Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 270747
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
347 AM EDT Tue Sep 27 2016

.SYNOPSIS...
A cold front will pass through early this morning before stalling
just to our south and east later today. Weak high pressure will
build overhead tonight. A strong upper-level low will impact the
area later Wednesday through Thursday night...bringing the
potential for heavy rain. The upper-level low will slowly move
away from the area Friday through the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
A cold front will slowly pass through the area early this morning
and it is expected to stall out just to our south and east across
the eastern shore of Maryland into central Virginia. A band of
showers along and ahead of the front near the I-95 corridor will
slowly slide south into southern Maryland by mid-morning. Patchy
fog has also developed ahead of the boundary early this morning.
Vsbys should improve behind the boundary as drier air filters in.

Weak high pressure will build in behind the boundary for most
areas today...bringing dry and seasonable conditions along with
increasing sunshine. The one exception will be across southern
Maryland where more clouds and showers will persist through much
of the day...closer to the boundary. Max temps will range from the
lower 70s in extreme southern Maryland due to more clouds to the
mid and upper 70s across most other locations.

Weak high pressure will remain overhead tonight while the boundary
remains stalled just to our south and east. Most areas will
remain dry and seasonably cool. Min temps will range from the
lower and middle 40s in the the Potomac Highlands to near 60 in
Washington and Baltimore as well as southern Maryland. The one
exception will be across extreme southern Maryland into central
Virginia. More clouds and even a few showers are possible closer
to the boundary. Patchy fog is expected due to radiational
cooling.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
A strong cutoff upper-level low will drop south from the Great
Lakes into the Ohio Valley and Midwest Wednesday and the
upper-level low will continue to drop south toward Kentucky and
Tennessee Wednesday night through Thursday. This system will
remain nearly stationary Thursday night.

This system will be strong enough to cause surface low pressure to
develop nearby over the Ohio Valley into the southeastern Conus.
Latest 00z deterministic guidance has come into much better
agreement and the 00z GEFS has also come to better agreement. This
increases the confidence for heavy rainfall and potential flash
flooding.

For Wednesday: A southerly flow aloft will allow for warm and
moist air to overrun the relatively cooler air in place. This will
cause clouds to quickly increase along with an increasing chance
for showers. The boundary that stalled to our south and east is
also expected to move into the area. This means that warm and more
humid air to the south and east will also move overhead.
Therefore...a few thunderstorms are possible as well especially
near and east of the Blue Ridge Mountains during the afternoon and
evening.

For Wednesday night through Thursday night: The upper-level low
will continue to slowly drop to our south and west during this
time. This will allow for subtropical air to move into the area.
Overrunning and a strengthening low-level jet will be the primary
lifting mechanisms for showers and possible thunderstorms. Locally
heavy rain is likely as subtropical moisture gets drawn into the
area. Also...with a south to southeast flow aloft there is the
potential for training convection over the same areas. Several
inches of rain are possible during this time. The exact location
of the heaviest rain remains uncertain at this time...but as of
now it appears that northern and central Maryland into northern
Virginia toward the Washington and Baltimore metropolitan areas
has the best chance for the heavier rainfall. Again...timing and
exact of location of heaviest rainfall remains uncertain so please
stay tuned to latest forecasts for updates. Despite the recent dry
conditions...flash flooding is possible during this time for
reasons mentioned above.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
GEFS family continues to converge on ECMWF solution...with upper-
level low to our west at 12z Friday. Low becomes nearly vertically
stacked Friday into Saturday as it begins to weaken and slowly
retreat to the north. While the heaviest rain should be winding down
during this time...shower chances (possibly still locally heavy at
times) continue until dry slot works into the area. When this occurs
at any one location will partially depend on the longitude of upper
level low as it retreats back northward (e.g., a further east/west
solution would mean an earlier/later end to rainfall). Thus,
locations across northeast Maryland have the best chance of rain
lingering into Saturday. OOz ECMWF/GFS is further west/east and ends
QPF Saturday/Friday.

Improving conditions across all areas late Saturday into early next
week with high pressure building into the area. Temperatures near
seasonable normals.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
A cold front will slowly pass through the terminals early this
morning. Patchy fog and areas of low clouds are expected ahead of
the boundary. Showers across the eastern terminals will gradually
shift south and east early this morning as well. IFR conditions
are possible until the boundary passes through.
However...confidence is low. The boundary should clear KMRB by 9z
and the other terminals between 9 and 11z.

VFR conditions are expected later this morning through this
afternoon. High pressure will remain overhead tonight...but patchy
fog is possible.

An upper-level low will impact the terminals Wednesday through
Thursday night. Increasing clouds along with possible showers are
expected Wednesday. Widespread showers and possible thunderstorms
are expected to impact the terminals Wednesday night through
Thursday night. Exact details remain uncertain at this time...but
locally heavy rainfall along with subvfr conditions are expected.

Sub-VFR conditions remain possible Friday and perhaps into Saturday
as as upper level disturbance brings showers to the region.
Conditions becoming VFR from south-to-north by late Saturday and
into Sunday as high pressure builds in.

&&

.MARINE...
A cold front will pass through the waters early this morning. Will
continue with the Small Craft Advisory until 6 am. Winds should
diminish behind the boundary due to a weakening gradient.

High pressure will briefly build overhead later today into
tonight. Low pressure will impact the waters later Wednesday
through Thursday night. Widespread showers and isolated
thunderstorms are expected during this time which will cause
locally lower visibilities. A small craft advisory may be needed
for portions of the waters Wednesday night through Thursday night.

Potential for Small Craft Advisory continues Friday. Winds fall
below SCA criteria sometime Saturday and remain below criteria into
Sunday.

&&

.TIDES/COASTAL FLOODING...
Winds will turn to the northwest early this morning behind a cold
front. This should cause tidal anomalies to decrease. Will
continue with the coastal flood advisory for the Annapolis area
early this morning...but water levels should fall below flood
stage by sunrise.

An onshore flow will develop tonight into Wednesday and it will
strengthen Wednesday night through late this week. Tidal
anomalies will increase and minor flooding is possible.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Coastal Flood Advisory until 8 AM EDT this morning for MDZ014.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 AM EDT early this morning for
     ANZ530>534-536>543.

&&

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


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