Flash Flood Warning 60, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
327 AM EDT Thu Sep 10 2020

.SYNOPSIS...
A cold front will slowly approach from the Great Lakes while a
tropical disturbance lurks off the Carolina coast through
this afternoon. The cold front will pass through our area
overnight into Friday before stalling out to the south through
Saturday. The boundary will return north as a warm front
Saturday night into Sunday before a cold front passes through
the area Sunday night into Monday. Canadian high pressure may
impact the area for the middle portion of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A southeast flow throughout the low and mid-levels of the
atmosphere has allowed for tropical moisture to advect into the
area around high pressure over the Atlantic. A weak upper-level
trough will remain in place between the subtropical high over
the Atlantic and another subtropical high over the Tennessee
Valley. At the surface, and inverted trough extends from
eastern Virginia northeast into southern Maryland this morning.

Showers and isolated thunderstorms have developed with shortwave
energy over the eastern shore into the Virginia Tidewater
region. This activity will move north and northwest into
southern Maryland toward the I-95 corridor early this morning.
PWATS over 2 inches along with some instability suggest that
heavy rainfall will accompany this activity, and this does pose
the threat for flooding and flash flooding. Farther northwest,
scattered to numerous showers are expected, but the threat for
flooding is lower due to a little less instability and less
low- level lift being northwest of the surface trough.

For later this morning and afternoon, more showers are likely
along with the possibility of a few thunderstorms. The most
widespread activity is expected near and east of Interstate 95,
closer to a surface trough. Again, with deep moisture in place,
heavy rainfall is the main threat from this activity and that
will increase the threat for flood/flash flooding.

Some guidance suggests that the surface trough may build a bit
farther northwest into north-central Maryland this afternoon
into this evening, and convergence along this trough may
increase a bit as the cold front approaches from the northwest.
At the same time, the upper-level trough may center itself over
this area as well. Therefore, there is a threat for heavy rain,
especially later this afternoon into this evening across north-
central Maryland, but confidence is too low for a headline at
this time since Flash Flood Guidance is higher and convection
to the southeast may rob some of this moisture during this time.
Will continue to monitor throughout the midday to see if the
Flash Flood Watch needs to be extended for these areas.

Overall, the forecast for total rainfall amounts is uncertain
given the convective nature. Average amounts of 1-3" are most
likely near and east of Interstate 95 with a half to one inch
farther northwest, and perhaps a little less west of the Blue
Ridge Mountains. Having that been said, with a tropical airmass
in place, rainfall rates around 1-3 inches per hour are likely,
and if there is training convection (which the pattern does
favor multiple storms over any one particular area), localized
rainfall amounts can be significantly higher. The 24 hour max
QPF from the 00z HREF ensembles (today and tonight) shows max
QPF with localized amounts around 4-8". This certainly cannot be
ruled out in this pattern.

Warm conditions with high humidity are expected with max temps
in the lower to middle 80s for most locations this afternoon.

The cold front will move into the region tonight from the
northwest. Drier will begin to filter into areas west of the
Blue Ridge and Catoctin Mountains, but a few showers may hang
around farther southeast ahead of the boundary. Do think that
the heavy rain threat across these areas will diminish overnight
as continental air begins to build back into the area (instead
of the tropical air mass).

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
The cold front will continue to pass to our south through midday
Friday, and drier air will push into the area from the north.
However, the bulk of the dry air is most likely to be near or
north of the Mason-Dixon Line. Therefore, a few showers are
possible, especially across central Virginia into southern
Maryland.

The boundary will remain stalled to the south Friday night
through Saturday, and again a few showers are possible,
especially across our southern areas into the Potomac Highlands.

The boundary will begin to move north as a warm front Saturday
night. More showers are possible along with low clouds and
patchy fog.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
A strong cold front will approach and affect our region from the
west on Sunday and linger through Monday morning. Surface flow ahead
of the front will be weak out of the east but a strong flow at 850mb
off of the Carolina coast will help transport moist aloft into our
region. This should help moisten the layer allowing more efficient
warm rain processes as the front moves through our region. There
will be a threat for some thunderstorms on Sunday but coverage and
strength should be limited due to lack of good daytime heating and
upper level forcing. Temperatures Sunday and Monday should remain
mild in the upper 70s to lower 80s due to cloud cover and on shore
flow.

The cold front should shift south of our region by Monday afternoon
and high pressure builds into our area from the northwest Tuesday
through the rest of next week. Daytime temps are forecast to return
back to the mid 70s with overnight lows in the 50s to lower 60s.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
Low clouds underneath the subsidence and nocturnal inversions
will result in IFR conditions most of the time through mid-
morning. Cigs/vsbys will gradually improve later today.
However, the likelihood for showers will persist during this
time, and with the tropical airmass in place any showers may
briefly reduce vsbys below 2 miles at any time into this
evening.

The showers should push southeast of the terminals later
tonight, and drier air will begin to work its way into the
area. However, there will still be plenty of moisture around so
more low clouds are likely. Current forecast shows MVFR
conditions since there will be some drier air working in, but
IFR or even subIFR cigs cannot be ruled out along with fog.

The boundary will stall to the south for Friday through Saturday
before returning north as a warm front Saturday night. VFR to
perhaps MVFR cigs are most likely later Friday morning through
Saturday, but lower clouds will likely return Saturday night.

SubVFR conditions expected Sunday into early Monday as a front moves
through all the terminals bringing periods of heavy rain and strong
winds. VFR conditions should return for all terminals by Monday
evening.

&&

.MARINE...
Showers and perhaps a few thunderstorms are expected into this
evening. Low-level convergence along with relatively warmer
waters through this morning and convection moving overhead may
lead to the development of a waterspout this morning. Also,
vsbys will be reduced below one mile at times in heavier showers
and thunderstorms.

A cold front will pass through the waters late tonight into
Friday, and the gradient will strengthen with high pressure
building to the north. Therefore, east to northeast winds may
gust past SCA criteria later Friday through Saturday night.

Small Craft Advisories may be needed Sunday and Monday as the front
moves through our marine areas.

&&

.TIDES/COASTAL FLOODING...
An onshore flow will continue to bring elevated water levels
through Friday. However, the flow will be light so the
probability for minor flooding is low at this time. However,
the onshore flow will strengthen later Friday into the weekend.
This may bring a better chance for minor tidal flooding.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch through this evening for DCZ001.
MD...Flash Flood Watch through this evening for MDZ011-013-014-
     016>018-504-506-508.
VA...Flash Flood Watch through this evening for VAZ052>057.
WV...None.
MARINE...Flash Flood Watch through this evening for ANZ535.

&&

SYNOPSIS...BJL
NEAR TERM...BJL
SHORT TERM...BJL
LONG TERM...JMG
AVIATION...BJL/JMG
MARINE...BJL/JMG
TIDES/COASTAL FLOODING...BJL

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
105 AM EDT THU SEP 10 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1234 AM     FLOOD            2 SE WOLF TRAP          38.93N  77.26W
09/10/2020                   FAIRFAX            VA   911 CALL CENTER

            OLD COURTHOUSE ROAD FLOODED NEAR BESLEY ROAD.

&&

EVENT NUMBER LWX2002431

JMG