Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 211858

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
258 PM EDT SAT MAY 21 2016

Low pressure near the mouth of the Chesapeake Bay will move
northeast toward the Canadian Maritimes through Sunday. A second
area of low pressure will develop off Cape Hatteras Sunday; this
too will head northeast through Monday. High pressure will build
for the middle of the week.


Stratiform rain ended across the area by midday. A
shortwave/trough axis has been supporting a band of developing
convection this afternoon-- mostly showers but there are a few
areas of embedded thunder as well-- to the west of the
Appalachians. As the shortwave nears, heights will drop, which
will support a return of showery conditions late this afternoon
into early this evening. In addition, still expect a
reinvigorationof showers across northeast Maryland late today as
easterly winds in advance of the coastal low bring moisture
inland. Through sunset, likely PoPs will be confined to the
Potomac Highlands (70 percent) and metro Baltimore (60 percent).
Any rainfall totals will be less than this morning; generally 1/10
to 2/10th inch.

As the trough axis crosses the area later this evening into the
early overnight hours, it will bring sufficient forcing for likely
if not categorical PoPs to the entire area. Lift most favorable in
eastern West Virginia and northern Maryland.


The 500 mb trough axis sharpens Sunday into Monday as shortwave
energy continues to undercut the low. There will be enough energy
to spawn a second coastal low by late Sunday. Unfortunately, have
needed to make some pessimistic adjustments to the forecast.
Cannot accurately time showers specifically in such a pattern,
but am becoming increasingly convinced that there will be
measurable rainfall each day. Have increased PoPs to likely most
areas. Even have a bit of 80 percent probability in the Potomac
Highlands Sunday.

In regards to thunder chances...there will be a meager amount of
instability in the mountains Sunday (no shear). The CAPE setup
looks a little better areawide on Monday as heights will be lowest
at that time. Am keeping thunder a slight chance at best (for one
thing, dewpoints rather low), in the mountains with diurnal
heating Sunday and areawide on Monday.

Lowered high temperatures tomorrow given the expected cloud
cover. Although Monday won`t be a sunny day, there should be at
least a few glimpses. Rainfall won`t be all day, either. The
added insolation may be enough to bump temps upward. Guidance
fairly insistent on upper 60s/lower 70s. Low temperatures have
been consistently on the higher side due to atmospheric moisture.
No substantive changes made there.


Couple of quieter weather days Tuesday and Wednesday, as influence
of vertically-stacked closed low shifts off to the NE and mid-level
heights rise. Only real exception would be Tuesday morning, where a
few lingering showers are possible (especially across eastern
portion of CWA). Depending on how quickly we clear out, highs
Tuesday might finally be near normal (M/U70s). With the help of
downsloping westerlies we warm a few degrees Wednesday, though
remain dry, with most places into the L80s by afternoon.

By late Wednesday, developing Bermuda high offshore will begin to
promote southerly flow over the area, which will increase low-level
moisture rather quickly. Though, absence of large scale lift or
significant upper level feature through at least Saturday suggest
mesoscale forcing mechanisms will be primary driver of any diurnally
driven thunderstorm activity through Saturday. As such, best
thunderstorm chances will remain over the higher terrain...with
lower chances to the east. While instability will be moderate-to-
high, kinematic flow is poor (flow generally less than 30 kts at all
levels), which would keep any pulse-type stronger storms generally


MVFR/IFR conditions persist this afternoon. Steady rainfall has
pushed east, but there is plenty of low level moisture left
behind. In easterly flow, wouldn`t rule out a little drizzle,
although that`s not explicitly in the TAFs. There are also a
plethora of showers west of the mountains. The overall trend will
be for these to head east tonight. Timing problematic. Have
forecast IFR ceilings. Wouldn`t rule out brief sub-IFR vsbys, but
cannot skillfully forecast them at this time.

There won`t be much change in the pattern Sunday-Monday. Numerous
showers possible each day, but timing restrictions explicitly a
challenge. Have MVFR forecast through the end of the TAF. Lower
restrictions certainly possible. Further, ceilings should be on
the rise, perhaps reaching VFR category.

VFR conditions expected Tuesday and Wednesday with high pressure
remaining in control. A few isolated thunderstorms possible Thursday
afternoon as humidity increases over the area.

Winds did increase earlier today, and have justified SCA in
effect. Am keeping the current profile in place. After this
evening, mixing looks poor with the upper low approaching. That
covers the synoptic pattern for both Sunday and Monday. In terms
of direction, these winds will mainly be coming from the north.

With high pressure under control, westerly winds will remain light
Tuesday and Wednesday. Winds remain sub-SCA Thursday, though
they shift to southerly as Bermuda high develops offshore.

MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>534-
     Small Craft Advisory until 7 PM EDT this evening for ANZ535-


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