Area Forecast Discussion
Issued by NWS Wakefield, VA

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FXUS61 KAKQ 270629
AFDAKQ

Area Forecast Discussion
National Weather Service Wakefield VA
229 AM EDT Thu Jul 27 2017

.SYNOPSIS...
High pressure slides off the Northeast coast overnight. 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 THIS MORNING/...
High pressure cntrd right along the srn New England coast will
slide ewd overnight, and should maintain dry wx acrs the
region. SE winds will become S, resulting in warmer dewpoints.
Moisture return and modest isentropic upglide over the Piedmont
could result in an isolated shower twd morning, but best
coverage expected to remain west of the local area. Otherwise,
the sky will average mostly clear to partly cloudy, with some
patchy stratus/ground fog possible late. Lows will range fm the
mid 60s to around 70.

&&

.SHORT TERM /6 AM THIS 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 /07Z THURSDAY THROUGH MONDAY/...
A leeside trough develops over the region this afternoon as a
cold front crosses the Midwest into the Ohio Valley today.
Shortwave energy/moisture interacting with the lee trough will
result in the development of thunderstorms primarily this
evening into the overnight hours. Main impacts will be heavy
rainfall, locally strong wind gusts, and brief periods of
CIG/VIS reductions to MVFR/IFR. With storms not expected to
impact TAF sites until closer to 28/0000Z, have kept thunder and
CIG/VIS reductions out of 06Z TAF issuance and will likely do
the same for the 12Z issuance as well. The cold front slowly
crosses the Mid Atlantic Region on Friday with more widespread
thunderstorms anticipated. Very heavy rainfall, locally strong
wind gusts, and rapid CIG/VIS reductions to IFR (possibly LIFR)
will be the primary impacts. Some storms could become severe
with gusts in excess of 50kt due to additional instability/lift
from an upper trough digging over the Ohio Valley as it
intensifies.

The cold front is expected to stall over far SE VA/NE NC on
Saturday before shifting southeast of the area late Saturday
night. This will keep showers/storms focused along the coast
with possible wraparound moisture moving into the area from the
north. Once again, primary threats from any thunderstorms will
be moderate to heavy rainfall and locally strong wind gusts.
with rainfall intensity waning, any brief CIG/VIS reductions
will likely be MVFR.

Upper trough sits over the Mid Atlantic Region Sunday through
Monday...exiting the coast late Monday night/Tuesday morning.
Periods of clouds and showers/storms should be anticipated.

&&

.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 approaching 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...SAM
NEAR TERM...SAM/TMG
SHORT TERM...SAM
LONG TERM...MPR
AVIATION...BMD
MARINE...MPR/JEF



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