Area Forecast Discussion
Issued by NWS Wakefield, VA

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FXUS61 KAKQ 271450
AFDAKQ

Area Forecast Discussion
National Weather Service Wakefield VA
1050 AM EDT Tue Sep 27 2016

.SYNOPSIS...
A cold front tracks across the region today, stalling along the
coast tonight. The front remains along the Mid-Atlantic coast
through the end of the work week, resulting in unsettled weather
conditions into the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1030 AM Tue...Updated fcst through rest of today to fine
tune pops/weather/wind grids. Best precip coverage depicted well
in morning grids with regional radar showing light to ocnl
moderate rain showers across nwrn third of the FA. Latest high
res model suite (NAM12/HRRR/RAP) indicate area of enhanced
925-500mb fgen/omega developing and spreading southeastward across
the central FA in a SW- NE oriented fashion roughly from KEMV to
KMFV by this afternoon. Have fine tuned area of categorical pops
to this region, while maintaining slght chc thunder as weak
instability will develop despite the widespread stratus. Lower
pops will flank the NW and far SE zones as precip will be much
more widely sct in nature. Max temps look on par with latest low
level thickness fcst 1370-1390 meters.


Previous Discussion...Latest GOES water vapor imagery centers an
anomalous upper-level low over the upper Great Lakes region. A
plume of high water vapor also visible spreading into the Mid-
Atlantic region ahead of a cold front located over the central
Appalachians. Lead shortwave moving across the region has produced
a line of showers along the front. The showers and spreading into
the northwest Piedmont. Another area of showers has pushed into
central Virginia to the Virginia Eastern Shore. Best chances for
precip this morning will remain north of a line from Farmville to
Richmond and Snow Hill. Have retained categorical POP`s for that
region.

The cold front reaches central Virginia by mid/late morning, and
then the coast by early evening. The front then stalls along the
coast in deep layer southwesterly flow. Meanwhile, the deep upper-
level low drops southward over the Great Lakes. Best chances for
showers will remain along and ahead of the front thanks to added
height falls and increasing winds aloft. Theta-e advection and
temperatures warming into the upper 70`s to around 80 will result
in marginal instability across the southeast this afternoon.
Effective shear is also marginal, around 20-25 knots. However,
given the dynamics and moisture, expect scattered to numerous
showers and thunderstorms to develop ahead of the front as it
reaches the southeast this afternoon. Hi-res guidance backs this
scenario up well. Convective elements will help produce locally
heavy rainfall. Previous forecasts were concerned about a
connection to deeper moisture located over the Gulf stream, but
that appears unlikely at this time. Rainfall amounts will
generally be around one half inch, with locally higher amounts
across the southeast. Have opted to hold back on flood products,
even with the heavy rainfall of last week. Trended highs under
guidance due to precip and cloud cover, ranging from the low 70`s
to upper 70`s.

The upper-level low centers over the Ohio Valley tonight as the
frontal boundary remains stalled along the Mid-Atlantic coast.
Shortwave energy pushes offshore as low pressure lifts offshore of
the Mid-Atlantic coast. Expect a generally downward trend in
precip tonight, but based on the air mass in place and the
dynamics provided by the deep upper low, will keep chance POP`s
through the overnight period. GFS/NAM isentropic surfaces also
indicate low level lift behind the front. Lows range from the
upper 50`s northwest to mid 60`s southeast.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY/...
Anomalous/deep upper-level low settles over the Ohio Valley
Wednesday as today`s front aligns along the southeast coast. A
brief lull in precipitation is expected Wednesday morning as the
associated surface low lifts offshore. However, precipitation
returns in earnest Wednesday afternoon as diffluent flow upstream
of the upper low results in favorable upper vertical motion along
an area of deep moisture convergence over the Piedmont/central
Virginia. This represents a westward shift in the highest POP`s
compared to the previous forecast. Thermal profiles and cross
sections indicate the upper levels will begin to dry, with
precipitable waters dropping to around 1-1.25 inches. However, the
low levels remain quite moist with 1000-500mb relative humidity
values over 75%. Deep lift indicated in the models will be enough
to overrule limited moisture, resulting in widespread showers
Wednesday afternoon through Wednesday night. Showers may be heavy
at times. Went below guidance on temps Wednesday due to a cloudy
sky and precip, with highs forecast around 70 northwest to around
80 southeast. Few thunderstorms also possible Wednesday afternoon
due to elevated instability (dry air aloft). Lows Wednesday night
in the low to mid 60`s.

