Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 220057

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
857 PM EDT Wed Sep 21 2016

High pressure will build over the region through early
Thursday then weaken through Friday. A cold front will move
through the area early Saturday. High pressure builds again over
the region early next week.


Low pressure continues to spin across the coastal Carolinas.
Associated showers this evening have been confined to southeast
Virginia. With northeast winds, don`t believe there will be
support for these showers to spread north again; therefore, it
will be a dry night.

However, the same cannot be said with confidence regarding low
clouds. There are mixed signals in synoptic and mesoscale guidance
at this point. NARRE ceiling probabilities carry about 25% chance
of low clouds moving in overnight, to about I-95. Have made the
tonight forecast much cloudier, and am carrying this into Thursday

If a solid maritime deck does moves in tonight, it will be harder
to dislodge it in late September heating. That could affect the
Thursday daytime forecast. West of the stratocumulus, mostly sunny
still likely. Therefore, am making no additional changes at this


Weak high pressure in control during this period. Temperatures
are expected to warm up into the upper 80s and possibly reach 90F
in a few spots on Fri.


Surface cold front will likely have passed south of the region by
Saturday morning, being pushed along by a 1030+ mb high pressure
center originating near Hudson Bay Canada. The center of this high
and coolest air will pass over New England, but much cooler
weather is expected over the Mid-Atlantic as well especially
during the second half of the weekend into early next week.

Onshore flow may result in elevated humidity and low clouds and
drizzle especially early next week. The operational GFS runs have
been the quickest with an approaching cold front next week but have
been trending slower towards a GEFS/EC blend. A slower solution
seems more likely given the large blocky nature of the high pressure
center that will be stationed just northeast of the region next
week. This should keep more substantial rain chances at a minimum
through much of the forecast period.


Forecast currently not looking as clear-cut as it was earlier. An
IFR stratus/stratocumulus deck is moving inland, and has made it
to RIC as of 00Z. This is more aggressive than most guidance. NAM
seems to have caught on, but am not sure if its solution is now
overdone, as it plunges DCA into IFR for the morning push, and
brings the western edge to IAD and BWI for that same time frame.
Meanwhile, NARRE only indicates about a 25% chance of IFR
ceilings along I-95. Have therefore kept any IFR ceilings just
southeast of DCA/BWI, and thus out of the TAFs, but am giving the
potential much more weight than most models. This deserves close
attention tonight.

Once the low clouds and fog-- whatever there is-- burns off, the
remainder of the TAf period will feature VFR.

Mainly VFR expected Saturday with northerly winds 10 to 15
knots. Sub-VFR possible later in the weekend with lower clouds as
winds become easterly around 10 knots.


Winds will gust to around 15kt in the mid Bay through midday Thu
before they begin to diminish.

A pressure surge, aided by northerly channeling, will likely
push winds into Small Craft Advisory territory on Saturday. Winds
should gradually diminish and veer to the east Saturday night into


Water levels running less than a foot above astronomical normals.
Nonetheless this is proving to be enough to flirt with minor
inundation thresholds at sensitive locations. Forecast has been
running right on track...adding confidence in projected water
level at high tide tonight.

Current Advisory status...DC and Alexandria 11pm-2am, Anne
Arundel County til midnight.

Elevated water levels persist through the rest of the week due to
low pressure off the Carolina coast.


DC...Coastal Flood Advisory until 2 AM EDT Thursday for DCZ001.
MD...Coastal Flood Advisory until midnight EDT tonight for MDZ014.
VA...Coastal Flood Advisory until 2 AM EDT Thursday for VAZ054.


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