Area Forecast Discussion
Issued by NWS Sterling, VA

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478
FXUS61 KLWX 200755
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
355 AM EDT Sat May 20 2017

.SYNOPSIS...
A cold front will stall out to the south of the area through Sunday.
The front will then lift north into the area as a warm front late
Sunday before another cold front passes through from the west
Monday. High pressure will briefly return for Tuesday before another
cold front impacts the area Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 330 AM...cold front currently draped from west-to-east across
the northern half of the forecast area (i.e., just north of DC) as
depicted by surface observations and local area radar data. Front
will continue to drop south of the area through daybreak...and
remain south of the area through much of the weekend...leading to a
couple of days of below normal temperatures.

As the front remains to our south...low pressure over the plains
will eject northward into the Great Lakes area...while a surface
high sinks southward across the northeast beneath upper-level
ridging. These features will foster persistent onshore flow...with
extensive cloudiness...as a saturated marine layer develops between
2-4 kft AGL. Presence of cloud cover will greatly inhibit quality
insolation...with highs today 20+ F below yesterday. Cold front will
also shunt moisture to the south...with dewpoints dropping into the
low 50s by Sunday morning.

While most areas will remain dry...showers and a few thunderstorms
are possible closer to the cold front across the SW (e.g., mainly
Highland, Augusta, and Nelson counties) this afternoon. Any
thunderstorms that do develop across this area would weaken rapidly
as they move into a convectively-poor environment north of the
front.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Easterly onshore flow continues through much of Sunday as CAD wedge
continues to dominate with surface high pressure centered off the
NY/NJ coast. Once again...cloud cover will hinder insolation and
keep temperatures in the 60s.

Warm front will begin to lift back northward late Sunday...with
southerly return flow developing to its south. Rain chances return
into Monday as moisture quickly streams back into the area ahead of
frontal passage. Poor timing suggest limited instability ahead of
cold front. One exception could be across central VA...where some
destabilization could develop with a few thunderstorms possible.

We begin to dry out late Monday afternoon/evening as cold front
shifts away from the area.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
A deep, large scale trough axis will be dropping into the Central
Plains Tuesday, and sharpening as it heads toward the East Coast
Wednesday and Thursday. The only differences are in the fine-scale
details, which will drive whether a secondary cutoff low can form in
Appalachia (GFS) or the parent low rotates south (ECMWF).
Regardless, they dynamic nature of this system should be able to
induce cyclogenesis. Therefore, the midweek is looking wet.

Tuesday will be the driest day of the stretch, as weakening high
pressure passes over the area. Guidance has been consistent that the
day would start out sunny, but clouds would be advancing through the
day. Latest guidance, however, is suggesting that showers will be on
the doorstep of central Virginia by the end of the day. Occasional
showers will be the rule through Thursday. Areal coverage (scattered
vs numerous) will be the biggest challenge...something that can`t be
sorted out at this time frame. For the most part, will be
emphasizing a precip type of showers. However, as the potential
upper low nears, there should be sufficient instability for some
thunder as well.

The deepening cyclone should migrate toward New England by the end
of the week, for considerable improvement by Friday.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
Cold front will stall to our south through Sunday...with high
pressure sliding to our NE. This will allow for persistent easterly
onshore flow...which will keep the area entrenched with low clouds.
Long-lasting MVFR ceilings appear likely with cloud bases near 2-4
kft AGL. Warm front lifts northward into the area late Sunday and a
cold front will cross the area Monday. These features will allow
rain to develop with periods of sub-VFR with low ceilings.
Improvement is expected later Monday as cold front pushes east of
the area.

Tuesday should be VFR for most of the day. Showers will spread north
late Tuesday or Tuesday night, and will be scattered or numerous
through Wednesday. Flight restrictions quite likely, but details
uncertain this far out.

&&

.MARINE...
Winds will continue to increase over the waters through morning as
north/northeast winds develop with pressure surge translating across
the area behind departing cold front. Small Craft Advisory remains
in effect for the potential for low-end SCA gusts near 20 knots
through afternoon. Pressure surge weakens by evening...with winds
diminishing overnight.

Winds turn southerly late Sunday as warm front lifts back north
through the waters...wind gusts near SCA criteria are possible (the
low probability) as this occurs. Low-end SCA gusts remain possible
Monday as cold front slides across the area.

Transitory high pressure Tuesday will be replaced by showers
associated with an upper low Tuesday night into Wednesday. Gradient
flow not that strong, and mixing should be poor. Therefore, no
headlines anticipated at this time.

&&

.TIDES/COASTAL FLOODING...
Easterly onshore flow develops this weekend as high pressure nudges
into the area from the northeast. Flow turns southerly by Sunday
night as warm front lifts into the area. Water levels will gradually
increase today through Monday, with anomalies supporting
elevated/minor coastal flooding at Annapolis, Straits Point, and DC
by Sunday night.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 PM EDT this evening for
     ANZ531>534-536-537-539>543.
     Small Craft Advisory until 11 AM EDT this morning for ANZ530-
     535-538.

&&

$$
SYNOPSIS...MSE
NEAR TERM...MSE
SHORT TERM...MSE
LONG TERM...HTS
AVIATION...MSE/HTS
MARINE...MSE/HTS
TIDES/COASTAL FLOODING...MSE



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