Flooding in Virginia, November 11 to 13, 2020

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
926 PM EST Wed Nov 11 2020

.SYNOPSIS...
A slow-moving cold front will move through the region early
Thursday. High pressure will briefly build in from the Great
Lakes as the front stalls just to the south at the end of the
week, then another frontal system will likely impact the region
by the end of the weekend.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
The surface cold front remains west of the Appalachians this
evening with anomalously moist area ahead of it. Dew points are
well into the 60s to near 70. The precipitable water value of
2.07" on the 00Z IAD sounding is by far a record for the date
per SPC climatology. Shortwave trough, right entrance of jet
streak, a weak surface wave, and a sharpening frontal zone will
enhance lift across the area through the overnight hours. The
radar is filling back in as moderate to heavy rain pushes
northeastward from central and southwest Virginia. Instability
has actually increased (though is still meager) over the past
several hours ahead of the shortwave and moisture advection
continues. Thus some thunderstorms are embedded which are
enhancing rainfall rates.

It continues to look like the heaviest rain amounts overnight
will be focused from central Virginia to southern Maryland,
more or less where the Flash Flood Watch is in effect. There's
still some potential heavy rain elements may push farther
northward into the Baltimore metro, though latest radar/NWP
trends at this time suggest the watch area is in good shape.
Localized training convective elements producing 1"/hour rain
rates at times could ultimately end up in a few spots seeing
storm totals upwards of 5 inches, though the CWA average of 1-3"
is still the most likely range. The rain will gradually shove
to the southeast through the night, though many areas near/east
of I-81 will still be seeing at least some light rain through
daybreak.

In the breaks between the rain, some dense fog developed, though
expect visibility to be variable as rainfall increases mixing at
times. Expect overall visibilities to remain reduced through the
night though.

There will be gradual cold advection through the night as the
cold front drifts southeastward, with temperatures in the 50s to
the west, but remaining in the 60s in many locations east of the
Blue Ridge.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/...
A steadier rain will taper to light and intermittent rain or
occasional showers and drizzle Thursday morning as the lifting
mechanisms push eastward and drier air works in from the
northwest.

As Eta moves well southeast of the area Thursday night, some
overrunning moisture will work northward, so low clouds and
perhaps some light rain or drizzle will spread back across the
area, particularly east of the Blue Ridge. Northwest winds will
bring drying conditions and clearing skies by Friday afternoon.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
High pressure will be in place across the area Saturday before
exiting off the coast by late Sunday evening. This will keep
conditions dry and fairly calm for this duration. The remnants of
Eta are forecasted to remain SE of our CWA which will allow us to
remain dry for the early start to the extended period. A low
pressure system associated with an upper level trough situated just
north of the Great Lakes early Sunday will eventually drive a cold
front through our area Sunday night into Monday. A few showers
expected Sunday night for some of our western zones before migrating
eastward during the day on Monday. Rainfall amounts look to be
fairly light with a quick exit of the front. Winds will be moderate
on Monday with 10 m winds forecasted to be around 15-20 kts. Dry
conditions expected for most of Tuesday before a cutoff low could
drape across the NE US, allowing for a few showers possible across
central MD late Tuesday night.

Temperatures for the extended will start out in the mid 50s for
Saturday and eventually warm up a bit for Sunday and Monday into the
low 60s. Tuesday, after the cold front passes will inhibit some
colder temperatures. Monday night, below freezing temps are possible
for our western zones along with mid 30s to 40s elsewhere. Any
available moisture with the frontal passage could induce the chance
for wintry precip for those extreme western zones. Confidence is low
at this point. We only warm up about 10-15 degrees into the upper
40s to low 50s for the day on Tuesday.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
IFR to LIFR conditions will continue through the night due to
additional moderate to heavy rain. A few thunderstorms may near
CHO, but don't think they will have much direct impact. Fog may
be dense at MRB and CHO at times as well. As a cold front drifts
southeastward, some guidance suggests MRB could have improving
conditions late tonight, but this is uncertain. Otherwise, conditions
slowly improve Thursday, with MVFR or even VFR possible by the
afternoon. Expect low clouds (MVFR to perhaps IFR) to redevelop
Thursday night into Friday morning as low level moisture
returns. Light rain or drizzle is also possible. Skies should
clear Friday afternoon.

VFR conditions expected Friday night through Sunday with the
possibility for brief sub-VFR conditions Sunday night for some
western terminals depending on rain persistence into the area.

&&

.MARINE...
Southerly flow continues for a large part of tonight, with
occasional SCA conditions near southern Maryland. A few
thunderstorms with gustier winds are possible, but SMWs look
unlikely at this time.

The cold front will cross early Thursday morning. NW winds
behind the front may gust near SCA levels if some of the hi-res
guidance is correct, but the front will be slow moving and
winds may fall off a bit. A secondary NW push is possible
Friday, though the gradient is somewhat marginal then as well.

