Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180736

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
336 AM EDT Tue Apr 18 2017


High pressure will build over the area through early Wednesday.
The next cold front will cross the area Friday. High pressure
builds over New England for the weekend while low pressure
moving in from the west will impact the Mid Atlantic Sunday.



As of 3am, surface high pressure is across the NErn CONUS and Ern
Canada under an upper trough (axis is Burlington, VT to Pittsburgh).
High clouds persist over the LWX CWA as we are in the right entrance
region of the SWly jet on the east side of the trough. The upper
trough axis will shift east off the northern Mid-Atlantic coast
around 12Z, which should make for a mostly sunny sunrise.

The trailing surface ridge axis will shift east across the area late
morning through midday. A light onshore flow will develop through
this time - the onset of cold air damming. Abundant sunshine will
overcome light mixing today, low 70s I-95 and west, upper 60s east
of I-95 where the onshore flow will be most notable (Bay water temps
are upper 50s).



Ongoing light rain over SWrn VA should enhance development of cold
air damming across the area by this evening. A good general rule is
24 hours from onshore flow onset to drizzle onset. Therefore, patchy
drizzle and chances for rain were added to the forecast Wednesday
afternoon. Max temps decreased to around 60F inland. The GFS and
ECMWF have the wind shifting S to SWly Wednesday night. However,
another good rule is to not believe models that erode CAD overnight.
Therefore, the rain and drizzle were maintained until 6am Thursday.

Low pressure develops as it shifts east across the Great Lakes
Thursday. An associated warm front will lift north across the
central Mid-Atlantic with a threat for elevated and becoming surface
based thunderstorms spreading in from the SW through the day. 12 to
14C 850mb temps overspread the area with MaxT in the 80s.



A cold front will be slowing southward through the Mid Atlantic
Friday. This will produce low chances for showers...but
temperatures should still be able to climb to the lower 80s
south/mid to upper 70s north.

Saturday an induced high pressure wedge will move over the
region allowing for a partly sunny and cool day. Highs will be
below normal - in the lower to mid 60s.

Sunday not looking too great: high pressure will move off to the
east allowing upper level low pressure to move into the
Appalachians. The atmosphere should be stable so am forecasting
the p-type as straight rain. Likely PoPs. Highs only in the
upper 50s/lower 60s.

The upper low may be slow to dislodge itself from the east
coast, keeping the area mostly cloudy Monday. Highs still in the



VFR under high pressure today as light Nly flow shifts east. Onshore
flow then into Wednesday night as cold air damming sets up. IFR
conds/rain/drizzle developing Wednesday afternoon and continuing as
late as Thursday morning.

A warm front lifts over the area Thursday with southerly flow and
scattered thunderstorms possible over the DC metros in the

Showers expected Friday, then VFR Saturday.



Light Nly flow becomes onshore today as high pressure moves east
over the area. Onshore flow then through Wednesday night as cold air
damming sets up.

Warm front lifts north through the area Thursday with afternoon/
evening thunderstorms possible.

Small craft advisory may be needed for Friday into Friday night
with wind gusts reaching near 20 kt.




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