Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221911

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
311 PM EDT Mon May 22 2017

A cold front will move through the area later today. High
pressure will briefly return tonight before low pressure
develops over eastern Carolina Tuesday and moves northeast off
the DelMarVa Tuesday night. Upper-level low pressure will move
toward the area Wednesday through Friday.


MCV is slowly moving through SE VA/NE NC this afternoon. That
feature will pull the last of this morning`s rain away from
southern MD. Cold front is currently near the spine of the
Appalachians but only slowly moving eastward. In between the
front and the rain/cloud-limited airmass across
Baltimore/Washington, some limited instability has been able to
develop due to insolation. As the front continues eastward,
scattered showers and perhaps a few thunderstorms may develop.
Have carried chance POPs eastward through this evening, but if
the stable airmass never scours out, combined with sunset, any
shower/storm may have difficulty reaching I-95.

If the front makes it through the area tonight, it won`t be by
much, but drier air should eventually filter in. Before it does
so, some patchy fog may develop, mainly in southeastern areas.
Lows will be in the 50s to lower 60s.


Tuesday`s forecast has become increasingly tricky due to a gap
between the synoptic and mesoscale models handling low pressure
development along the front to our southeast. Have chatted with
WPC, and they think the GFS/ECMWF could be suffering from
feedback, and think the NAM/WRFs simulations may prove more
accurate unless an unmodeled MCV pulls the surface low farther
north. With better precipitable water values to our southeast,
think heavy rain threat is limited, but areas from
Charlottesville to St. Marys could have a period of moderate
rain. It is more uncertain how much rain falls in the metros.
The low will be progressive though, with any rain ending for
most areas Tuesday evening.

A closed upper low will be moving south into the Mid-west Wednesday
while a sfc low and associated cold front moves eastward across the
Ohio Valley. A brief period of subsidence is expected across the Mid-
Atlantic Wednesday leading to mostly dry conditions for most of the
day. Weak instability and lift increase across the southwest by
late afternoon. If current trends hold, widespread rainfall will
occur Wednesday night as a boundary lifts northward into the
area. There may be some weak instability, resulting in a few
rumbles of thunder.

Have not strayed too far from model consensus for temperatures.
Diurnal ranges will be muted due to clouds and precipitation,
with lows in the 50s to lower 60s and highs in the upper 60s to
mid 70s.


A slow-moving low pressure system will allow for the chance for rain
showers and a few thunderstorms to linger across the region Thursday
into Thursday evening.

The chance for rain showers shift to the Mason-Dixon line and
adjacent areas Friday as the storm system finally departs northeast
into New England.

By Friday night and Saturday, weak high pressure will move into the
region. Dry conditions and a gusty breeze should ensue.

Rain showers develop in the region again for Sunday as the next
storm system pivots into the region. Temperatures will be warmer.


Cigs are lifting into the MVFR category at mid afternoon, and
are expected to rise to VFR by this evening. A weak cold front
will be crossing the area. Scattered showers are possible near
the front, and perhaps a thunderstorm, but any impacts would be
very local/targeted, so have only included VCSH in the TAF.

Tuesday forecast a little uncertain with guidance spread in
position of low pressure and moderate rainfall. Have hedged
toward a more optimistic solution of MVFR for now, but IFR isn`t
out of the question. A break in precipitation is expected
Tuesday night through most of Wednesday, although with continued
uncertainty of how ceilings will respond.

Showers and perhaps a storm are possible Wed night-Thu night as
a warm front moves across the terminals. Sub-VFR conditions
possible during this time.

Ifr conditions Thursday. Winds southwest around 5 knots Thursday.
Mvfr conditions Thursday night. Winds becoming west 5 to 10 knots
Thursday night. Vfr conditions Friday into Friday night. Winds
increasing northwest 10 to 15 knots gusts 20 knots Friday,
diminishing to 5 to 10 knots Friday night.


Winds have dropped to less than 10 kt. Mixing should be rather
poor today in vicinity of showers. Southerly flow will become
west/northwest this evening with a cold frontal passage. Will
have to monitor the potential of thunderstorms reaching the
waters this evening, although it is looking more doubtful that
any will reach the waters.

The pressure gradient will tighten Tuesday as low pressure
moves up the coast. Have raised a Small Craft Advisory for the
mid Bay and lower Potomac, where the gradient will be the best.
The winds should subside Tuesday night as the low moves away.

Low pressure will be moving west of the area Wednesday night
into Thursday, lifting a warm front into the area. Southerly
flow may reach SCA criteria.

No marine hazards Thursday night. Small craft advisories
possible Friday. No marine hazards Friday night.


Anomalies have continued to fall to around 1/2 ft except on the
Potomac. Flooding looks unlikely for the next couple cycles at
least. However, a coastal low Tuesday provides some
uncertainty. Guidance ensembles then diverge substantially
through midweek, although increasing water levels look more
likely by Wednesday night.


MARINE...Small Craft Advisory from 2 PM Tuesday to midnight EDT Tuesday
     night for ANZ533-534-537-543.


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