some spread in guidance) could
have a role in temperature/dew point trends on Friday, as the flow
will be more northwesterly behind the front. Lower to mid 90s should
be common though. Heat indices will be in the 100-105 range east of
the Blue Ridge. Although there will be some weak confluence along
the boundary, there will be a lack of deep moisture, and the
strengthening mid/upper ridge will help put a cap on convection.
Have removed mentionable POPs except over the southern terrain.

After the front slides to our south and largely dissipates, high
pressure will build to the north. The northerly low level flow will
knock a few degrees off the temperature and dew point, because
otherwise, the mid and upper level ridge will nearly be centered
over VA. Above average temperatures and dry weather expected.


Guidance remains in relatively good agreement in the long term.
Sunday morning a ridge of high pressure aloft will be sitting
squarely over the region. A weak shortwave passing to the north
will weaken the high a bit on Monday but it will remain over the
region. Another shortwave may then slowly slide southward into the
area by Wednesday...possibly accompanied by a weak cold front.
While we will start mostly dry early Sunday with the surface high
sliding to the northeast...increasingly southerly flow will
promote moisture advection which will help some terrain convection
going. This will be aided by the weak disturbance and front by
Wednesday. This so far ignores any potential impacts of any
tropical systems which various pieces of guidance are depicting
impacting the United States. Uncertainty becomes very high as we
head into the middle of next week.


Increasing high clouds this morning, which should put a damper on
fog, although MRB is nearing its dew point. Today`s forecast is
somewhat uncertain in regards to convection, as activity currently
over Ohio should diminish as it crosses the mountains, although a
few showers could survive, especially to MRB. Very little storms may
redevelop however, and ones that do should be isolated. Mention will
not be included in the TAFs at this time. Due to possibility of
cloud cover and a light wind, fog formation for tonight is also in

A weak front will slip through the area on Friday, with little more
than a wind shift. While a rogue storm can`t be ruled out, it`s a
very low probability. VFR conditions expected Saturday with high
pressure to the north.

Mainly VFR in the long term.


Southerly channeling continues on the waters with occasional 20 kt
gusts. Hi-res guidance indicates that although there may be a lull
this morning, winds may strengthen again by afternoon. Thought the
simpler message would be to carry the SCA through the day and
evening. Gusts will be last to subside on the southern waters late

A weak front will slide through on Friday. Flow will gradually veer
through Saturday, eventually becoming E, as high pressure moves to
the north. At this time, sub-SCA conditions expected.

Winds mainly sub SCA in long term.


Increased southerly flow will cause water levels to rise today.
By tonight, caution stage will be reached at several sites on the
Bay. Annapolis could approach minor flood, although not all
guidance sources support that solution.

Winds will become west on Friday, so there should not be a concern
beyond the Thursday night cycle.


MARINE...Small Craft Advisory until 2 AM EDT Friday for ANZ531>533-
     Small Craft Advisory until 6 AM EDT Friday for ANZ534-543.


TIDES/COASTAL FLOODING...RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.