Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 300714

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
314 AM EDT Thu Jun 30 2016

High pressure will build over the area this morning and move
offshore this afternoon and evening. A warm front across the
Carolinas will lift north tonight into Friday. A cold front will
move into the region Friday night. High pressure will return to
the Great Lakes and northeast for the weekend. A frontal boundary
is expected to hover over the Mid Atlantic Sunday night and


The synoptic pattern for the local area is quiet this morning,
and is expected to remain that way for the day. The nearest
frontal boundary is quasi-stationary across the Carolinas, and it
may slowly sink a bit further south today. There is still
sufficient positive vorticity advection across the Mid Atlantic
early this morning. The impacts of these two features overlap
across southern/central Virginia, manifesting in ample lower
clouds. This cloud deck has spread as far north as DCA as of 3am.
Do not anticipate these clouds to last locally much past sunrise
as the vort and front both depart, to the east and south

High pressure will be over the region today, but return flow will
commence by late in the day. Anticipate dewpoints will start to
creep up late in the day within a strengthening theta-e ridge,
but will be more tangible (into the mid and perhaps upper 60s)
tonight. As this occurs, instability and subsequently thunderstorm
chances will follow. Am keeping 20-30 percent chance for late
in the day today, generally southwest of the Potomac River, but
have scattered precipitation spreading north in isentropic
advection along the I-95 corridor in the evening. Will attempt to
preserve a diurnal component as CAPE only a couple hundred joules;
thus just have a few showers after midnight and a dry pre-dawn
period. Clouds should be plentiful, but the increased humidity
also makes fog a possibility by early Friday morning.


Friday`s forecast will be a little bit complicated. There will be
the isentropic ridge in the eastern (metro) part of the forecast
area. Meanwhile, a shortwave and surface cold front will be
crossing the northeastern US. Both features would be capable of
supporting convection given daytime heating. The former will be
pushing east ahead of the front, primarily affecting the Eastern
Shore/Delmarva; while the latter doesnt arrive until Friday night.
Area may end up in between the two, but wish to broaden the range
in forecast grids to account for uncertainty. Highest PoPs will be
in the east. Will introduce chances in the west for the afternoon
and early evening. While shear will be present, the other factors
either aren`t aligning well or are poorly timed. Thus, will be
forecasting scattered, non-severe convection. Will need to monitor
the strength of the shortwave/surface convergence though-- the
setup is quite similar to Tuesday.

The cold front forecast to push through Friday night. (How often
can one say that that for successive cold fronts in the Mid
Atlantic in early July?) Given support of numerous model runs, am
fairly confident that this will lead to a partly-mostly sunny
Saturday...even though this solution is a departure from
climatology. The only wrinkle may be the extent of high level
clouds, as the jet stream will be overhead.


Zonal flow aloft is expected Sunday as a stationary front is located
across the southern Mid-Atlantic region. The front will waver into
Monday and showers will be possible in vicinity of the front...
mainly across central VA and southern MD. The front is expected to
retreat northward as a warm front into Tuesday. Showers may increase
in coverage Monday night into Tuesday night. A shortwave trough may
impact the region Wednesday and Thursday. Temps should stay below
normal Sunday-Monday and near normal Tuesday-Wednesday.


VFR flight conditions should generally prevail today. There may be
a little early morning fog, but confidence in its extent/areal
coverage low, and have omitted it from the TAFs at this time.

A warm front will approach the terminals tonight. With it,
thunderstorm chances increase. Scattered areal coverage preclude
TAF mention at this time. However, the increase humidity may yield
areas of fog. Will only be forecasting local predawn MVFR Friday
morning, due in part to anticipated cloud cover. There is the
potential for it to be worse.

A cold front will approach the terminals on Friday, and cross
Friday night. Scattered thunderstorms will be possible, especially
during the afternoon and evening hours. Local IFR would be
possible within any thunderstorms, but their coverage in doubt at
this time.

High pressure/VFR returns for Saturday. VFR conditions expected


Light winds (less than 10 kt) prevail on the waters at this time,
and anticipate that will be the case for much of the day.
Northerly flow will gradually become south by late in the day. The
gradient will tighten on Friday as a cold front approaches.
However, do not have much confidence in any strong winds outside
of thunderstorms.

There are a couple of possible period of storms... tonight as
humidity returns, and Friday ahead of the cold front. The storms
on Friday likely will be stronger, and may be capable of producing
storms requiring Special Marine Warnings.

The cold front will clear the waters Friday night, with high
pressure returning Saturday. If mixing is strong enough, there may
be a window of opportunity for Small Craft Advisory gusts behind
the front Friday night-Saturday morning.

Sub-SCA expected on the waters Sunday-Monday.




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