Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 220115

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
915 PM EDT Mon Aug 21 2017

High pressure will remain offshore tonight and a strong cold
front will approach our area Tuesday before passing through
Tuesday night into Wednesday. Showers and thunderstorms are
possible ahead of this cold front. The front could stall to our
southeast, while a second area of high pressure builds in from
the west later this week.


Most of the convection from this afternoon and early evening has
dissipated. However...very humid conditions persist and a weak
surface trough remains near the Blue Ridge Mountains.
Therefore...a popup shower or thunderstorm cannot be ruled out
through late this evening.

High pressure will remain offshore overnight. A southerly flow
will continue to usher in warm and quite humid conditions. Min
temperatures will range from the 60s in the Potomac Highlands
to the mid and upper 70s in downtown Washington and Baltimore.
Most places should turn out dry overnight as the nocturnal
inversion strengthens. Patchy low clouds and fog are
possible...especially in sheltered valleys and areas that have
received rainfall from thunderstorms earlier.


A very warm day on tap Tuesday with highs in the lower to mid
90s. Will have to keep an eye on dewpoints Tuesday...because a
Heat Advisory may be needed for portions of the
area...particularly along and east of the Interstate 95
corridor. As of appears that the low-level flow may go
slightly west of south and this will cause winds to be better
aligned with height...increasing mixing. Therefore...dewpoints
may drop into the upper 60s and lower 70s. If this occurs...then
heat indices will be closer to 100 degrees. However...if not
then dewpoints would hold in the middle 70s causing heat indices
closer to 105 degrees. Will mention the possibility of heat in
the Hazardous Weather Outlook for now.

A cold front will move into the Ohio Valley by Tuesday
evening...bringing a chance of thunderstorms to the western part
of the forecast area late in the afternoon/evening. Given the
amount of CAPE that will be available severe weather is a
possibility, especially west of the Blue Ridge. SREF has been
consistent on weakening the storms after 00Z Wednesday as these
attempt to track east of the mountains - we`ll have to see.

The front is expected to move through the forecast area Tuesday
night into Wednesday morning. This will bring a drop of about
ten degrees to the region Wednesday, and the start of some very
pleasant weather that will go into the weekend.


High pressure passing from the Great Lakes into New England will
control the weather pattern over the Mid-Atlantic late this week
through the weekend. Temperatures and humidity will be below normal,
more reminiscent of late September than late August. Precipitation
chances will be minimal during this time.


VFR conditions are expected for most of the time through
Tuesday. Patchy fog is possible early Tuesday morning. Do have a
period of MVFR vsbys for the western terminals...but current
thinking is that alight gradient wind will prevent dense fog
from developing.

Thunderstorms will again be possible Tuesday night. MRB may be
the most at risk.

VFR again Wednesday.

Mainly VFR expected w/ light N flow as high pressure moves
to the north Thu-Fri.


Southerly winds will channel up the Bay and lower Tidal Potomac
River tonight. A Small Craft Advisory is in effect. There has
been a lull in the gusty winds this evening...but wind gusts
around 20 knots are expected overnight as the gradient

Another SCA is in effect Tuesday afternoon/evening ahead of an
approaching cold front.

Generally light flow is expected over the waters late in the
week as high pressure moves by to the north.


Tidal anomalies are around one half to three quarters of a foot
above normal this evening. These anomalies should hold nearly
steady through tonight with a southerly flow in place. The next
high tide overnight into early Tuesday will be the higher of the
two astronomical norms...and water levels will be close to minor
flooding thresholds. A Coastal Flood Advisory is in effect for
Anne Arundel County where confidence is highest near Annapolis.
Elsewhere...will have to watch Straits Point in St Marys
County...Baltimore and Washington DC. Did not issue an advisory
at this time since the surface flow may go just west of south.

A south to southwest flow will persist through Tuesday and into
Tuesday night. More minor flooding is possible near times of
high tide Tuesday night. An offshore flow will develop behind a
cold front Wednesday.


MD...Coastal Flood Advisory from 4 AM to 9 AM EDT Tuesday for
MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ531>534-537-
     Small Craft Advisory from noon to 11 PM EDT Tuesday for


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