The upper low drops southward over the southern Appalachians
Thursday as a prevalent dry slot works into the forecast area.
Showers will lift northward through the day, with the best chances
confined north of Interstate 64 late Thursday. However, based on
the amount of forcing and lingering moisture, will keep chance
POPs across the remaining forecast area for light showers. Highs
Thursday range from the low to mid 70`s inland to upper 70`s
southeast. May begin to see the sun late Thursday afternoon across
the south as the sky begins to clear.

Per the latest GEFS standardized anomalies, best moisture
transport expected to remain north of the region through the short
term. However, even with limited moisture over the region, upper
level dynamics still expected to result in produce a widespread
1-2 inches inland. Less across the southeast.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Medium range period continues to feature the presence of a deep
anomalous upper level low centered over the TN/OH Valley Thu
night/Fri, gradually lifting N and slowly weakening/filling across
the Great Lakes by late Saturday-Sunday. At the surface, high
pressure centered over eastern Canada will also weaken/slide E
through the period as a surface low locates close to the upper
low, with a secondary area of low pressure over the Carolinas/VA
lifting N of the local area by Fri night/Sat. Fairly good
agreement exists among the operational GFS/GEFS mean and the ECMWF
with this overall scenario. Thus, precipitation chances will
initially begin high (especially across the N) Thu night where
50-70% chances have been added to the forecast. Fri will see
diminishing POPS (with southern 1/2 of the CWA only having about a
20% POP with at least partly sunny skies developing). Will
maintain 30-50% chances N closer to the retreating frontal
boundary. Will continue with a mainly dry forecast from fri night
through the weekend (except for a 20-30% POP across the NE).
Temperatures will be close to seasonal averages with highs
generally in the 70s and lows in the 50s W to the 60s along the
coast.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
Cold front approaching from the northwest will spread showers and
lower ceilings into the region this morning. MVFR ceilings
expected to spread across the Piedmont into central Virginia by
12-15Z, remaining over the region through late Tuesday morning.
VFR conditions prevail KORF and KECG. MVFR (possibly IFR)
visibilities expected to light to moderate showers. Surface winds
generally out of the south below 10 knots.

Showers will reach southeast Virginia/northeast North Carolina by
12-15Z as the front pushes across the region. Front expected to
stall along the coast late today, with showers lingering near the
coast. Thunderstorms possible along the coast. Surface winds at
or below 10 knots today.

Unsettled weather conditions expected through the end of the work
week as waves of low pressure lift along the cold front. Showers
are possible each day.

&&

.MARINE...
Have dropped SCA headlines across the remaining Bay zones, though
a few gusts to 20 KT will persist through daybreak (and an MWS may
be issued if showers to the west push over the Bay and enhance the
winds briefly). Otherwise, The approaching cold front drops into
the area and stalls near SE VA tonight and Wed. Waves of low
pressure forming along the front Wed night into Fri should
gradually lift the boundary back to the N Thu-Fri (though
confidence on exact timing is uncertain). Winds will become mainly
ENE across northern areas and S/SE over southern areas Wed-Fri.
SCA conditions are most likely over northern coastal waters Thu-
Fri as the gradient tightens between the High to the N/NE and the
front lifting back to the N. Still a fair amount of uncertainty
and if the front is slower to lift to the N, SCA conditions could
occur over most of the marine zones. Overall winds will avg 10-15
KT with mainly 1-2 ft waves over the Bay and 3-4 ft seas over
coastal waters through this period, with seas to 5-7 ft possible N
of Parramore Island.

&&

.AKQ WATCHES/WARNINGS/ADVISORIES...
MD...None.
NC...None.
VA...None.
MARINE...None.

&&

$$
SYNOPSIS...SAM
NEAR TERM...TL/SAM
SHORT TERM...SAM
LONG TERM...LKB
AVIATION...TL/SAM
MARINE...LKB



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