Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Wakefield Virginia
1057 am EDT Monday Apr 21 2014

Synopsis...
low pressure off the Carolina coast...will track slowly northeast
into the Atlantic through today...as high pressure slides off the
New England coast. A cold front will cross the area Tuesday
afternoon into Tuesday night. High pressure returns for Wednesday
and Thursday.

&&

Near term /until 6 PM this evening/...
latest water vapor imagery shows southern stream upper low located off
the southeast coast...slowly pushing out to sea. At the
surface...coastal low remains anchored off the Carolina coast as high
pressure remains ridged over the local area (ongoing NE flow). Low
clouds/stratus still hanging on over southeast Virginia/NE NC...but will thin/erode
by late morning. Sunny elsewhere. Radar returns remain confined
off the NC coast...and moving offshore.

For today...southern stream speed maximum will begin to push the upper low
and associated moisture farther offshore...resulting in dry
conditions and sunny-mostly sunny skies. Split flow over the
Continental U.S. Will feature a northern stream shortwave over southern Canada and a
southern stream disturbance over the Southern Plains. These features will
begin to phase over the central Continental U.S. Today as they March
eastward...pushing an associated cold front out in advance. Amplifying
flow will build a shortwave ridge over the local area today with
the upper flow becoming northwesterly this afternoon. However...low
level/surface north-NE flow will persist. Increasing low level
thickness/warm air advection will warm temperatures into the low 70s over interior
Virginia/nc(regardless of northeasterly flow)...but NE flow near the coast will only
produce temperatures in the middle-upper 50s.

&&

Short term /6 PM this evening through Wednesday/...
cold front tracking into the Ohio Valley Monday eveng...will push east and
over the mountains Tuesday morng. That front will bring increasing clouds from
west to east Tuesday...along with a chance for showers. Most of if not all the
showers may hold off until the afternoon west...and toward late afternoon/eveng east
on Tuesday. Will maintain a slight chance of a thunderstorm...though dynamics
weakening with time...as the front enters the area. Low temperatures Monday
night will range from the middle 40s to near 50. Hi temperatures on Tuesday will
range from the middle to upper 70s inland...to the upper 60s to lower 70s at
the CST.

The cold front will push out to sea Tuesday night...with significant drying
and a period of cooling thereafter (fm aftr midngt Tuesday night through wed).
Maintaining 30-40% probability of precipitation eastern portion of the forecast area Tuesday eveng...15-25%
elsewhere...then clearing aftr midngt...as winds become northwest. Near
seasonable/clear-prtly cloudy weather then expected Wednesday with gusty northwest
winds...mainly eastern portion. Low temperatures Tuesday night ranging from the middle
40s northwest to the lower 50s southeast. Hi temperatures on Wednesday will range through the 60s.

&&

Long term /Wednesday night through Sunday/...
progressive flow will result in near seasonable normal temperatures and
generally dry conditions through the extended period forecast. A
cold front will bring a chance of showers to the region Friday
afternoon into Friday night.

Trough axis will push off the NE coast Wednesday night as an upper
level ridge builds eastward over the central-eastern Continental U.S.. at the surface...cool
Canadian high pressure will build southeastward from the Great Lakes into
the middle-Atlantic region Wednesday night-thurs. This will result in
another cool day Thursday (highs generally in the middle-upper 60s) as
flow remains NW-W. Surface high will slide over the local area Thursday-
late Thursday...locating offshore Thursday night. This will veer the
flow around to the south as the upper ridge axis builds overhead.
Medium range guidance is coming into better agreement...but
continue to struggle with the evolution of an upper trough over
the intermountain west into the northern plains Thursday-Thursday night.
However...guidance is in fairly good agreement with lifting a warm
front over the region in the amplifying flow. Trailing cold front
forecast to cross the region Friday afternoon. Westerly flow over the
Southern Plains into the Gulf states will limit the amount of moisture
advection into the region...especially east of the Appalachians.
Dewpoints only prognosticated to warm into the low-middle 50s Friday. Best
dynamics will also remain north of the local area as zonal flow
prevails over the local area. However...increasing speeds aloft
and small amplitude shortwaves will be enough to mention slight
chance-low end chance probability of precipitation along the front at this time. With the local
area in the warm sector Friday...temperatures will warm into the middle-upper
70s (near 80 inland). Warm temperatures combined with low level Theta-E
advection will maintain a marginally unstable airmass over the
region Friday afternoon. Thus will carry a slight chance of thunder.
Southern portion of the front will likely stall over the Carolinas Friday
night-Sat in westerly flow aloft. Cool Canadian high pressure will be
slow to build over the region as the trough remains over the NE
states through the weekend. This will result in another warm day Sat
with highs in the middle-upper 70s in westerly flow. Cooler Sunday as cool
Canadian high pressure finally builds southeastward over the region.

&&

Aviation /15z Monday through Friday/...
the pressure gradient between high pressure over New England and low
pressure well off the Carolina coast will continue to slowly
slacken through the taf period. Have had a brief window of IFR at
orf/ecg this morning, and accounted for that through the next few
hrs, with ceilings quickly improving to VFR by 15-17z. Some gusts to
20 knots through midday before easing as gradient slackens.

Outlook...some patchy fog is possible late tonight/ Tuesday morning
but current probs do not point to widespread IFR. A cold front
moves across the region late Tuesday...and there is a good chance
for showers and a few thunderstorms Tuesday afternoon and evening with
a brief window of flight restrictions possible. High pressure
returns Wednesday and Thursday. There will once again be a chance
for showers and thunderstorms on Friday.

&&

Marine...
latest observation reflect continued NE winds over the waters this morning.
Winds have diminished slightly over the past few hours as low
pressure slowly pushes east-NE away from the southeast coast this
morning and high pressure pushes off the NE coast. As surface low
is still slowly pushing farther offshore, Small Craft Advisory will remain in place
over the Bay and lower James until middle-morning, and the mouth of
the Bay (for seas and gusts to 20 kt) and Currituck Sound through
this afternoon. Still noting seas of 7-11ft over the coastal waters
at 11z, and seas will be slow to diminish today through tonight.
Thus Small Craft Advisory remains in place over the Atlantic coastal waters until
late tonight.

Surface low will push farther out to sea through tonight as high
pressure remains ridged over the waters. Seas will subside to 4-8 feet
by this afternoon. Flow veers around to the south tonight, in advance of
a cold front approaching from the west. Seas finally fall below 5ft
by Tuesday morning. Could have a few gusts to low end Small Craft Advisory over the
Bay/sand in S-SW flow ahead of the front on Tuesday, but expect
predominate conditions to be sub-sca. The front crosses the waters
Tuesday night, with a period of Small Craft Advisory expected with resultant north-northwesterly
cold air advection surge late Tuesday night and Wednesday. After a short lived
secondary surge Wednesday ngt, quieter marine conditions are expected as
high pressure builds over the area from the west Thursday.

&&

Akq watches/warnings/advisories...
Maryland...none.
NC...none.
Virginia...none.
Marine...Small Craft Advisory until 4 PM EDT this afternoon for anz633-
634.
Small Craft Advisory until 6 am EDT Tuesday for anz650-652-654-
656-658.

&&

$$
Synopsis...tmg
near term...Sam
short term...alb/tmg
long term...Sam
aviation...mam
marine...mam