Area Forecast Discussion
Issued by NWS Wakefield, VA

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FXUS61 KAKQ 262001
AFDAKQ

Area Forecast Discussion
National Weather Service Wakefield VA
401 PM EDT Wed Jul 26 2017

.SYNOPSIS...
High pressure slides off the Northeast coast tonight. Low
pressure affects the region Friday through Friday night, with a
cold front pushing across the region Friday night. High pressure
builds into the region through early next week.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
A shortwave depicted on water vapor and RAP analysis continues
to produce isolated to widely scattered showers from the
Northern Neck to the Maryland Eastern Shore this afternoon.
Expect this activity to diminish by 5-6 pm as the shortwave
slides offshore and low level instability wanes. Otherwise, high
pressure located along the Mid-Atlantic and Northeast coasts
resulted in onshore flow and more comfortable conditions today.
Temperatures have generally warmed into the mid 80`s today, with
locales along the immediate coast only in the upper 70`s. Subtle
energy in the northwest flow aloft has resulted in afternoon
fair weather cu, but dry conditions persist. High pressure
slides offshore tonight, resulting in return/southerly flow and
warm dewpoints. Moisture return and modest isentropic upglide
over the Piedmont could resulted in an isolated shower late
tonight, but best coverage expected to remain west of the local
area. An isolated shower is also possible over the Maryland
Eastern Shore late tonight as another upstream shortwave
approaches from the northwest. Sky averages mostly clear to
partly cloudy, with some patchy stratus/ground fog possible
late. Still mild and generally comfortable, with lows in the
upper 60`s to around 70.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/...
Increasing confidence for widespread beneficial rainfall
through the short term, especially Thursday night through Friday
night.

Initial shortwave drops over the Ohio Valley Thursday and into
the Mid-Atlantic region late Thursday into Thursday evening in
west to northwest flow aloft. Pressure falls ahead of a frontal
boundary over the Ohio Valley will induce a lee/thermal boundary
over the Piedmont Thursday afternoon. Modest mid-level lapse
rates and dewpoints in the mid to upper 60`s result in only
marginal instability, with mixed layer CAPE values progged
around 500-1000 J/kg. Moisture returns in earnest, with guidance
indicating precipitable water values increasing to around 2
inches by Thursday afternoon. The result will be isolated to
scattered afternoon showers and thunderstorms over the Piedmont.
Highs Thursday generally in the mid to upper 80`s.

Attention turns upstream late Thursday for convection
developing over the Ohio and Tennessee Valleys. Guidance
indicates this activity should organize ahead of the boundary
and cross the central Appalachians into the Mid-Atlantic region
Thursday evening. Energy in the northwest flow aloft in tandem
with increasing moisture and elevated instability (negative
showalter values along with an elevated mixed layer) will aid
the showers/thunderstorms dropping over the higher elevation
into the local area Thursday evening through Thursday night.
Strong winds are possible along the leading edge of the
convection due to deep layer shear around 25-30 knots. SPC has
downgraded the slight risk to a marginal risk. There still
remains some uncertainty in the guidance, especially in the
placement of the upstream low pressure system, so will continue
to monitor future model trends for severe weather risk Thursday
evening. Mild, with lows in the 70`s under a mostly cloudy to
cloudy sky.

