Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 232152

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
552 PM EDT Fri Jun 23 2017

Tropical air will envelop the area today. A cold front will
cross the area late tonight into Saturday. High pressure will
return Sunday and persist through early next week.


In spite of pockets of insolation, atmosphere within the forecast
area capped, as evidenced by the lack of agitated cumulus on
satpix and just showers on radar. The same cannot be said across
western Pennsylvania down the Ohio Valley, in vicinity of the
frontal zone. Storms in that region shear-driven, and that will
be our concern tonight.

Recent runs of HRRR/RAP both depict some enhancement of the
showers in Virginia. Given 1500-2000 J/kg SBCAPE, expect some
thunder to develop by mid-late afternoon. That area should be
scattered in nature and have a diurnal component to it.

By 21-22 UTC, the line of thunderstorms, and scattered storms
ahead of the line, will be approaching the Potomac Highlands.
With loss of daytime heating and residual inhibition, expect a
weakening trend in the strength of convection. But, the overall
coverage in the Highlands will be increasing. Precipitable
water above 2 inches suggests that heavy rain can be expected
within any thunderstorm.

Latest model guidance (12 UTC cycle) offers a quicker timing.
The southwesterly low level jet will be substantive (60-70 kt at
850 mb), so anticipate that some thunder can be expected tonight
even without daytime heating. As has been discussed the past
couple of days, the question remains whether these storms can
root in the boundary layer east of the Appalachians. If they
can, it will be busy, with low-topped rotating updrafts. While
heavy rain can be expected too, the system overall remains
progressive (quicker than previously modeled), which will
preclude greater issues.

Bottom line... have categorical PoPs crossing forecast area
between midnight and 6 am (likely beginning mid-evening), with
heavy rain possible. With dewpoints in the lower 70s, low
temperatures follow suit.


Latest GFS/NAM prog timing to be so quick that all precip will
be east of the forecast area by the start of the period. Am a
little bit caution of that solution, and have lingered some
precip into the Saturday morning hours. Then we`ll need to
assess how quickly front leaves area; there are some suggestions
in may get caught up across southern Maryland or the Virginia
tidewater. Otherwise, high pressure will be building across the
area this weekend, with no other weather hazards. Perhaps there
will be a reinforcing boundary Sunday, but moisture looks scant
for even clouds.

Accepted a MOS blend for temps. Sunday will be the cooler day,
with lower dewpoints also advecting in Saturday.


Surface high pressure sets to the west on Monday as a slow
moving upper level trough slides east into Tuesday with surface
westerly flow. Below normal temperatures, with showers and
thunderstorms possible during this period.

Conditions should become drier as surface high settles overhead
Tuesday night. The high will move offshore on Wednesday and continue
moving west into Thursday as flow becomes more southerly.
Temperatures will be on the rise as dry conditions continue into
Friday, when guidance suggest a boundary will bring showers and
thunderstorms over our area.


VFR flight condition across the terminals this afternoon. THere
is an outside chance of a shower or tstm, but odds much too low
for a TAF inclusion.

A cold front, infused with tropical moisture, will cross the
terminals tonight, most likely after midnight but before dawn.
At this point, kept restrictions at MVFR. There is a decent
potential of local/brief IFR, probably due more to reduced vsbys
in heavy rain. There may also be localized wind gusts. Where
these gusts aren`t realized, there is a 50-70 kt wind max
several thousand feet off the deck, so low-level wind shear will
be a concern.

The front will be east of the terminals by Saturday morning. VFR
conditions will prevail for the weekend.

Mostly dry/VFR conditions expected Monday into Wednesday as
surface high pressure settles in. Some scattered showers and
thunderstorms possible Monday afternoon into Tuesday as upper
air disturbance moves through, which could cause periods of sub-
VFR conditions.


Update...A Gale Warning is in effect for all waters late
tonight into Saturday morning. Showers and thunderstorms will be
capable of producing gusty winds on the waters during this

Southwest winds 10-15 kt with some gusts to 20 kt. Those winds
will be increasing ahead of a cold front tonight. Small Craft
Advisories remain in effect. There will be a 50+ kt low level
jet several thousand feet off the waters overnight. Do not
believe this will mix down in the gradient flow, but local
punches may near heavier showers. Marine Warnings may be
required tonight.

Northwest flow Saturday will contain better mixing. The
gradient will decease Saturday night. Have not extended the
Advisory further at this time.

Mostly dry conditions expected Monday into Wednesday as surface
high pressure settles in. Some scattered showers and
thunderstorms possible Monday afternoon into Tuesday as upper
air disturbance moves through. Winds gusts will be below the
small craft advisory criteria.


As mentioned above, precip waters will be exceeding 2 inches, so
heave rain expected in storms tonight. However, only 0.1 to 0.3
inches fell this morning, so soils were not saturated. Further,
storms look to be progressive. So, while heavy rain expected,
believe that the soil will be able to handle it overall, based
on flash flood guidance. Thus, will not post any Watches at this
time. Local minor flooding is possible, especially in the
Potomac Highlands.


Tides have started to spike, especially in the northern Bay,
where departures are now greater than a half foot above normal.
The early morning tide cycle will be susceptible for minor
flooding. Beyond that, northwest winds should lead to levels
returning close to normal.


MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>543.
     Small Craft Advisory from 8 AM to 6 PM EDT Saturday for
     Gale Warning from midnight tonight to 8 AM EDT Saturday for


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