Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 230746

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
346 AM EDT Tue May 23 2017

Low pressure will develop over eastern Carolina today and move
northeast off the DelMarVa tonight. Upper-level low pressure
will move toward the area Wednesday through Thursday, and then
depart on Friday.


Latest surface analysis indicates that broad high pressure remains
across the eastern third of the United States early this morning.
Yesterday`s cold front can be found just offshore. Drier air really
hasn`t infiltrated too far east, with dewpoints along and east of
the Blue Ridge in the lower and mid 60s. This has resulted in fairly
widespread low clouds and/or fog. In those locations experiencing
fog, visibility hasn`t been consistently reduced to quarter-mile,
although am seeing some indications that may change. May need to
launch a Dense Fog Advisory for a few counties northeast of the
Potomac River. Will be monitoring obs and act accordingly.
Whatever does develop will lift after sunrise.

There has also been a stray shower or two in central Virginia.
Mesoscale guidance hasn`t been maintaining much. Moisture and
overrunning are both present though, so its worth an isolated PoP
through dawn.

HRRR/RAP/NAM/GFS all have slowed down precip onset today, and paint
a sharp northwest edge just inside of the forecast area boundary.
The responsible shortwave (there isn`t a distinct surface reflection
as of yet) is approaching the Tennessee Valley as of 3am. There is
copious moisture and vertical velocity across the southeastern
United States. As the moisture advects northeast, the warm conveyor
will encounter upglide from east/northeasterly surface flow. As the
shortwave hits the coast, cyclogenesis will develop, supported by
jet dynamics as well. That deformation band will be the feature
responsible for the rapid end to the precip.

Have categorical PoPs late afternoon/early evening for central
Virginia and southern Maryland. QPF will range between a quarter and
half inch, although locally higher amounts likely. There is
substantial uncertainty along the Maryland/Pennsylvania border.

The overnight hours will be dry in the wake of this low. But,
residual low level moisture, wet ground and rather high dewpoints
suggest that fog could once again be a concern.

Have not gone far from a model blend for temperatures forecasts
today. Highs will be trapped in the 60s today (matching nearby water
temperatures) under mostly cloudy skies and onshore winds. Lows
tonight will match the the 50s.


For Wednesday and Thursday, the forecast area will be affected
by a deep trough axis and closed upper low pivoting across the
eastern United States. The area will start out in a lull
Wednesday morning, then height falls, shortwave energy, and a
700-500 mb speed max will pave the way for showers to overspread
the area late afternoon and evening. The nighttime hours will
be wetter than the day, as that will be when the aforementioned
forcing features will be at their max. Would not rule out
elevated thunder either, but mean layer instability will be
minimal so doubt that there will be any storms rooted in the
boundary layer.

A second round of showers and thunderstorms will cross the area
Thursday afternoon and evening. The 500 mb low will be passing
through about this time, so there is a better chance at developing
instability and thus stronger storms. That said, stronger is a
relative term here. Daytime heating will be fleeting, so its unclear
whether stronger storms actually will manifest as strong storms.

Again, not straying too far from model consensus for
temperatures through the period.


Closed upper low will be moving off the southern New England
coastline Friday morning. Post frontal conditions expected across
the Mid-Atlantic region Friday with partly sunny conditions
expected. A few showers are possible Friday as upper level
energy rounds the backside of the upper level trough. Amts will be
scattered and light. High pressure will briefly move overhead
Fri night-Saturday and dry conditions are expected.

Unsettled conditions return Saturday afternoon as fast moving
shortwave trough moves across the region. A warm front will also
approach the Mid-Atlantic during this time and rounds of showers are
likely Saturday afternoon/evening through Memorial Day.

Temperatures will be near normal Memorial Day weekend.


Low clouds and fog (sub-IFR) have been lurking east of the terminals
early this morning. The next cloud layer is mid deck (around 8000
ft). The low deck has been wavering across DCA/BWI/CHO, resulting in
highly changeable conditions. Mesoscale guidance have been
struggling to capture current conditions, so there is substantial
uncertainty in its output. Drier air should be filtering east, which
may be why guidance hasn`t been exhibiting its usual bias of
outright tanking, especially at CHO.

Whatever does develop should lift in the morning, before onshore
flow ahead of a developing coastal low brings in another low cloud
deck. There is uncertainty at the timing and height of today`s
cloud. Am going on the more pessimistic side of options, which
introduces MVFR by early afternoon and IFR this evening.

Rainfall associated with this low will pull out this evening. But
believe that low clouds and fog will be left behind, and am taking
rather substantial flight restrictions (aob IFR) through the night.
These will gradually improve Wednesday. The cloud deck Wednesday
morning will be on the thin side. So, if it breaks apart, that rapid
VFR could result. Do not have high confidence in this solution.

Two additional periods of showers and thunderstorms anticipated
though midweek. Flight restrictions will be numerous, but there
should be some periods of embedded MVFR if not VFR.

VFR conditions expected Fri-Sat. Periods of rain moves back into the
terminals Sat-Mon.


Low clouds will result in poor momentum transfer though the midweek.
Coastal low pressure will move past the Delmarva this afternoon and
tonight. Am maintaining Small Craft Advisories for the mid Bay and
lower Potomac. There will also be opportunities for higher winds in
southerly flow Wednesday night (mixing will be in question) and on
Thursday in vicinity of thunderstorms.

NW winds expected Friday behind a cold front. SCA condtions are
likely. Winds diminish into the weekend and rain returns.


Water levels between a quarter and half foot above astronomical
normals. No issues expected today or tomorrow. However, deep low
pressure will cross the area Thursday, with southeast winds picking
up Wednesday night. The early Thursday morning tide cycle will need
to be monitored, as the preponderance of evidence suggests that
minor flooding will be a concern. Still considerable spread in model
solutions though.


MARINE...Small Craft Advisory from 2 PM this afternoon to midnight EDT
     tonight for ANZ533-534-537-543.


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