SCAs likely Sunday into Monday ahead of low pressure system moving
eastward with a strong cold front associated with it. No other
marine hazards are expected at this time.

&&

.CLIMATE...
The record high minimums are still in play for IAD and DCA for
November 11. BWI dropped to 58 degrees earlier, eliminating its
chance.

Day    IAD high/high min BWI high/high min DCA high/high min
11/11: 78(2006)/55(2002) 77(2006)/59(1970) 78(1949)/58(1970)

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch from midnight EST tonight through Thursday
     morning for DCZ001.
MD...Flash Flood Watch from midnight EST tonight through Thursday
     morning for MDZ013-014-016>018-504.
VA...Flash Flood Watch through Thursday morning for VAZ025-026-029-
     036>040-050-051-507-508.
     Flash Flood Watch from midnight EST tonight through Thursday
     morning for VAZ052>057-501-502-506.
WV...None.
MARINE...Flash Flood Watch from midnight EST tonight through Thursday
     morning for ANZ535.
     Small Craft Advisory until 1 AM EST Thursday for ANZ534-537-
     543.

&&

SYNOPSIS...KLW
NEAR TERM...ADS/KLW
SHORT TERM...KLW/ADS
LONG TERM...ADM
AVIATION...ADS/KLW/ADM
MARINE...ADS/KLW/ADM
CLIMATE...LWX

Hazardous Weather Outlook
National Weather Service Baltimore MD/Washington DC
933 PM EST Wed Nov 11 2020

ANZ530>543-DCZ001-MDZ008-011-013-014-016>018-508-VAZ052>057-
130245-
Chesapeake Bay north of Pooles Island MD-
Chesapeake Bay from Pooles Island to Sandy Point MD-
Chesapeake Bay from Sandy Point to North Beach MD-
Chesapeake Bay from North Beach to Drum Point MD-
Chesapeake Bay from Drum Point MD to Smith Point VA-
Tidal Potomac from Key Bridge to Indian Head MD-
Tidal Potomac from Indian Head to Cobb Island MD-
Tidal Potomac from Cobb Island MD to Smith Point VA-
Patapsco River including Baltimore Harbor-
Chester River to Queenstown MD-Eastern Bay-
Choptank River to Cambridge MD and the Little Choptank River-
Patuxent River to Broomes Island MD-
Tangier Sound and the inland waters surrounding Bloodsworth
Island-District of Columbia-Cecil-Southern Baltimore-
Prince Georges-Anne Arundel-Charles-St. Marys-Calvert-
Southeast Harford-Prince William/Manassas/Manassas Park-Fairfax-
Arlington/Falls Church/Alexandria-Stafford-Spotsylvania-
King George-
933 PM EST Wed Nov 11 2020

This Hazardous Weather Outlook is for the Maryland portion of the
Chesapeake Bay, Tidal Potomac River, and adjacent counties in
central Maryland and northern Virginia as well as the District of
Columbia.

.DAY ONE...Tonight

A Flash Flood Watch is in effect for much of northeastern
Virginia, the District of Columbia, and southern Maryland tonight.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

A Flash Flood Watch is in effect for much of northeastern
Virginia, the District of Columbia, and southern Maryland through
11 AM Thursday morning.

.SPOTTER INFORMATION STATEMENT...

Please report flooding and heavy rain measurements to the National
Weather Service according to standard operating procedure.

MDZ001-003>006-501>507-VAZ025>031-036>040-050-051-501>508-
WVZ050>053-055-501>506-130245-
Garrett-Washington-Frederick MD-Carroll-Northern Baltimore-
Extreme Western Allegany-Central and Eastern Allegany-
Northwest Montgomery-Central and Southeast Montgomery-
Northwest Howard-Central and Southeast Howard-Northwest Harford-
Augusta-Rockingham-Shenandoah-Frederick VA-Page-Warren-Clarke-
Nelson-Albemarle-Greene-Madison-Rappahannock-Orange-Culpeper-
Northern Fauquier-Southern Fauquier-Western Highland-
Eastern Highland-Western Loudoun-Eastern Loudoun-
Northern Virginia Blue Ridge-Central Virginia Blue Ridge-
Hampshire-Morgan-Berkeley-Jefferson-Hardy-Western Grant-
Eastern Grant-Western Mineral-Eastern Mineral-Western Pendleton-
Eastern Pendleton-
933 PM EST Wed Nov 11 2020

This Hazardous Weather Outlook is for eastern West Virginia,
northern and central Virginia, and central and western Maryland.

.DAY ONE...Tonight

A Flash Flood Watch is in effect for the Virginia Piedmont and
central Shenandoah Valley tonight.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

A Flash Flood Watch is in effect for the Virginia Piedmont and
central Shenandoah Valley through 11 AM Thursday.

.SPOTTER INFORMATION STATEMENT...

Please report flooding and heavy rain measurements to the National
Weather Service according to standard operating procedure.