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Area forecast discussion...updated 
National Weather Service Baltimore Maryland/Washington District of Columbia
907 PM EDT Wednesday Sep 17 2014

Synopsis...
weak high pressure will move off the coast tonight. A larger area
of high pressure will build over the upper Midwest on Thursday...
while an upper trough axis passes overhead. The high will drift
across the middle Atlantic region to close out the week...with a cold
front expected to cross the area late Sunday into Monday.

&&

Near term /through Thursday/...
while one high pressure moves off the coast...another will begin
to build in from the west. The one moving off the coast will
actually bring some weather to the area on Thursday. Until then...the
position of the high currently is allowing the earlier cloud cover
to dissipate off and clear out over much of the area. Mild temperatures
from earlier this afternoon have slowly dropped off into the 60s in
recent hours...but will only drop down into the l-m50s.

Tonight and into Thursday...the eastern and western halves of the County Warning Area will
have some opposing conditions. While moving toward and eventually
off the coast...the light wind field will begin to bank relatively
moist air back onshore into early Thursday. This will keep temperatures
moderated over the I-95 corridor/metropolitan areas while the west will
see temperatures dropping back into the 40s - over the Appalachians.

Heading into the Post-dawn hours on Thursday...the low-middle level stratus
layer will rebuild again and help to develop some isolated
showers later in the day. The NAM is fairly aggressive west/ the
moisture increase - possibly too aggressive as it completely
saturates the column along and east of the Blue Ridge as if this were
a strong cold air advection/overrunning situation were occurring. The likely more
reasonable GFS and to a lesser extent/Euro making the convergence
zone bisecting the area moderately moist west/ broken sky cover and a
few isolated showers. The opposing wind field over the area will
allow the I-95 corridor to see temperatures poking back into the M-u70s -
though cloud cover dependent...and u60s/l70s over the shen
Valley/Mountains from a light northwest wind.

&&

Short term /Thursday night through Saturday/...
the cold front will drop to our south Thursday night before
stalling out over the southeastern Continental U.S. Friday through Saturday.
High pressure will build into New England Thursday night through
Friday before moving off the coast Saturday. The surface ridge
axis will remain over the region during this time.

The ridge axis should remain close enough to bring dry conditions
to the area during this time but clouds may hang around due to an
onshore component to the low-level flow...especially across the
Shenandoah Valley into the Potomac Highlands. An east to northeast
flow Friday will cause maximum temperatures to be below climatology...but winds
should turn toward the southeast Saturday allowing for warmer
conditions.

&&

Long term /Saturday night through Wednesday/...
high pressure will remain wedged southward between the Appalachian
Mountains and East Coast Saturday night...keeping dry conditions in
the region.

A cold front will move from the eastern Great Lakes to the East
Coast Sunday and Sunday night. There is a chance for showers along
the front west of the Blue Ridge Sunday and in all areas Sunday
night. Upper level support should pivot northeast into eastern
Canada by Sunday night...the front could stall across the Delaware-Maryland-Virginia
peninsula and Tidewater of Virginia briefly.

A reinforcing push of upper level energy Monday should push the
front offshore by late Monday. High pressure will gradually build
into the region Monday night through Wednesday to allow for the next
push of colder air.

&&

Aviation /00z Thursday through Monday/...
VFR conditions and light winds are expected into Thursday. Some patchy
valley br is possible late tonight/early Thursday...then a broken middle
stratus deck and some stray showers possible tmrw afternoon.

High pressure will return for Friday and Saturday...bringing more
VFR conditions. VFR conditions Saturday night through Sunday with
light and variable winds.

&&

Marine...
high pressure will slowly build toward the coast tonight...into
Thursday. An isolated shower is possible Thursday afternoon. High
pressure will build toward the waters Thursday night and Friday. A
pressure surge will cause north-northeast winds to increase. A
Small Craft Advisory is in effect for the Maryland Chesapeake Bay
and lower tidal Potomac River late Thursday night through Friday
morning.

The flow will turn toward the southeast later Friday through
Saturday. Winds may be close to Small Craft Advisory criteria at times...but gusts
have been capped at 15 knots for now since the gradient is
expected to weaken a bit.

No marine hazards Saturday night. Small craft advisories are
possible Sunday with an area of low pressure near the Tidewater of
Virginia and the main Stem Chesapeake Bay.

&&

Tides/coastal flooding...
tidal anomalies running close to a feet above astro norms at this time at nearly
all sites except in the upper Potomac. However...suspect those
departures will increase as well with the incoming tide /theyre at
low tide now/.

Based on these values...the coastal Flood Advisory looks good for St
Marys...Calvert...and Anne Arundel counties. Would need departures
of 1.1 to 1.3 feet for cbofs guidance to verify. Dont think we will get
there...and once again shaded forecasts under that guidance. Based
on that logic...Baltimore City will come real close to minor
criteria. Most likely scenario seems to be a peak just under
threshold. Will opt to not issue an advisory...but confidence not the
greatest. If trends prove otherwise...there may need to be a
short-fused advisory. Dont think Harford will rise above caution stage
due to their higher thresholds.

In the Potomac...hec-Ras guidance suggests a peak of 4 feet as
wasd2... which would be a 1 feet anomaly. Do not believe we will be
higher than that. Thus...the advisory wont be expanded up the
Potomac for this cycle.

A weak cold front will pass through the waters Thursday. High
pressure will build over New England behind this front. An easterly
component to the wind is expected to continue during this time and
it will likely last through the weekend. Therefore...minor tidal
flooding is possible near times of high tide through the weekend.
The best chance for minor tidal flooding will be during the late
evening and morning high tide cycle which is the higher of the two.
We would need 2 feet departures for MDT flooding to be realized. Do
not forsee that happening. If it did...Annapolis and St George
Island would be the most susceptible.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...coastal Flood Advisory until 4 am EDT Thursday for mdz014-017-
018.
Virginia...none.
WV...none.
Marine...Small Craft Advisory from 2 am to noon EDT Friday for
anz530>534-537-539>541-543.

&&

$$
Update...gms
previous discussion...bjl/klw
tides/coastal flooding...hts/bjl

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