Area Forecast Discussion
Issued by NWS Wakefield, VA

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742
FXUS61 KAKQ 191109
AFDAKQ

AREA FORECAST DISCUSSION
National Weather Service Wakefield VA
709 AM EDT Fri Apr 19 2019

.SYNOPSIS...
Low pressure at the surface and aloft will approach from the
west today bringing the chance for strong to severe storms to
the region. A cold front will cross the area tonight. An upper
level low will be slow to lift away to the northeast over the
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 400 AM EDT Friday...

...STRONG TO SEVERE STORMS ARE POSSIBLE TODAY AND TONIGHT...

A potentially volatile combination of wind shear and instability
across the region today. A deep trough aloft is currently
digging southward across the central part of the country with a
deepening surface low located over Tennessee. This surface low
will translate north and eastward today as the upper trough
evolves into a closed low across the Southeast. Deep layer
southerly winds will continue to transport copious amounts of
Gulf and Atlantic moisture northward into the region this
morning helping to set the stage for strong to severe
thunderstorms.

The Storm Prediction Center has outlined varying risk areas across
the region today and tonight. A Moderate Risk encompasses the
southwestern half of the area, along and southwest of a line from
Edenton NC northwest to Petersburg, Richmond, and Palmyra VA. An
Enhanced Risk closely surrounds the Moderate Risk area with a Slight
Risk in place for the remainder of the region.

The degree of destabilization this afternoon and tonight remains the
biggest question mark for this event with substantial variation
among models. General consensus shows MLCAPE values of 500-1000
J/Kg, with pockets of 1000-1500 (up to as much as 2000) J/Kg most
probable across the southern half of the area by afternoon.
Extensive cloud cover and generally poor lapse rates in the mid
levels are responsible for keeping instability in check. However,
deep layer wind shear will be increasing through the course of the
morning with 850mb winds increasing to 45-50 knots by this afternoon
and potentially 55-60 knots by this evening, leading to lengthy
clockwise curved hodographs across the region. Southerly low level
flow will also transport rich moisture northward with dewpoints
in the low to mid 60s and the potential for upper 60s to around
70 degree dewpoints pooling along and just ahead of the cold
frontal boundary.

Two rounds of potentially strong to severe storms are expected
across the region today. The first is associated with low level warm
advection and a leading wind speed maximum aloft. This area is
forecast to develop by mid to late morning across southwestern
portions of the area and spread north north-eastward into the
afternoon hours. This initial round of storms has the potential to
be quasi-discrete in nature and will pose both a damaging wind and
an isolated tornado threat as deep layer shear vectors are initially
out of the west-southwest. This initial wave will also benefit the
most from any pockets of greater heating that can be realized this
morning into the early afternoon.

The second round of storms will be strongly forced by excellent
upper divergence and low level lift along the cold front this
evening as deep layer wind shear continues to increase. Uncertainty
remains regarding the degree to which the first round of convection
disrupts the favorable convective environment but it appears the
atmosphere will remain largely supportive of strong to severe storms
along the cold front. Deep layer shear vectors will become south and
southwesterly with time this afternoon, supporting a more linear
storm mode with the second round of storms. The main threat with the
latter round of storms will come from the potential for significant
damaging winds, especially in any bowing line segments that form.
However, model forecast soundings continue to indicate 100 to as
much as 300 m2/s2 of 0-1km storm relative helicity, which would
support a continued threat for embedded supercellular structures and
associated tornado potential within the convective line. The line
will move eastward across the area late this afternoon into the
overnight hours with areas west of I-95 impacted from roughly 4-
10PM, areas east of I-95 from 6PM-2AM, and areas near the coast
and Eastern Shore seeing the greatest threat from 10PM-4AM.

In addition to the severe weather potential, model guidance shows
1.5 to nearly 2 inches of precipitable water (+2 to +4 Std Dev above
normal) across the area prior to the cold frontal passage this
evening, leading to the potential for isolated areas of flash
flooding. WPC has outlooked our area for a Slight Risk of flash
flooding today and tonight. Storm total QPF for this event
ranges from 1-1.5 inches with generally less than an inch for
the Eastern Shore.

Temperatures will be warm today with southerly flow, generally mid
to upper 70s with the potential for a few low 80s east of I-95 and
away from the immediate coast/Eastern Shore. Overnight lows
will bottom out in the upper 50s and low 60s.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
As of 400 AM EDT Friday...

