Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 222324

Area Forecast Discussion
National Weather Service Wilmington NC
724 PM EDT Mon May 22 2017

Waves of low pressure will develop along stalled cold front
producing increased chance of showers and thunderstorms through
mid week. A cold front trailing from a deep low pressure system
over the Ohio Valley on Thursday will push through bringing the
final round of storms to the area. High pressure will build in
on Friday maintaining quieter weather for much of the weekend.


As of 545 PM Monday...This early evening forecast update
increased forecast PoPs to 100 percent for almost the entire
forecast area tonight with the expectation of a widespread 0.50
to 1.00 inches of rain with embedded 4 inch amounts. If rainfall
amounts over the past two weeks weren`t so low, I would probably
have issued a flash flood watch given the potential for such
heavy rainfall this evening in a short period of time. Severe
weather potential is low given fairly low CAPE values and a
saturated airmass. The HRRR model is performing best with the
convective outflow boundary massing along a line from Florence
through Dillon, Whiteville, and just north of Wilmington.

Two week rainfall totals and percentages of normal (from this
morning) over the past two weeks:

Wilmington, NC         0.01"...<1 percent of normal
Lumberton, NC          0.22"...16 percent of normal
North Myrtle Beach, SC 0.42"...29 percent of normal
Florence, SC           1.05"...76 percent of normal

Discussion from 300 PM follows...

Waves of low pressure will move along a stalled frontal
boundary which appears to be running from SW to NE west of local
forecast area. A deep W to SW flow of warm and moist air with
dewpoint temps near 70 degrees will persist ahead of this front.
The moisture profiles show RH greater than 80 percent from near
the sfc up through the mid to upper levels. Pcp water values
remain above 1.75 inches increasing up near 2 inches for most of
tonight. With deep moisture present, just need a forcing
mechanism to help to produce showers and thunderstorms over the
area. Shortwave energy rotating around a deepening mid to upper
trough digging down from the Upper Great Lakes combined with
increased jet dynamics will produce periods of showers and
thunderstorms through the period focused along and ahead of this

Overall expect plenty of clouds but enough instability to
initiate showers and storms especially along the stalled front,
sea breeze front or any other localized boundaries. The HRRR
depicted the sea breeze development and other shwrs/tstms inland
this afternoon and shows more widespread activity developing
along waves of low pressure along front just along the west to
northwest periphery of our local forecast area closer to the
vicinity of I-95 corridor later this aftn into early this
evening and shifting eastward toward the coast overnight. SPC
shows marginal risk for severe storms over western zones and
farther west and south with main threat of damaging wind gusts.
WPC has issued a MPD for localized pcp amounts greater than 3
inches focused along the stalled front nosing into our inland
SC zones, mainly into Darlington, Marlboro, Florence area.

Not much will change for Tuesday as the front remains wavering
over the area as shortwave energy moves around deepening upper
trough right across our area. The models show a MCS type feature
with the strongest storms and heaviest rain exiting the area
off to the northeast by noon on Tues, but clouds and shwrs
should persist through Tues aftn.

A very warm and moist airmass along with clouds and pcp will
keep temps well above normal overnight, remaining around 70
degrees most places. High temps tomorrow will only gain about 10
degrees making it to around 80 degrees.


As of 300 PM Monday...Front remains stalled in the area Tue
night, moving a little closer to the coast by Wed morning.
Although moisture will be on the high side there will be a lack
of upward motion. In fact there may be a lull as the passage of
a surface wave Tue evening and its associated shortwave will
lead to a period of subsidence Tue night. Forecast soundings
depict this rather well showing a mid level subsidence inversion
developing and precipitable water values dropping close by as
much as 0.50 inch overnight. Mid level trough axis remains well
west of the area Wed which will keep the front lingering in the
region. Passage of the surface wave Tue night may briefly push
the front south of the area early Wed but it is quick to return
north as a warm front around midday Wed.

