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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
739 PM EDT Wednesday Jul 30 2014

Synopsis...
surface high pressure persists over the mid-Atlantic. Upper level
low pressure over eastern Ontario continues to spin cool air into
the region. A coastal front will develop this weekend and last
into early next week.

&&

Near term /tonight/...
radar shows rain showers/thunderstorms and rain mainly in PA...then a split in convection
developing over northern WV...and a storm cluster diving south with
outflow in southwestern WV. The 21z hrrr shows almost this exact
placement...three hours from now. Timing differences aside...the
evolution shown in the hrrr is reasonable...with activity
continuing to slide across PA...and the WV activity falling apart
aside from the convective cluster in southwestern WV.

Will have to watch right along the Mason-Dixon line...and perhaps
slightly further south...throughout the evening for potential
development of showers. Thunder looks increasingly less likely as
we reach sunset...plus instability is more limited further
east...and already the cloud-to-ground lightning is half what was occurring an
hour ago.

Slightly increased probability of precipitation along the Mason-Dixon line with the 730 PM
update. Will probably be able to lower probability of precipitation further south in the
next couple hours if current trends continue.

Made no change to temperatures at this early stage. Still expect some
mainly valley fog to develop overnight...mainly south of
Martinsburg and west of the Blue Ridge.

&&

Short term /Thursday through Friday night/...
high pressure moves further offshore Thursday...with return flow
over the region. Not expecting any notable lifting mechanisms over
the region...so dry weather likely for most. Some terrain
circulation thunderstorm activity possible in The Highlands. Temperatures rebound
back into the 80s for most areas...including possibly the upper
80s in some urban locations.

A more unsettled pattern sets up Thursday night through Saturday.
500mb trough over the Ohio Valley strengthens. An offshore
stationary front may retrograde westward...with a wave of low
pressure developing along it by Friday night. Periods of showers
and thunderstorms will be possible Thursday night through Saturday. Daytime
temperatures Friday and Saturday should hold in the low 80s due to cloud
cover. Low temperatures should be near normal.

&&

Long term /Saturday through Wednesday/...
upper level trough over the Great Lakes/Mississippi Valley will
slowly fill and shift off Atlantic coast by Saturday afternoon.
Surface low pressure trough along the Atlantic coast...positive vorticity advection
associated with 500mb trough over the Ohio Valley and moisture
brought into the County Warning Area by south/southwest winds associated with Bermuda
ridge will support development of showers/thunderstorms Saturday and
Sunday. Not so much Monday and Tuesday when dry airmass associated
with high pressure centered over the Midwest builds into County Warning Area.
Chances of precipitation look to increase again at midweek when a
cold front from Great Lakes approaches.

&&

Aviation /00z Thursday through Monday/...
VFR conditions expected through tonight...except at kcho and
perhaps kmrb where a brief period of MVFR fog is possible early
Thursday morning. Light S-SW flow expected through Thursday
afternoon.

Periods of showers or thunderstorms possible Friday through early next
week...but VFR conditions likely in between any shra/tsra.

&&

Marine...
winds generally 10kt and less over the waters this evening. Low level jet
is expected to develop tonight with 15-20kt S-SW winds aloft. Not
expecting this to gust down at this time as the inversion sets
up overnight...but will keep an eye on it. Southerly winds
expected again Thursday. No Small Craft Advisory expected...but some gusts in the
teens possible near the confluence of the tidal Potomac and ches
Bay.

Southeasterly winds Friday through Sunday with large high
pressure in the western Atlantic...but close enough to keep winds
fairly light. Periods of rain showers/thunderstorms and rain possible each day.

&&

Climate...
chilly morning with a record low of 48 at Dulles. This is the
coolest morning low in July since July 12th 2002. Not to be
outdone...even in the urban center...the morning low of 60 at Reagan
was the coolest in July since that same date in 2002.

At Dulles...we are on track for the coolest July since 2001...and
the year-to-date average temperature of 51.5 degrees is the 5th
coolest on record /dating back to 1962/.

At BWI...a record low was set of 55 degrees...the coolest in July
since July 3rd 2007.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...none.
WV...none.
Marine...none.

&&

$$
Synopsis...KS/je
near term...je
short term...Kansas
long term...cem
aviation...je/KS
marine...je/KS
climate...je

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