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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
308 PM EDT Friday Jul 3 2015

low pressure will advance east-northeast up the Ohio Valley
tonight. A secondary low will develop over Virginia and head
east-northeast into the Atlantic Ocean on Saturday. High pressure
will build across the area on Sunday. Another low pressure will
move into the area Monday and linger in the area for a few days.


Near term /through Saturday/...
as of 18z...a stationary front was located along the borders of
North Carolina and Virginia...stretching west along the borders of
Kentucky and Tennessee...and the borders of Missouri and
Arkansas...finally terminating in Oklahoma. A wave of low pressure
was located along this front near the Kentucky/Tennessee
border...while high pressure dominated the Great Lakes.

Most guidance depicts this low as moving east-northeast up the
Ohio River valley towards southwestern Pennsylvania
tonight...with a secondary low developing over Virginia and
heading east-northeast into the Atlantic on Saturday. This is
certainly more of a winter-like synoptic pattern than what you
would expect to see in July...but the results will be fairly
similar...with some warm advection/isentropic lift induced rain
moving into the region tonight...then perhaps dry slot with
lingering drizzle developing over the area on Saturday. As the
upper trough associated with this system moves east across the
area later Saturday...forcing will likely allow showers to pop
back up across the area. Given the much more humid air mass
compared to winter...and the unusual dynamics for July...isolated
thunderstorms are a possibility...but enough stability appears
present to prevent any sort of strong to severe storms from
developing. There are hints in some guidance of a significant
upslope flow producing excessive rain across our northwestern
zones...but confidence is not high and some modeling solutions in
fact include these areas in the dry slot later on causing them to
miss out. Therefore...decided to hold off on any flood watches
until there is a better signal for excessive rain. Lows tonight
will drop back into the 60s in most areas...while highs on
Saturday will stay mostly in the 70s thanks to
clouds...drizzle...and showers.


Short term /Saturday night through Sunday night/...
low pressure moves away from the area Saturday night...but upper
trough may still kick off a few showers. Otherwise...conditions
should generally be improving...but if there is clearing late some
patchy fog may develop. Lows will again be in the 60s.

Northward progression of the frontal boundary hovering near the
Virginia/NC border will be minimal on Sunday as high pressure builds in
at the surface under a developing ridge aloft. Low pressure riding the
front should exit the area by 12z sun...with dry conditions then
expected the remainder of the morning. Depending on where exactly
the front aligns...a stray shower or thunderstorm could move into the
southern portion of the County Warning will just continue with the chance
probability of precipitation for now. By Sunday evening...12z GFS and European model (ecmwf) both
indicating ridge axis aligns far enough east to allow progression
of a southern stream shortwave to progress along the Appalachians.
Still unsure how far north the precipitation will progress during the night
with any surface boundaries looking to linger further south...and not
pushing north till the beginning of next work week.

Warm and moist airmass in place during the day Sunday. 850mb
temperatures right around 13-14c...with most of the area pushing into
the 80s. Dew points generally hovering in the middle to upper 60s.
Enough instability should be generated to result in
thunder...wherever any convection does develop. However...lack of
shear should keep thunderstorms below severe levels.


Long term /Monday through Friday/...
upper level cut-off low opening up and flow becoming zonal over
middle Atlantic region as trough on northern track moves eastward.
Next trough to affect region Friday. Areas of weak to moderate positive vorticity advection
present through the forecast period.

Surface low pressure trough over region will slowly fill through
Tuesday night and be replaced by weak high pressure ridging.
Dynamics support convective development during the day Monday with
drying overnight. More development due as a front from the west
crosses the region Wednesday through Thursday night.


Aviation /19z Friday through Wednesday/...
storm system moving in will result in widespread rain showers with
ceilings and visible lowering to IFR at times late tonight and Saturday
morning. Conditions should improve during the day Saturday as the
low begins to move away. Thunderstorms are possible this evening
and again Saturday...but coverage is expected to be no higher than
isolated. Winds should generally remain below 10 knots through the
next 36 hours. Some potential for reduced ceilings/visible if patchy fog
forms in the wake of the rain Saturday night.

MVFR ceilings possible initially Sun morning...with conditions improving
to VFR at all taf sites during the day. Kcho has the best chance of
seeing any precipitation sun-Sun night. Winds will be light...less than 8

VFR conditions should prevail through next week. Showers and
thunderstorms will remain a concern.


low pressure passing to the south will intensify winds up to low-
end Small Craft Advisory levels tonight into Saturday morning. Otherwise...main
concern is lowered visibility in rain showers tonight into
Saturday. Thunder is possible but should be isolated. Winds should
diminish Saturday night.

Weak pressure gradient will result in a light flow sun-Sun
night...keeping sub-Small Craft Advisory conditions in place. Slight chance of showers
Sun night...though with minimal impact on visibility.

Sub-Small Craft Advisory conditions should prevail through next week. Showers and
thunderstorms will remain a concern.


some guidance is showing some significant rainfall across the
Appalachians. Thought was given to the potential to issue a Flood
Watch...but at this time feel precipitation will fall slowly
enough and be light enough to not cause significant flood
concerns. Will continue to monitor.


Tides/coastal flooding...
easterly flow around a passing low pressure system will help
elevate water levels along tidal portions of the Potomac and the
Chesapeake Bay shore. At this time think levels will stay just
below minor flood...but coastal flood advisories would be
necessary with only a modest increase in projected water levels.
Will continue to monitor.


Lwx watches/warnings/advisories...
District of Columbia...none.
Marine...Small Craft Advisory from 6 PM this evening to 6 am EDT
Saturday for anz534-536-537-543.


near term...rcm
short term...Sears/rcm
long term...cem
tides/coastal flooding...rcm

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