Better rainfall chances develop later Wed and Wed night as potent
shortwave dropping into the 5h trough early Wed helps kick the mid
level pattern in motion. The 5h trough rotates east-northeast Wed
into Wed night, driving a stronger cold front into the region Wed
night. Increasing southwest flow ahead of the front, as well as
strengthening low level jet, increases deep moisture in the region.
Pwats rise to near 2 inches Wed afternoon and evening and mid level
lapse rates steepen. PVA ahead of advancing shortwave, strong
divergence aloft, and the abundance of moisture coupled with low
level dynamics should yield a healthy line of convection Wed
afternoon and evening. Although severe parameters are not
particularly high there will be at least some potential for severe
weather given the strengthening low level jet and modest shear.
Large dry slot sweeps in late Wed night as the trough axis moves
east, ending any precip prior to daybreak Thu. Cold front likely to
be moving into the region as the period ends so cooler and drier air
will not arrive until after the end of the period. High temperatures
will continue running below climo with lows above to well above
climo due to clouds and moisture.


As of 300 PM Monday...Deep frontal moisture to be offshore by the
start of the period. However the main trough axis and strongest
shortwave will be crossing the area and this should manage to
squeeze a few light and short-lived showers. Continued dry advection
and the lifting of the trough Thursday night should keep us dry
despite one last final and moderately strong vorticity center
streaking by. Zonal flow will keep the weekend dry and fairly
seasonable with only gradually increasing surface dewpoints.


As of 00Z Tuesday...A cluster of heavy thunderstorms just west
of Myrtle Beach is merging into a line of showers and embedded
storms stretching across the Wilmington area. This will create a
massive problem for aircraft operations in the ILM area for the
next several hours with prevailing IFR visibilities continuing until
this thunderstorm cluster pushes offshore. Elsewhere, mainly
light rain falling from mid-level clouds aloft should prevail
through the evening hours with temporary heavier convective
showers potentially developing at times.

Cloud ceilings should lower to the MVFR/IFR category late
tonight as surface temperatures cool. Scattered showers may
linger all night, with scattered thunderstorms redeveloping
Tuesday during the heat of the day.

Extended Outlook...Reduced flight categories will be possible
in periodic showers and thunderstorms through Thursday. The
strongest convective activity should occur Wednesday.


As of 545 PM Monday...No thunderstorms have reached the coastal
waters yet, but this will change over the next several hours as
widespread storms inland advance east and offshore. Visibilities
reduced below 2 miles in heavy rain with frequent lightning are
expected. Prevailing southwesterly winds may turn northwesterly
and gusty in and near storms. Discussion from 300 PM follows...

South to southeast flow will increase from 10 to 15 kts up to
15 to 20 kts as gradient tightens between cold front to the west
and high pressure to the east. This persistent southerly push
will produce an increase in seas into early Tues morning from 2
to 4 ft this afternoon up to 3 to 5 ft heading into Tues. WNA
model data shows a fairly sharp rise Tues with the greatest
winds and seas occurring through the day on Tues. May see a
period of SCEC conditions late Tuesday.

As of 300 PM Monday...Front remains west of the waters through
the period, maintaining southwest flow. Speeds start to increase
later Wed and Wed night as the front moves to the coast and
slowly strengthening low moving into the OH valley helps tighten
the gradient. Southwest flow on the high end of the 15 to 20 kt
range Tue night will increase to a solid 20 kt around midday
Wed with 20 to 25 kt expected late Wed and Wed night, likely
requiring SCA headlines for all waters. Seas will build due to
prolonged and increasing southwest flow. Seas 4 to 5 ft at the
start of the period will build to 4 to 6 ft during Wed,
eventually reaching 7 ft Wed night.

As of 300 PM Monday...Small Craft Advisory will be in effect on
Thursday just ahead of a cold front that will pinch the
gradient. Within the agitated wind field of the pinched gradient
a strong upper disturbance will traverse the area possibly to
enhance wind gusts. In the wake of this boundary there will be
west winds to round out the period of gradually diminishing
speed. Wave heights will similarly abate somewhat slowly.


SC...Beach Hazards Statement from 7 AM EDT Tuesday through Tuesday
     evening for SCZ054-056.
NC...Beach Hazards Statement from 7 AM EDT Tuesday through Tuesday
     evening for NCZ106-108.



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