Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210653

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
253 AM EDT Sat Oct 21 2017


High pressure will move east into the western Atlantic through
Monday. Low pressure and its attendant cold front will sweep
through the area Tuesday. Troughing will remain overhead for
much of the second half of the week.



Surface high pressure squarely over the area this morning will
begin to shift eastward into the western Atlantic by this
afternoon. Winds will be light and variable, then become
southeast this afternoon as the high moves offshore. With
ridging at all levels of the atmosphere, not much in the way of
any clouds are expected, with just a little cirrus spilling into
the region later this afternoon. Highs will be about 10 degrees
above normal, well into the 70s similar to recent days.



Ridging shifts eastward into the North Atlantic during the
second half of the weekend. Meanwhile, an amplifying ridge over
western NOAM will result in lowering heights over the eastern
half of the CONUS by late Monday. Southern stream shortwave
moving northeastward out of the lower Mississippi River Valley
will interact/phase with digging northern stream wave crossing
the Great Lakes resulting in the development of a surface low
which will bring widespread (much needed) rain to the area
starting late Monday night. Until then, only a modest and
gradual increase in clouds with above normal temperatures will



The big story in the long term will happen right off the bat as
a cold front crosses the region. Have upped the PoPs to 90%. The
question at the moment is one of timing - models actually appear
to favor the first part of the day for precip while the
numerical guidance is more in the afternoon. For now will keep
PoPs at 90 for the entire period. Fine tuning will come later.
If this does come through earlier this will limit thunderstorm

Highs Tuesday should reach the lower 70s. Afterwards Mid
Atlantic will be on the backside of upper level cyclonic flow.
This will bring temperatures back down to more normal later
October values: highs in the 60s, low in the 40s.



Patchy fog possible in outlying areas early each morning. Lower
ceilings possible by late Monday night as low pressure
approaches and atmosphere moistens. Showers develop west to east
late Monday night. Low-level wind shear is possible as well by
Tuesday morning as a southerly low-level jet (~40 knots at 2kft)
develops. Latest model guidance remains consistent and in
reasonably good agreement at this time range.

Tuesday will be an interesting day as a cold front brings
showers and isolated thuderstorms to all TAF sites. Periods of
sub-VFR ceilings will be likely, as well as the potential for
low level speed shear as well as a wind shift.

Conditions should improve Tuesday night and Wednesday.



Light winds are expected as high pressure remains in control
of the weather over the waters through the weekend. Small Craft
Advisories may be needed as early as Monday afternoon due to
tightening pressure gradient/increasing southerly winds ahead of
developing low pressure.

Tuesday winds will definitely reach SCA levels with the
potential for gales, especially on the wider part of the MD
Ches Bay. SCA will continue to be possible Tuesday night and




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