Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 280755
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
355 AM EDT Fri Jul 28 2017

.SYNOPSIS...
Upper-level low pressure system will intensify as it moves into the
area through tonight. The low will slowly pass through the area
Saturday before gradually moving away from the coast Saturday night
into Monday. High pressure is expected to build overhead early next
week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Active near-term forecast as potent/compact upper-/mid-level low
dives south from the Great Lakes and interacts with nearby frontal
boundary to induce cyclogenesis over the area. Strong
frontogenetical forcing amidst an anomalously moist (PWATs surging
near 2 inches by afternoon/evening) and modestly unstable airmass
will foster convective elements that are very efficient rainfall
producers...with heavy rainfall likely over a rather large area.

In coordination with NWS AKQ...expanded the Flash Flood Watch just a
bit further south into central VA...as latest guidance suggests a
slight southward shift with the heaviest rainfall. Otherwise, Watch
appears to be in good shape, with most guidance suggesting a belt
of heavy rainfall across the area near/north of the surface low
track. PWATs near 2 inches suggest rainfall rates potentially
in excess of 2 inches per hour in the heaviest activity...with a
storm total of 2- 4+ inches possible over the Watch area.

In terms of timing...spotty shower and thunderstorm coverage through
early morning. Coverage expected to increase by afternoon, with the
heaviest rainfall expected from late afternoon into the overnight as
the surface low rides crosses the area as it traverses the frontal
boundary. While some areas may temporarily dry out early Saturday as
the area of precipitation near the surface shifts east of the area,
TROWAL precipitation (some of which may be of moderate intensity)
may shift south into parts of the area.

While Hydro issues are the primary threat today...some severe
thunderstorm potential does exist...with a Marginal risk from SPC.
The primary threat is isolated strong wind gusts due to modest
instability and relatively strong shear profiles.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
Surface low remains east of the area Saturday with TROWAL
precipitation remaining over the area (some of which could be of
moderate intensity). Soils expected to be saturated over much of the
area...so while precipitation Saturday should be a bit lighter in
intensity...any additional precipitation could exacerbate hydro
issues.

Cloud cover and nearby precipitation will keep temperatures in the
60s to L70s Saturday.

Drying out Sunday as low moves far enough to our east keeping
precipitation east of the area.
&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
High pressure will be building across the Mid Atlantic for the
beginning of next week, providing an extended period of tranquil
weather.  The only concern will be whether a cold front will drop
down into the area to provide a focus for showers/thunderstorms
Thursday.  There will be plenty of time to resolve that detail.

Temperatures will start the week mild-- almost cool-- for the last
day of July, and gradually warm through the week.
&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Low pressure will approach the terminals today before moving
overhead and intensifying tonight. Widespread showers and
thunderstorms are expected to develop with locally heavy rain and
subsequent flight restrictions at times. Some storms may also
contain gusty winds. More rain is likely Saturday as the low stalls
out near the DelMarVa Peninsula. Gusty north to northeast winds are
also possible during this time. Improving conditions Sunday as
surface low shifts well east of the area.

No aviation concerns anticipated under building high pressure.

&&

.MARINE...
Low pressure will approach the waters today before strengthening
overhead tonight. The low will likely stall out near the DelMarVa
Peninsula Saturday through Saturday night. Gusty winds are possible
with the low nearby...especially tonight through Saturday night.
Issued a Small Craft Advisory with this package for tonight through
Saturday night for the Bay and for the 2/3 of the Tidal Potomac for
the day Saturday. Gale force winds are possible Saturday
afternoon/evening...though probability of occurrence is currently
too low for a warning.

During afternoon mixing in the wake of low pressure, momentum
transfer may be sufficient for Small Craft gusts Monday. Aside from
that, no flags expected as high pressure builds.

&&

.HYDROLOGY...
A Flash Flood Watch remains in effect this afternoon through
Saturday afternoon for the northern 2/3 of the forecast area.
Most likely rainfall amounts during this time will be around 2
to 4 inches (with locally higher amounts). This may cause
instances of flash flooding.

PQPF from WPCs 70 member ensemble shows storm total precipitation of
1/3/5 inches at the 10/50/90th percentiles. Long-term nature of
the threat suggests that river flood risk could arise over the
weekend. Though, current MMEFS guidance suggest any threat could
be localized...particularly across the Potomac River Basin.

&&

.TIDES/COASTAL FLOODING...
Water level departures starting to ease lower...a trend that`s
likely to continue in light of anomalies of a half to 2/3rds of a
foot in the Virginia waters. While onshore flow should be a
mitigating factor today, the duration of such winds will be minimal,
and will be followed by northerly blowouts. In the end, believe that
tides will reach Action Stage at worst today, and that would be for
vulnerable sections of the coastline such as Straits Point and
Annapolis. This solution supported by not just ETSS but also SNAP-Ex
Ensemble guidance. Subsequent tide cycles should be unaffected.
&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch from 2 PM EDT this afternoon through
     Saturday afternoon for DCZ001.
MD...Flash Flood Watch from 2 PM EDT this afternoon through
     Saturday afternoon for MDZ003>006-011-013-014-016>018-
     501>508.
VA...Flash Flood Watch from 2 PM EDT this afternoon through
     Saturday afternoon for VAZ027>031-038>040-050>057-501-502-
     505>507.
WV...Flash Flood Watch from 2 PM EDT this afternoon through
     Saturday afternoon for WVZ050>053-055-501>504.
MARINE...Small Craft Advisory from 5 AM Saturday to 7 AM EDT Sunday for
     ANZ531>533-539>542.
     Small Craft Advisory from 10 AM to 7 PM EDT Saturday for
     ANZ530-535-536-538.
     Small Craft Advisory from 11 PM this evening to 7 AM EDT
     Sunday for ANZ534-537-543.

&&

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



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