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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
301 PM EDT Tuesday Sep 30 2014

low pressure over the Great Lakes will track east across the
middle Atlantic region tonight and Wednesday. High pressure will
return to the East Coast later Wednesday and remain in place until a
cold front approaches the area Friday. A cooler and drier airmass
will follow for the weekend and early next week.


Near term /through Wednesday/...
Upper Cutoff low will gradually pull out of the Great Lakes region
and move across the middle Atlantic and northeast tonight and
Wednesday. Upper dynamics associated with the upper low will
continue to spread east across the forecast area. Laps analysis
shows the best instability west of the Blue Ridge and over the
northwest third of the forecast area where convective available potential energy are running at
500-800 j/kg with no cin. Best chances for showers and isolated
storms will occur over these areas and along the Mason/Dixon line.
Cooling aloft from the upper low will lead to some steepening of
middle level lapse rates. A few of the storms could produce small
hail and gusty winds but not expecting severe thresholds. Will
also keep wording out of the severe weather potential statement.

Low clouds and fog hung in long enough to limit instability from
cho to iad and areas east. A few breaks in the overcast will occur
for a few hours this afternoon but thinking is that destabilization
will be minimal across these areas. Have left probability of precipitation out of this
area but evening shift will have to monitor to see if anything can

Convective chances should diminish overnight although a few light
showers/sprinkles may be possible. Bigger story overnight will be
the potential for fog development. Low level moisture should
remain in place for fog development after midnight. Some of the
fog could be locally dense...similar to the patchy dense fog of
this morning. Have included the potential in the weather grids.

Upper low will pull away to the north and east on Wednesday. Fog
and low clouds will mix out by late morning/midday with a few
afternoon showers possible especially over the high terrain.
Afternoon highs from the lower to middle 70s can be expected.


Short term /Wednesday night through Thursday night/...
high pressure to the north nosing into the region will keep an
easterly low-level wind fetch in place Wednesday night into
Thursday. This will result in more clouds than clear sky through the
period...especially east of the Blue Ridge mountains.

As the high pressure moves out into the northern Atlantic...mid- and
high-level clouds approaching from the west will lower and thicken
in advance of an approaching trough Thursday night. A couple of
showers preceding this trough could approach the western slopes by
daybreak Friday...but the bulk of precipitation is expected to hold
off until Friday afternoon and night.


Long term /Friday through Tuesday/...
a seasonably strong upper trough and cold front will move towards
the middle-Atlantic Friday. Widespread clouds will continue to lower
and thicken through the day...with a band of showers preceding the
front expecting to move across the area during the afternoon and
into the overnight. Most areas east of the Blue Ridge mountains
should remain predominantly dry during the day. Models are still
divergent on timing/strength of the trough which will determine
exactly who gets how much rain and when. Forcing with the trough is
strong but there is not much instability to work while a
few rumbles of thunder cannot be ruled out the current thinking is
that widespread thunder is unlikely. Brief heavy downpours and
perhaps gusty winds in stronger/heavier bands of rain could be
realized especially if the trough ends up being on the stronger side.

Models are in better agreement by later Saturday after the front
moves through. A much cooler and drier airmass will move into the
region and stick around for the remainder of the weekend...with
temperatures falling to several degrees below normal...perhaps
isolated freezing temperature readings along the western ridges
Saturday night if winds slacken a tad quicker than guidance is
currently suggesting.

The next storm system approaches the area from the west early next
week but models are highly divergent on when it and its impacts
reach the area.


Aviation /19z Tuesday through Sunday/...
mostly VFR the remainder of today...still a few areas seeing MVFR
ceilings but those will continue to mix out. Scattered
showers/isolated thunderstorms will be possible the remainder of
this afternoon into the evening hours especially west of the Blue
Ridge and across the Mason Dixon line. Confidence in any storm
directly impacting a terminal site is low...therefore mention has
been left out of the tafs. Low ceilings and fog a good bet overnight.
Some patchy dense fog will be possible but not widespread. Low
ceilings and fog should mix out by late morning or midday Wednesday.

Ceiling and possible visibility restrictions late Wednesday night-Thursday am and again Thursday
night into Friday am with moist onshore flow portrayed in all the
guidance. Climatology is favorable for fog/stratus this time of the year
so that lends to moderate confidence. Showers/isolated thunderstorms in one or more
bands ahead of a cold front possible west/ gusty winds Friday-Friday night. VFR
Sat as winds turn out of the west or northwest.


winds are expected to remain below Small Craft Advisory criteria the remainder of
today through Thursday.

Solid Small Craft Advisory conditions looking increasingly likely late this week into
this weekend both ahead of /southerly flow/ and behind
/northwesterly flow/ a seasonably strong cold front. Winds/seas
gradually abate as the weekend progresses.


Lwx watches/warnings/advisories...
District of Columbia...none.


near term...krw
short term...dfh
long term...dfh

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