Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 141440 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
940 AM EST Tue Nov 14 2017

High pressure will move across the area through Wednesday. A
cold front will cross the region late Wednesday night into early
Thursday. High pressure will return briefly Thursday night into
Friday before a strong cold front moves through during the first
half of the weekend.


Made some significant changes to sky grids for today. Animation
of visible imagery shows stubborn low clouds which have been
expanding southeastward over the past couple of hours. Given
the strength of the subsidence inversion and relative light
wind flow from the north or north northwest with little
downslope component believe these clouds will be very difficult
to erode today. So, have made sky grids significantly more
pessimistic. Guidance not particularly helpful/useful this
morning, so evolution of when if clouds will break up is
uncertain. Lowered temps a couple of degs from earlier fcst.
Highs in the mid to upper 40s only rising a few degs from where
they are right now.


Transient ridging at the surface and aloft glides directly over
the area tonight. With light winds and clear skies, anticipate
patchy fog in the typically favored locations (river valleys,
central VA piedmont). Low temperatures will drop close to
freezing in many locations outside the major cities with a
favorable radiational cooling setup.

Wednesday will be a touch warmer than today but still a few
degrees below normal for high temperatures despite mostly sunny
skies for much of the day. The high moves offshore by late
afternoon, though, so high clouds should start filling in pretty
quickly from west to east by sunset.

Various GEFS/SREF/EC members are in good to very good agreement
on the timing of a cold front Wednesday night. There is only a
narrow zone of moisture in the low-levels, and that should cause
most showers to dwindle as they cross the mountains, though some
sprinkles or showers are still possible east of the higher
terrain (particularly near the Mason-Dixon line). Rainfall
amounts should be very light, generally a tenth of an inch or
less, and model guidance is in very good agreement on this as
well. Temperatures should be above freezing through the low
levels which will allow any precipitation to fall as liquid.

The surface front departs east of the area Thursday, but the
upper trough axis lingers. This will likely result in a mix of
sun and clouds, a few lingering upslope showers (rain, possibly
mixing with snow by afternoon) and perhaps a sprinkle east of
the mountains along the PA/MD border. With downsloping NW flow
and cooler air lagging a bit behind the front, temperatures
should be noticeably warmer Thursday as compared to previous
days (mid 50s to near 60).


Surface high pressure and a shortwave ridge aloft will be
traversing the area on Friday. While it will be cooler than
Thursday, temperatures should remain within several degrees of

The weekend will become more active as a strong low pressure
system lifts northeast from the Great Lakes and the trailing
cold front pushes toward the east coast. There is still some
uncertainty in the timing, but showers -- perhaps moderate or
briefly heavy -- will become likely sometime Saturday or
Saturday night. Thunder chances are low but not zero. This
system will have plenty of wind energy, although the strongest
winds (low level jet) prior to the front will not be able to
mix completely to the surface. The timing/magnitude of the winds
will need to be monitored, but breezy conditions are likely
behind the front in a cold air advection regime. Upslope snow
showers will be possible, and it`s not out of the question some
flurries could spill east of the higher terrain on Sunday.

Model spread in timing continues on Monday with differences on
how quickly high pressure builds to the east coast. Given a
cutoff upper low developing over eastern Canada, this process
probably won`t happen quickly, making chilly and breezy
conditions the more likely forecast outcome.


Some patchy fog has developed across the central VA piedmont as
well as northeastern Maryland early this morning. This fog
should abate as some stratus sneaking in from south-
central/southeastern Pennsylvania advects south, but levels
should be low-end VFR (3500-4500 feet or so).

VFR as high pressure crosses today with generally light north
winds. These winds will go calm tonight as the high moves
overhead. Patchy fog is possible after midnight for MRB/CHO/IAD,
perhaps BWI/MTN depending on how low level moisture profiles
evolve over the next 12-24 hours (NAM, as is typical, is more
aggressive with fog potential. MVFR most likely, but spotty IFR
can`t be ruled out toward daybreak Wednesday). VFR returns
Wednesday into Wednesday night. A spot shower or two can`t be
ruled out Wednesday as a cold front crosses, but probability of
flight restrictions appear minimal at this time.

Mainly VFR expected Thursday and Friday with winds around 10
knots or less. A strong cold front will bring showers and
potential for sub-VFR conditions sometime Saturday or Saturday
night. Gusty winds will also be possible with and following
this system.


Narrowed down SCA timing to 4-10 AM this morning with just
enough of a gradient/mixing combo around daybreak to probably
get a few gusts to 20 knots near Point Lookout/Tangier Sound
areas. Otherwise winds will be light through Wednesday night as
high pressure moves overhead.

SCA will likely be needed Thursday into Thursday night in
the wake of a cold front (NW flow). Winds will diminish Friday
as high pressure crosses the area.

A strong cold front will sweep through on Saturday or Saturday
night. SCA will likely be needed (at a minimum), with strong
winds possible both before and (especially) after the front.
There is still a wide range of possibilities regarding the
magnitude of the winds.


Tidal anomalies of around one foot are being observed near the
mouth of the Chesapeake Bay as of early this morning. Light
northerly flow should keep this excess water to our south
through much of the day today, but as winds go light tonight
into Wednesday, some of this may work its way back north.
Currently looks like action stage at Straits Point/Annapolis
during the middle of the week, but typically more aggressive
CBOFS has minor at both sites and can`t discount that


MARINE...Small Craft Advisory until 10 AM EST this morning for ANZ533-
     Small Craft Advisory until 1 PM EST this afternoon for ANZ534-


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