Potent shortwave/compact upper low digs over the Ohio Valley
into the northern Mid-Atlantic region Friday into Friday night,
and then drops over the region Saturday before pushing offshore
Saturday night. At the surface, low pressure lifts over the
Kentucky Valley into the northern Mid-Atlantic region Friday.
Height falls will result in modest deepening of the surface low
over the northern Mid-Atlantic region Friday night, before the
low pushes offshore Saturday. A trailing cold front pushes
across the region Saturday night. There remains a great deal of
uncertainty with the overall evolution of convection Friday, not
only due to model uncertainty but due to the potential of
convection Thursday evening. A fair amount of clouds are
expected Friday, which along with modest mid-level lapse rates
will limit overall instability. If any dry air wraps into the
system Friday morning, we could see better instability. However,
given the warm/moist air mass and strong dynamics, scattered to
numerous showers and thunderstorms are expected Friday
afternoon and evening. POPs ramp up to categorical late Friday
into Friday night inland with the arrival of the strongest
height falls. Increasing mid-level flow will result in deep
layer shear around 30 knots, which will result in better
organization and stronger storms. Gusty winds and heavy rainfall
remain the main threat. SPC maintains a slight chance of severe
weather across the region. There is also a slight risk for
excessive rainfall across the northeast local area Friday, near
the strongest onshore 850mb anomalies. Highs Friday generally in
the low to mid 80`s.

Front pushes south of the region Saturday as the surface low
lifts along the Northeast coast. Strong height falls over the
region in tandem with 1000-500mb relative humidity values around
80-90 percent will result in afternoon showers. Instability
wanes behind the front, but perhaps a few rumbles of thunder are
possible near the coast. Cooler, with a north wind of 10-15 mph
(gusts 25 mph Eastern Shore). Highs generally in the low to mid
80`s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Low pressure pushes farther NE and away from the area Saturday night
with the trailing front stalling ivof the Gulf Stream early next
week. Lingering moisture keeps chc pops going along the coast Sat
eve. Otw, pt cldy with lows from near 60 NW to around 70 SE as high
pressure approaches the mts.

Models have a shortwave riding ENE along the boundary, but the
latest data suggests the high pressure over the mountains suppresses
the bulk of the moisture SE of the AKQ fa. This low is progged to
deepen out over the Gulf Stream by Tue, which will bear watching
this time of the year. Kept the forecast dry through the period
except for some slght chc pops ivof the Albemarle Sound Mon night
and Tue where a limited amount of moisture spreads north.

Temps below normal to start with readings returning to near normal
by mid week. Highs 80-85 Sun, low-mid 80s Mon, mid-upr 80s Tue and
upr 80s-lwr 90s Wed. Lows in the 60s Mon/Tue, mid 60s-lwr 70s
Wed.

&&

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
VFR conditions through the 18z TAF period with areas of cu sct-
bkn035 will persist through the afternoon giving way to more
sct040 to sct070 this evening. Flow remaivs E to SE across the
region and conditions VFR overnight into Thursday.

Primarily VFR conditions expected through Thursday. A cold
front approaches the area late Thursday into Friday bringing the
threat for periodic flight restrictions in showers and
thunderstorms Thursday night and Fri. Showers and thunderstorms
may linger into Saturday, but drier conditions arrive for the
second half of the weekend.

&&

.MARINE...
No headlines in the short term as the E-SE onshore flow remains aob
15 kts. Seas briefly hit 4 ft across srn beaches today but since
have backed off and are now avgg 2-3 ft. Return S-SW flow sets up
Thu into Fri ahead of an apporaching cold front. Guidance continues
to show winds aob 15 KTS through the period. Seas avgg 2-3 ft.

A fairly strong cold front (by mid summer standards) crosses the
area early Sat switching the winds to the NW then N. Rather decent
surge noted behind this feature for winds to be forecasted in the
SCA range attm (although it is to early for any headlines). To
complicate matters even further, low pressure is forecasted to
deepen a bit as it moves off the coast later Sat. 925MB winds
suggest a few gusts to around 30 KTS possible across the northern
coastal waters late Sat. Seas will build to at least 5 ft, and will
likely depend on the exact track/strength of the low as to how much
NE swell develops behind it. Have capped seas at 6 ft for now. Could
be looking at some beach hazards this weekend given this scenario.
Winds subside Sun but remain onshore through Mon.

&&

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

&&

$$
SYNOPSIS...MAM/SAM
NEAR TERM...SAM
SHORT TERM...SAM
LONG TERM...MPR
AVIATION...MAM/JAO
MARINE...MPR



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