Improving conditions for Saturday with chance and slight chance PoPs
confined to eastern portions of the area Saturday morning. The upper
level low responsible for all the unsettled weather will be slow to
lift northeast through the weekend keeping party to mostly cloudy
skies in the forecast. Greater clearing is expected by Monday with
temperatures in the low and mid 70s, upper 60s on the Eastern Shore.
Overnight lows will be in the upper 40s and low 50s through mid
week.

&&

.LONG TERM /TUESDAY THROUGH THURSDAY/...
As of 400 PM EDT Thursday...

Upper-level low will be over the area during the day on Sunday
and into early Monday morning. 20- 40 Pops were put in, with
the greatest chance in the northern parts of the area. There is
a chance of thunder as the cold pool core moves over the area,
but no thunder was added to the forecast at this time. As the
upper-level low moves off to the NE, a SW flow will begin ahead
of a front that will move south out of the Great Lakes. With
the upper-level flow out of the west early next week, the front
will stall and a chance of showers will be around for Tue-Thu.
However, models are in disagreements on were the boundary will
stall, therefore only 20-30 Pops were put in through Friday.

Temperatures will be cool on Sunday with the upper-level low over
the area. Highs will be in the 60s and lows in the 40s inland
to mid 50s along the coast and eastern shore. 80s will return
by Tuesday with a SW wind. Temps will remain mild for the
remainder of the week as a front stalls across the area, with
highs ranging from the 60s to near 70.

&&

.AVIATION /11Z FRIDAY THROUGH TUESDAY/...
As of 700 AM EDT Friday...

Largely VFR conditions in place across the region but
increasing/lowering clouds are advecting in from the south and
west with generally VFR/MVFR CIGs expected into early afternoon.
Synoptic southerly winds at 10-20 knots will continue today
with gusts 30-35 knots possible, especially at PHF, ORF, and
ECG.

Multiple rounds of showers and thunderstorms are expected today
with localized restrictions to MVFR/IFR in and around areas of
convection along with erratic and gusty winds. A cold front
will cross the area late tonight with continued showers and
MVFR/IFR CIG and VSBY restrictions in its wake.

Outlook: Upper low will be slow to exit the region, leading to
only gradual improvement in flying weather over the weekend.

&&

.MARINE...
As of 320 AM EDT Friday...

Sfc hi pres situated out over the wrn Atlantic and lo pres
tracking slowly NE through the TN Valley today into tonight will
result in a tightened pres gradient. S winds averaging 15-20 kt
to start will be increasing to 20-30 kt by late this
afternoon/evening as low level S jet intensifies. By late today
into tonight...both synoptic and (potentially higher)
convective gusts are expected. Wind probs continue to show the
highest % for gales (50-60%) on the ocean waters S of Parramore
Island (while other locations are generally aob 30%). Will start
the GLW now for that area and continue it through tonight.
Elsewhere...strong SCAs expected (25-30 kt)...w/ occasional
gusts to 35 kt possible (esp at elevated sites near the mouth of
the Ches Bay and on the nrn ocean waters). Waves to build to
4-5 ft on the Ches Bay...seas to 7-10 ft (by late today into
tonight). Winds are expected to diminish overnight tonight post
convection.

Lo pres will be slow to exit the region during the weekend. Will
maintain S winds 15-20 kt Sat for most marine areas (aob 15 kt
on the rivers)...becoming SW for Sat night/Sun w/ further
lowering of speeds expected.

&&

.TIDES/COASTAL FLOODING...
As of 320 AM EDT Friday...

Latest guid continues to be not as bullish w/ the threat for
moderate flooding. However, Bishops Head MD is forecast to just
reach minor flooding again w/ today`s high tide cycle then be a
bit higher than/exceed minor flooding with tonight`s high tide.
Tidal departures will average 1 to 1.5 FT above MLLW. Strong S
winds may result in other stages along the Ches Bay on the
Lower Md Eastern Shore to approach minor flooding thresholds
(tonight).

&&

.AKQ WATCHES/WARNINGS/ADVISORIES...
MD...None.
NC...None.
VA...None.
MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ635>638.
     Small Craft Advisory until 5 PM EDT Saturday for ANZ630>632-
     634.
     Gale Warning until 4 AM EDT Saturday for ANZ633.
     Gale Warning until 6 AM EDT Saturday for ANZ654-656-658.
     Small Craft Advisory until 6 PM EDT Saturday for ANZ650-652.

&&

$$
SYNOPSIS...RHR
NEAR TERM...RHR
SHORT TERM...RHR
LONG TERM...CP
AVIATION...RHR
MARINE...ALB/MPR
TIDES/COASTAL FLOODING...



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