Area Forecast Discussion
Issued by NWS Wakefield, VA

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FXUS61 KAKQ 111949

Area Forecast Discussion
National Weather Service Wakefield VA
349 PM EDT Fri Aug 11 2017

A warm front lifts north of the area tonight. A cold front crosses
the region Saturday night, then stalls across the Carolinas early
next week as a series of low pressure systems moves east along it.


Latest MSAS has the warm front snaking its way along the western
fringes of the Chesapeake Bay.  Meanwhile, weak energy impulse along
the southeast coastal areas north of the warm front resulting is the
ongoing convection across NE NC where an average of 1-2 inches of
rain has fallen since 12Z.

Latest high res data conts to back off on the amount of convection
in the warm sector until the next impulse of energy arrives after
22Z. Expect the convection along the se coast to decrease in coverage
over the next few hours as the first impulse exits off the coast.
So will keep chc pops going until then.

SPC has northern half of fa in marginal risk tonight mainly for
strong wind gusts and heavy rainfall. Guidance suggests convection
coverage increases across the Piedmont between 22Z-04Z so went ahead
with likely pops along and west of the I95 corridor. This is where
any strong storms may occur. Chc pops to the east. Activity then
shifts toward the coast after midnight where the pops will be the
highest. Thunder possible just about at any time. Patch fog is also
possible, especially in areas that do get the heavy downpours. Humid
with lows 70-75.

Low pressure lifts well north of the area Sat with the trailing cold
front weakening as it pushes across the mountains late in the day.
Strong height falls remain north of the region, but increasing winds
aloft and a strong influx of deep moisture associated with PW`s over
2 inches ahead of a lee/thermal trough will result in additional
scattered to numerous showers and thunderstorms. Chc to likely pops
Sat morning with high end likely pops all areas Sat afternoon. SPC
has once again put most of VA and all of MD in a marginal risk. The
degree to Saturdays severe threat will be whether any sun can occur
during the late morning and early aftn to destabilize the airmass.
The main threat again is strong wind gusts and heavy downpours.
Given the anomalous moisture feed and QPF forecast, went ahead and
added heavy rainfall in the grids for now. Highs 80-85.

Another consideration for Sat afternoon will be whether a short term
flash flood watch will be needed for nern NC given todays widespread
1-2 inch amounts.

The cold front crosses the fa Sat evening, then pushes offshore late.
Deepest moisture gets shunted offshore, but enough lingering moisture
to keep likely pops going across the east early, tapering off to chc
pops after midnight. Somewhat cooler behind the front with lows rainging
from the mid 60s NW to the mid 70s sern coastal areas.


Low pressure lifts over the Saint Lawrence River Valley
Saturday as the attendant, weakening cold front pushes across
the central Appalachians late Saturday. Strong height falls
remain north of the region, but increasing winds aloft and a
strong influx of deep moisture associated with precipitable
waters over 2 inches ahead of a lee/thermal trough will result
in additional scattered to numerous showers and thunderstorms
Saturday. Have retained likely PoPs for most of the area except
the far northwest. Heavy rainfall will be the main impact
Saturday given the anomalous moisture, but a lack of upper level
support expected to prevent widespread heavy rainfall. Warmer,
with highs in the mid 80`s under a partly cloudy to mostly
cloudy sky.

Cold front pushes across the region Saturday evening, pushing
offshore late Saturday night. Deepest moisture gets shunted
offshore, but will carry high end chance PoPs along the coast
through late Saturday night. The front progged to stall in the
vicinity of northeast North Carolina Sunday as flow aloft
remains southwest. High pressure builds over the Ohio Valley.
Mid level energy lifting over the TN Valley will develop a wave
of low pressure along the front Sunday, resulting in chances for
showers and thunderstorms across the southeast portion of the
forecast area Sunday. Given the uncertainty, have kept a slight
chance to low end chance PoP across the northwest Sunday. Highs
Sunday again in the mid 80`s.


The period begins with more wet weather expected from Monday night
into early Wednesday as a frontal boundary remains stalled across
the Mid-Atlantic States and the Carolinas.  The biggest question has
been the location of the front which the GFS and ECMWF have had a
hard time reaching a consensus as run to run consistency has waffled
the front from over us to areas south of the forecast area.  The 12z
ECMWF run of the models has the front and associated surface wave
lifting over the Mid-Atlantic states while in the 12 GFS the front
remains south of the area and holds the bulk of the rain further
south over the Carolinas.  This is a flip from yesterdays runs.  So
for now, have generally played a scenario where the front stall near
the area and the wave of low pressure moves along the front and
should impact much of the area on Tuesday.  Have added likely pops
to SE VA and NE NC as this area appears to have the best chance of
rain on Tuesday/Tues night.

By Wednesday...the front will get drug south of the area as the wave
moves off the coast.  This will allow high pressure to build into
the area as the flow aloft turns nw as the upper trough swings
through the area.  This high will provide a couple dry days before
the next boundary crosses the area on Friday as the upper ridge over
the lower MS Valley and Southern Plains builds eastward.  There
could be some scattered convection along this warm front moves
through the area.

For temperatures...did go a little cooler on Tuesday with the idea
of clouds and rain, but without having a better handle on the
frontal position could easily see either much cooler temps, in the
70s, than shown or perhaps if the front lifts further north, it
could be more int he mid to upper 80s.  Once the front clears should
see drier conditions and initially having temps in the low to mid
80s with a warming trend into the second half of the work week with
readings back toward near normal values in the mid to upper 80s.


Rather challenging forecast period ahead as a combo of abundant
low level moisture and advancing frontal boundaries will not
only keep the chc for convection through the period but low cigs
as well. About the only site confident enough to include thunder
is at ECG given current radar trends. Otw, VCSH for most sites
over the next several hours. Added VCTS to RIC arnd 22Z as the
models generally agree that sct tstms develop across the
piedmont after 18Z then slowly sag se.

Tsctns show widespread low and mid level moisture tonight to
keep CIGS in the MVFR range for now with local IFR CIGS toward
daybreak. In addition, next wave of low pressure approaches
ahead of the cold front Sat morning for shwrs to be included in
the forecast. Thunder also possible, but kept that out of the
forecast attm. Winds outside of tstms below 10 kts.

OUTLOOK...Unsettled conditions with periodic convection expected
into early in the week as the boundary stalls across the region
with a series of disturbances pushing across the area.


No headlines through Sun. Sfc hi pres invof LI weakens through
today. Meanwhile...weak sfc lo pres just E of DAB tracks NE off
the SE coast into Sat along a stall coastal boundary. SSE winds
will avg aob 10 kt through tonight...then SSW 5 to 15 kt Sat
into Sat night. A cold front will gradually push thru the nrn
2/3rds of the area late Sat night/early Sun morning...then
settle S to along the NC coast Sun afternoon. SW winds 10-15 kt
become N Sun. A possible short period surge in speeds possible
(esp over the Ches Bay/nrn ocean waters Sun morning) but lo
level CAA weak (so expecting conditions to remain blo SCA). Waves
through the weekend 1 to 2 ft and seas 2 to 4 ft.




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