Area Forecast Discussion
Issued by NWS Wilmington, NC

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771
FXUS62 KILM 230711
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
311 AM EDT Sun Jul 23 2017

.SYNOPSIS...
A Heat Advisory remains in effect today. A cold front will
approach from the north and may slip into the area mid week
before stalling and then dissipating during Thursday. Showers
and thunderstorms with heavy rain will increase in coverage
Monday through Wednesday and this will serve to knock down the
heat although the high humidity will remain. Drier air may
briefly work into the area Thursday, but the arrival of a
stronger cold front will bring more thunderstorms by the
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Sunday...Mid level pattern starts to change today as the
5h ridge weakens and is replaced by subtle 5h troughing. Minimal
amount of mid level subsidence present yesterday is gone today while
precipitable water values exceed 2.25 inches. Mid level forcing will
be weak to non-existent with only a weak shortwave passing north of
the area around peak heating. Main forcing today will be from
diurnal heating and the sea breeze/Piedmont trough. Given a more
favorable environment aloft anticipate some deeper convection along
the sea breeze with slightly higher coverage across western areas,
associated with the trough. Activity will be diurnally driven with
storms weakening in the evening and dissipating within an hour or so
of sunset. Nocturnal convection seems like a good bet overnight,
although coverage is likely to be limited given a lack of forcing.
Some of this activity may drift onshore and will carry a slight chc
pop along the coast overnight. 00Z GFS does show some weak PVA
pushing offshore late in the period but this could be feedback. The
18Z version had the same feature but in a much stronger state. The
latest Canadian, ECMWF, and NAM do not have this feature.

Temperatures will once again run above climo with highs ranging from
lower 90s along the coast to mid 90s inland. Cannot rule out an
isolated upper 90 degree reading in far western locations but
these should be isolated. Continued southerly flow will maintain
dew points in the low to mid 70s. This combination will push
heat index values above 105 this afternoon and require
continuation of the heat advisory. Temps well above climo will
continue overnight with lingering debris cloud and southerly
flow keeping lows in the mid to upper 70s.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 300 AM Sunday...This forecast period will be marked by
increasing shower and thunderstorm coverage. The hot ridge will
have retrograded back to the Southern Plains while the Bermuda
ridge remains offshore and suppressed to our S. This will allow
an east coast trough to dig which will coax a surface cold front
slowly southward. The front should be in close proximity by Tue
morning and will likely stall overhead by Wed. In addition, the
Piedmont Trough is expected to be displaced further E. This
scenario should allow for a likelihood of showers and
thunderstorms. These thunderstorms will be enhanced by mid-level
short wave troughs. Currently models are showing one of these
upper level impulses will move across the area Mon afternoon and
eve and another Tue afternoon and night. The later impulse may
actually close off across the Carolinas and linger. Rainfall
rates should be very high given precipitable water values will
be on the order of 2.25 inches or higher for much of the period.
Excessive rainfall is certainly possible given slow storm
motions and the expectation that cells will train along the
front. Rainfall amounts could easily reach 2 to 3 inches across
a majority of the area during this time with more rainfall
possible on Wed as well.

The clouds and widespread and significant rainfall will knock
down high temps, but the high humidity will remain. Highs both
days will be around 90, averaging just above 90 on Mon and just
below 90 on Tue. Heat index values will be in the 100 to 105
degree range Mon and around 100 Tue. Lows will be in the muggy
lower to mid 70s.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 300 PM Saturday...The upper air pattern will transition
to a cooler pattern for the eastern U.S. next week.

The upper ridge will retrograde to the Four Corners region by
Saturday. A downstream trough will develop along the East Coast,
enhanced by a shortwave that almost cuts off across the
Carolinas mid week, and a progressive shortwave that moves
through New England Friday.

At the surface, low pressure should sink down to coastal South
Carolina by Wednesday night. This will drag a weak cold front
down from the north. While yesterday`s ECMWF kept the front
across central North Carolina, today`s run is in agreement with
the GFS that the front will sink all the way down into the
Charleston, SC area. With low-level convergence focused along
the front and cooler upper-air conditions with the disturbance
aloft, expect thunderstorms to be fairly widespread into Wed.
There is the potential we`ll see 1-3 inches of rain area-wide.

As the upper disturbance kicks offshore Thursday afternoon
and drier mid-level air bleeds in from the west, the surface
front should dissipate. Friday should be the warmest day in the
extended period with highs popping back into the lower 90s and
heat indices over 100 degrees again. By Saturday, the GFS and
ECMWF are in surprisingly good agreement that a stronger cold
front will dive southward and into area, with another
enhancement in the potential of thunderstorms.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
As of 06Z...VFR conditions will dominate the period with brief
period of MVFR or even IFR possible from afternoon and evening
showers and thunderstorms. Both the sea breeze and the Piedmont
trough should be more active than in past days but coverage will
still be limited. Storm coverage and activity will diminish
quickly with the loss of heating as the sun sets. Bermuda High
will maintain southerly flow through the valid TAF period. Sea
breeze will bring gusts to coastal sites with potential for
speeds in excess of 20 kt from the southeast. Speeds tonight
will be around 10 kt, preventing any significant fog
development.

Extended Outlook...Possible MVFR from patchy stratus near
sunrise Mon. Brief MVFR/IFR possible in showers and
thunderstorms Mon thru Thu.

&&

.MARINE...
NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Sunday...Pinched gradient this afternoon and evening will
push speeds to 15 to 20 kt with potential for solid 20 kt at times.
Prolonged and increasing south to southwest flow will help build
seas to 3 to 5 ft with widespread 5 ft likely well away from shore.
Will maintain inherited SCEC for all zones for today continuing into
the overnight hours.

SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 300 AM Sunday...A cold front will sag south and be in
close proximity Tue and may stall overhead Tue night or Wed. The
eastward displaced Piedmont Trough will serve to keep a rather
tight pressure gradient in place and this will keep sustained
southwest winds near 20 kt Mon and Mon eve. As the cold front
gains proximity late Mon night and through the remainder of the
period, wind speeds will decrease to no higher than 10 to 15 kt
with 10 kt or less by late Tue night as the wind direction
becomes variable. Seas will be 4 to 5 ft through the first half
of the forecast period, subsiding to 3 ft or less during Tue
and Tue night. Thunderstorms will be increasing across the
waters during this time and mariners should expect poor
visibility in very heavy rain along with locally higher winds
and seas.

LONG TERM /WEDNESDAY THROUGH THURSDAY/...
As of 300 PM Saturday...A cold front will become increasingly
ill-defined on Wednesday, and by Thursday afternoon should wash
out as southwesterly winds increase ahead of a stronger front
that should affect the area next weekend.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for SCZ017-023-024-032-033-039-053>056.
NC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for NCZ087-096-099-105>110.
MARINE...None.

&&

$$

NEAR TERM...III
SHORT TERM...RJD
LONG TERM...TRA
AVIATION...III



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