Scientific Forecaster Discussion
fxus61 kakq 231519
Area forecast discussion
National Weather Service Wakefield Virginia
1119 am EDT Wed Aug 23 2017
a cold front will cross the area this afternoon through tonight,
with strong high pressure building north of the area from
Thursday into the upcoming weekend.
Near term /through tonight/...
latest surface analysis depicts a cold front over the northwest
Piedmont. Early morning showers have mostly dissipated over the
northern counties as the best forcing lags over the mountains.
Sunny conditions observed on visible satellite ahead of the
front, which will result in a quick warm up into early
afternoon. An already humid air mass is in place, with mixed-
layer cape values analyzed at 500-1000 j/kg already this morning
across southeast Virginia and northeast North Carolina. Across
the northwest, temperatures will hold nearly steady through the
The front slowly pushes across the local area through mid
afternoon, before slowly along the coast in southwest flow
aloft. The shearing vort lobe progged to ride along the front,
as winds increase aloft. The added forcing for ascent along the
boundary along with a narrow ribbon of high precipitable waters
will result in scattered to numerous showers and thunderstorms.
Westerly mid level flow and weak lapse rates will limit coverage
over the Piedmont, but as instability increases across central
Virginia eastward (mixed layer cape values of 1-2k j/kg), hi-res
guidance indicates that convection should become better
organized. Marginal shear (20-30 knots) expected as the best
upper level flow remains north of the region, but given
relatively dry mid-levels and an inverted-v sounding, damaging
winds are possible. The best chances for strong to severe storms
are across the southeast half of the local area where Storm Prediction Center
maintains a marginal risk for severe weather. Another concern
will be along the coast as the front slows and better upper
level support arrives. Impressive Omega along the front along
with precipitable waters in excess of 2 inches and mbe vectors
at or below 10 knots will result in locally heavy rainfall.
Flash flood guidance across the southeast is around 2-3 inches,
but given the overall progressive nature of the system, do not
anticipate widespread impacts from flooding. Headlines are not
Flow becomes north to northeast behind the front, as drier air
starts to spread into the area from the northwest later this
afternoon into tonight. Guidance indicates some linger moisture
over the Piedmont into central Virginia behind the front as
another area of convergence could result in additional showers
into this evening. Have increased cloud cover inland late today
through this evening and lingered chance pops inland through the
evening hours. Showers/thunderstorms diminish along the coast
through the evening as instability wanes and the shortwave
Highs today will range from the low 80's northwest to low 90's
southeast. Low tonight generally in the low to mid 60's
northwest to low 70's southeast.
Short term /Thursday through Friday/...
the front and waves of low pressure will push southeast of the region
and offshore overnight thru Thu. Those waves of low pressure
will maintain slgt chc to chc pops over extrm southeast Virginia and NE NC.
Lows tonight will range FM the lower 60s extrm northwest counties, to
the lower 70s extrm southeast Virginia and NE NC.
High pressure will slowly build in from the northwest Thu thru
Fri, with the front getting pushed well southeast of the area off the
southeast coast. This will result in dry, cooler and less humid
conditions for Thu night thru Fri. Highs on Thu in the lower
80s under a partly sunny sky. Lows Thu night in the 60s. Highs
on Fri in the upper 70s to lower 80s under a partly to mostly
Long term /Friday night through Tuesday/...
cooler and drier for the most of the extended period, as high
pressure builds acrs the Great Lakes and New England Fri night
thru Mon, and ridges southward into the region. Low pressure
moving north-northeast off the southeast coast could affect the area on Tue.
Medium range guidance in relatively good agreement with developing a
tropical system along and offshore of the southeast coast along the
old frontal boundary early next week. Kept weekend dry, with low
rain chances along the southeast coast by Monday. Looking ahead, rain
chances look to improve into the middle of next week, with moisture
from the remnants of Harvey looming to the SW. For temperatures,
high temperatures Sat-Mon will be at or just slightly below
normal through the period, mainly in the upper 70s to low 80s.
Early morning lows in the 50s inland to mid 60s along the coast.
Aviation /15z Wednesday through Sunday/...
early this morning, a cold front was moving into nrn/northwest
portions of the County Warning Area. That front will slowly push southeast acrs the
region and off the coast today into Thu morning. Flight
restrictions will be possible along and behind the front this
aftn into Thu morning FM showers and tstms. The front should
push southeast of NE NC during Thu, with lingering showers possible
over extrm southeast Virginia/NE NC. Conditions are expected to improve
Thu aftn thru Fri, as the front shifts farther southeast and high
pressure builds north of the region. Breezy NE winds expected
near the coast Thu and Fri.
a cold front will approach the waters from the northwest and
cross the area late this aftn through tonight. SW flow still
avearges 15-20 kt over the Bay and coastal waters, have the Small Craft Advisory
headlines through 7 am for the entire Bay and for the nrn coatl
waters. Waves avg 2 to 3 feet in the Bay and seas acrs the north are
4-5 ft. Winds shift to the north/NE 10 to 15 knots behind the front
by late this morning north and by aftn farther S (and with
convection this aftn winds will be locally much stronger but
this will be handled W/ mws/smw's as needed).
Strong high pressure builds north of the region Thursday and
into the weekend. Given the warm waters and some cold advection
late tonight/Thu morning, expect a marginal Small Craft Advisory event over at
least some of the Bay and probably the lower James Thu morning.
Did not raise any Small Craft Advisory headlines as this will be mainly a 3rd
period event (and still fairly marginal). Otherwise expect
diminishing winds later Thu aftn through Fri as sfc high
pressure builds a little farther south and the pressure gradient
weakens. Waves in the Lower Bay may briefly build to 3-4 ft Thu
morning, then will subside to 1-2 ft with coastal seas 3-4 ft.
Onshore (e-ne) flow for the weekend as high pressure builds north-northwest
of the local area and then shifts east to New England. Winds
remain elevated 10 to 15, occasional gusts to ~20 kt Sat, and
then increase more significantly Sunday into next week as
stalled front to the south lifts back north with low pressure
developing along the boundary. Seas remain choppy Sat, and then
will build to 5-7 ft or greater late sun into next week.