Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
356 PM EST Thursday Nov 20 2014
dry cold front pushes offshore this evening as high pressure
builds over the Ohio Valley. High pressure centers over the region
Friday and Friday night....pushing offshore late Saturday. A warm
front lifts over the region Sunday before a trailing cold front
crosses the region early next week.
Near term /until 6 am Friday morning/...
temperatures have warmed into the middle-upper 50s this afternoon thanks to
gusty SW winds of 20-30 miles per hour and modest warm air advection. However...the clipper
front is still yet to push through the region...which will bring a
reinforcing shot of cold air into the region tonight. The front will
push through the region and offshore this evening as a potent upper
shortwave tracks into New England. A few middle level clouds may
accompany the front...but skies remain generally mostly clear. Temperatures
will fall off behind the front as cold air advection ensues. Low-level thicknesses
drop off 50-60 meters this evening through early Friday
morning...resulting in temperatures falling into the middle-upper 20s inland
to low-middle 30s near the coast. Northwest winds generally 5-10 miles per hour
inland...with gusts up to 25 miles per hour along the immediate coast.
Short term /6 am Friday morning through Sunday night/...
persistent cyclonic flow over the eastern Continental U.S. Finally ejects eastward
off the NE coast as a strong 150+ knots upper jet exits the middle-
Atlantic coast. Strong ~1035mb surface high pressure builds over the
Ohio Valley early Friday before centering over the local area late Friday-
Friday night. Cold air advection wanes Friday afternoon...but 850 mb temperatures will have
reached -8 to -10c (-1 to -2 Standard dev). The result will be well
below normal daytime temperatures...generally in the middle 30s in the northwest to
middle 40s southeast (-2 Standard dev or nearly 20 degrees below normal). Some
record low maximum readings may be in danger. Northwest winds will gust up to
20-25 miles per hour during the afternoon before the pressure gradient
relaxes late. Surface high centers over the region Friday night. Calm-
light winds and clear skies will allow temperatures to drop back into the
middle-upper teens to lower 20s inland. Upper 20s- low 30s near the
coast. Return flow on the back side of departing upper/surface high
pressure Sat afternoon will result in moderating temperatures and
increasing thicknesses. 850 mb temperatures also recover quickly Friday night-
Sat...warming to around +6c by late Sat. Highs will be slow to
recover...warming only into the upper 40s northwest to low 50s southeast (still
10 degrees below normal).
Meanwhile...southern stream shortwave and associated surface low move into
the Southern Plains Sat as zonal flow develops over the local area. 850 mb
high pushes offshore late Sat with flow on the western periphery
tapping into Gulf moisture. Middle- upper clouds will increase late
sun into Sun night from the west as moisture advection
strengthens. Warm front lifts into the Tennessee/Kentucky valleys late Sat
night...extending into the southeast states. Shortwave ejects northeastward from
the Southern Plains late Sat night into Sun morning...lifting over the
central Appalachians sun. Models coming into better agreement with
timing of the precipitation...but as is typical with overrunning events
and the presence of an upstream 120+ knots upper jet...will keep probability of precipitation
early sun across the Piedmont into central Virginia. Main brunt of
dynamics/moisture/precip quickly spreads across the region Sun
afternoon-evening. Diffluence/divergence aloft and the lifting
shortwave will provide dynamic forcing for ascent over the region.
Overrunning commences as the warm front lifts into the region Sun
afternoon. Per gefs Standard anomalies...850 mb moisture flux is prognosticated
at +3 to +4 Standard dev with precipitable waters increasing to around 1.4 inches.
Soundings also depict a deeply saturated column. As a
result...locally heavy rainfall is possible. Have retained
categorical probability of precipitation Sun afternoon and sun evening as the warm front
lifts through the region. Brunt of precipitation will likely push offshore
Sun night as a dry slot punches into the region. Temperatures sun will
warm into the upper 50s northwest to middle 60s southeast. Little diurnal trend
expected Sun night as temperatures only fall a few degrees into the low 50s
northwest to upper 50s southeast.
Long term /Monday through Thursday/...
models differ with respect to timing of the cdfrntl passage across the area.
GFS about 6 hours faster than Euro Monday night. Either way...Monday
appears to be a warm and breezy day as forecast area becomes warm sectored
well ahead of cdfrnt. Kept low chance pop going for any shower that
develops in the warmer airmass. Highs u60s-m70s. Chance probability of precipitation Monday night
with frontal passage. Lows M-u40s...xcpt 50-55 southeastern coastal areas.
Kept low chance probability of precipitation across eastern areas Tuesday as a weak short wave moves NE along
the front as it slowly moves off the coast. Otw...drier and cooler
air moves in from the west. Highs u50s-l60s. Lows Tuesday night m30s-m40s.
Highs pressure from the west (pacific origin vs canadian) allows for a
quiet middle week prd. Dry weather through Turkey day along with a gradual cooling
trend. Highs Wednesday 50-55. Lows Wednesday night u20s-u30s. Highs Thursday
Aviation /21z Thursday through Tuesday/...
a coldfront will approach from the west this afternoon...crossing the
region tonght. SW winds this afternoon will gust 20 to 25 knots with the
approach of the surface trough. The coldfront clears the coast this
evening...with winds turning northwest and remaining gusty. High pressure
settles over the middle-Atlantic region Friday and Sat...moving well
offshore on Sunday.
A complex weather system is expected to bring periods of rain
Sunday through Monday with IFR conditions possible.
the gusty SW winds ahead of the front will dmnsh a bit this evening
before becoming west-northwest aftr midnite as strong cold air advection sets in Post frontal passage.
Latest data suggests the surge (15-20 kts) lasts till around 18z Friday
across the Bay and sand so have xnndd the Small Craft Advisory headlines across those
areas several more hours. Coastal waters a bit different as winds around 25 kts
should dmnsh aftr 12z. Seas having trouble reaching 5 feet across southern
coastal waters and may only reach those values out near 20 nm. Thus...will
let the Small Craft Advisory on coastal waters expire late tonite/Erly Friday there.
High pressure settles over the middle Atlantic region Friday night through
Sat night then moves well offshore Sunday resulting in winds and
seas below Small Craft Advisory levels. Next chance for sca's will come late sun into Monday as
srly winds increases ahead of next system approaching from the SW.
Tonights tides prognosticated about a half to three quarters of a foot
below normal. Tides expected to slowly return to near normal over
the next few days.
Marine...Small Craft Advisory until 1 PM EST Friday for anz630>634.
Small Craft Advisory until 4 am EST Friday for anz638-652-654-
Small Craft Advisory until 10 am EST Friday for anz650.