Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221835

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
235 PM EDT SUN MAY 22 2016

Low pressure will develop near Cape Hatteras tonight and head
northeast Monday and Tuesday. High pressure will slide
southeastward from the Ohio Valley on Monday into the southeast on
Tuesday and then move off the coast Wednesday, where it will
remain for the rest of the week.


As guidance progged, an area of rain has dropped south from
Pennsylvania into most of the forecast area today. One can see the
pivoting taking place on animated radar loops, as the 500 mb low
is positioned nearly overhead. On visible satellite pictures
cumulus clouds are developing on the western edge of the cloud
mass, which includes the Appalachians. Presently thunderstorms are
forming in West Virginia, but near term guidance suggests its
still possible for the mountains to get in on this. Have
transitioned database over to steadier rain, evolving to
occasional showers before days end. In addition, have preserved
the slight chance of thunderstorms for the western zones.

RAP/HRRR guidance suggests that there will be a decreasing trend
in radar coverage toward and especially after sunset. NAM/GFS
holds onto precipitation near the center of the low. That could be
drizzle. Regardless, there`s still a decent likelihood that
measurable precipitation may occur between midnight and dawn. So
PoPs will reflect an overall downward trend, but will remain quite
high in central Virginia and southern Maryland through the
nighttime hours.


The upper low will slip offshore Monday and begin its trek up the
coastline. Another coastal surface low will develop, and the
system will become vertically stacked. The forecast area will be
situated in a similar placement to where West Virginia is today.
There will be minimal instability (via CAPE) and better lapse
rates (6 C/km) between 850-500 mb. Anticipate redevelopment of
showers during diurnal heating, and cannot rule out thunder
either. Since mean layer RH will be a bit thinner, there will be
the potential for brief glimpses of sunshine, which will drive the
heating necessary for thunderstorms to form. MOS has been real
consistent suggesting that high temperatures will approach 70F
Monday, so will continue with that forecast.

Have a better chance that the nocturnal minimum will result in
showers ending overnight. Believe there still will be enough
influence from the upper low that skies will remain mostly cloudy.
In turn, that will influence low temperatures, keeping them a bit
warmer once again.

Ridging will begin to build across the area on Tuesday. Won`t
completely rule out a morning shower or afternoon thunderstorm along
the coastline. Instability looks a little better for convective
development compared to Monday. However, heights will be much
higher. Only the coast, nearer to the departing upper low, will
vertical velocity be enough to take advantage of this lift. Skies
will be partly sunny east of the Blue Ridge, due to mean layer
moisture. To the west, subsidence from the building ridge suggests
skies will be mostly sunny. The higher heights will also manifest
in the temperature forecast, which will feature a substantial the mid to upper 70s.


While temperatures climb into the 80s Wednesday, dewpoints remaining
in the M50s, on downsloping westerly flow, will keep it from
becoming too oppressive. Noticeable changes arrive late Wednesday
into Thursday, as surface high offshore to our SE promotes strong
moisture advection.

By Thursday afternoon, dewpoints forecast to rise at least 10F, with
M/U 60s dewpoints commonplace through the weekend. This will help to
increase afternoon instability into at least the moderate range,
though lack of lifting mechanism and nearby ridge axis aloft, will
likely keep thunderstorm activity diurnally driven and CI confined
to the higher terrain out west. Furthermore, lack of meaningful
steering flow could prevent thunderstorms from moving too far off
the higher terrain, but could also lead to a few hydro issues
(especially with increasing PWATs).

Places devoid of afternoon thunderstorm activity will remain hot and
humid, with afternoon highs at least 5-10F above normal. Increased
moisture will also keep nighttime lows elevated, with 60s across
most areas (near 70 in urban areas).


Flight restrictions prevail across the terminals. They have been
variable between MVFR and IFR, depending upon intensity of rain
and positioning of lower clouds. Will be in a similar pattern
through the night. Areal coverage of rain should become less,
especially MTN/BWI/MRB, but that could lead to drizzle and fog
instead. Suppose that LIFR possible, but have low confidence in

The upper low will begin its departure Monday, which should give
ceilings at least an opportunity to rise. Have VFR in TAFs by
afternoon. Confidence quite low in the details of its progression,
but believe we ultimately will get there...perhaps by afternoon.
In the heating that will result from the lifting clouds, there
will be the chance for diurnal showers, and maybe even a
thunderstorm. Renewed restrictions would result, which would be
more localized and of a shorter time duration.

Trends will continue to improve Monday night. Higher confidence
VFR by Tuesday as high pressure builds. Slim chance of a shower
along the Chesapeake Bay shoreline, which will be near the
periphery of the exiting low. This includes BWI/MTN.

VFR conditions Wednesday, with high pressure remaining in control.
While chances at any one location are low, a few thunderstorms are
possible Thursday and Friday, with locally sub-VFR in and around
storms, otherwise VFR expected.


Mixing will be quite poor through Monday, as low pressure remains
nearly overhead. Winds will below any critical thresholds. Flow
will be northerly tonight, but then become northwest in the wake
of the low.

While winds remain light through the end of the workweek, there is a
low-end chance for an isolated thunderstorm Thursday and/or Friday




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