Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 291910

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
310 PM EDT Fri Jul 29 2016

An area of low pressure will pull away from the coast this
afternoon and evening. A frontal boundary across the Carolinas
will return north Saturday. An area of low pressure will track
along the front on Sunday. The front will move southward again on
Monday, with high pressure building in early next week.


Radar across the area is quiet as of 2pm. Subsidence in the wake
of departing low pressure as well as slightly lower dewpoints are
contributing to the lack of activity. SBCAPE near 1000 j/kg per
mesoanalysis, so still believe a few showers or a thunderstorm
possible. Terrain circulations likely are the primary instigator.

Whatever develops should diminish upon sunset. However, 850 mb
return flow will arrive tonight as the front makes its return
approach. We will also have the leading edge of vorticity
advection arriving toward dawn from the Ohio Valley. Therefore,
shower or thunderstorm activity will become possible once again.
Have overnight chance PoPs west of I-95, highest in the mountains.


A trough axis will cross the area Saturday-Saturday night.
Anticipate moisture transport ahead of the trough, with the
southwest flow dragging the weak quasi-stationary boundary back
north again. So, we once again will be dealing with a marginally
unstable, sheared environment with weak surface and PVA forcing.
Therefore, expect conditional instability, the extent of which
will be predicated by how much daytime heating we will receive. In
the end, there is sufficient evidence to increase PoPs to
categorical, with locally moderate to heavy rain possible. SPC
outlook for Saturday carries a MRGL risk area to account for the
potential of wet microbursts. In addition, believe there once
again will be a local flood threat.

Believe the will be a break Saturday night, due in part to loss of
daytime heating. However, shortwave passage will also play a role,
so the timing may need to be adjusted. But, the lower heights
aloft likely to induce low pressure once again for Sunday. Again,
have focused highest PoPs with the diurnal cycle. That may need to
be adjusted.

High temperatures will be limited by lack of insolation and
anticipated rain Saturday, and slight cool thermal advection
Sunday. Minimal change anticipated for lows through the period.


The front that has been meandering over our area for the past
week or two looks like it will finally push offshore by Monday
night as a 1020+ mb surface high moves in from the north. Before
then, a few showers and thunderstorms will again be possible
Monday. But after that conditions look much more seasonably mild
and less humid through the middle part of next week.

The high will move offshore by the end of the week likely leading to
increasing heat and humidity once again.


VFR conditions through the late afternoon and evening hours.
Thunderstorms, if any, will be limited. There may be a slightly
better chance overnight as the recently departed cold front
returns as a warm front. However, odds still too slim to mention
at this time.

Probability for flight restrictions will improve Saturday as
shortwaves will act upon an increasingly warm, moist, and unstable
atmosphere. Still a bit too far out for specific timing. Have
focused on midday and afternoon hours in the TAFs, limited
restrictions to MVFR at worst. However, local downpours possible,
which could briefly create IFR or below. In addition, there also
will be a gusty wind threat.

Those storms likely to exit at some point in the evening on
Saturday. However, additional storms possible on Sunday...again
with possible flight restrictions.

Generally VFR expected Monday and Tuesday with northwesterly flow
around 10 knots.


Winds have diminished to 10 knots or less. Anticipate similar
speeds for the next several days.

Showers and thunderstorms return Saturday, some of which may
contain heavy rains and gusty winds. Additional thunderstorms
possible Sunday. Any hazards will come from these winds.

A cold front will finally push offshore early next week. A
brief period of Small Craft Advisory winds cannot be ruled out
immediately behind the front in northwest flow.


Water levels running about a half-foot above astronomical normals
at this time. There may be a slight decrease tonight, but that
will increase once again on Saturday as storms will return--
drawing moisture north. Sensitive locations may threaten minor
thresholds both Saturday and Sunday. The overnight tide cycle
will be the higher astronomically; that`s where the better risk




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