Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 190008

Area Forecast Discussion
National Weather Service Wilmington NC
808 PM EDT Tue Apr 18 2017

A cold front stalled over South Carolina will lift back north
as a warm front Wednesday into Thursday. The Bermuda High will
expand west over the region Thursday through Saturday. Another
cold front will move through Sunday night with high pressure
building in behind it into early next week.


As of 9 PM Tuesday...Long-lived convection over the SC portion
of our forecast area finally on its last legs, with no more
lightning in evidence and a transition to a remnant stratiform
rain shield ongoing. Will make some slight adjustments to pop
and weather grids, otherwise no changes needed to the forecast.
Previous discussion from this afternoon follows:

High pressure will ridge in behind cold front draped from NE SC
southwestward into SC. A moisture rich air mass but more stable
air mass behind the front over NC was producing low clouds and
spotty light rain in cooler northerly sfc flow behind the front.
Ahead of the front the winds were still S to SW with some mid
clouds mixing with the sunshine. Temps to the north were in the
mid 60s while to the south the temps were into the 70s. The
heating to the south was producing more unstable conditions this
afternoon and therefore included some thunderstorms into
northeast SC into early this eve. This activity will be focused
in more unstable air mas to the south of the front and will be
aided by convergence along sea breeze boundary just inland of SC
coast and by differential heating boundaries. Temps to the
north of the front were struggling to reach into the 70s while
temps to the south reached 75 to 80.

Models showing some decent isentropic lift over inland
Carolinas, mainly NC, tonight as southerly low level winds ride
over the shallow cool air behind the cold front. The lift will
be enhanced as a shortwave makes its way into the Carolinas from
the west. It looks like this pcp will remain mainly west of
local forecast area, but soundings and moisture profiles show
low clouds and fog a possibility. Also expect some pcp possible
as lingering boundary/trough over SC coast produces some pcp as
well. This trough/weak low will push inland on Wed as high
pressure remains wedged inland. Expect shwrs and iso
thunderstorms in convergence along this trough/lingering
frontal boundary as some shortwave energy rides through. May end
up being focused closer to the I-95 corridor Wed afternoon.
Lows tonight should be around 60, influenced by the lingering
clouds and moisture. Wed highs will reach the 70s most places
with some breaks of sun.


As of 300 PM Tuesday...Warm front will lift
slowly north across the area during Thursday as surface high
pressure settles off the southeast coast. This will pump warm
air into the region and highs on Thursday will climb into the
low 80s, mid 80s inland, after a mild Wednesday night with lows
around 60. As this warm front lifts north, surface convergence
along it will combine with a westward propagating sea breeze to
produce isolated to scattered convection on Thursday. The high-
res guidance is less aggressive with areal coverage of showers
and storms, likely due dry W/NW flow aloft, and the best chance
for tstms will be along the sea breeze as it penetrates well
inland thanks to combined synoptic SE flow. Most of the
convection is expected to be showers, but isolated tstms are
possible as MLCape increases to 500-800 J/kg despite relatively
weak ML lapse rates. Very warm temperatures are forecast
Thursday as warm southerly flow persists, and minimums will only
drop into the mid 60s, a full 10+ degrees above climo.

One other thing to note is a weak surface trough/low that may form
along the warm front and just offshore. This feature is likely to be
weak, but could provide slightly more lift and moisture that QPF
along the coast, especially in any tstms, would be enhanced. Still
do not expected widespread or heavy convection, but WPC has a
focused area of heavier rainfall, albeit still only around 0.25
inches, along the Cape Fear coast.


As of 300 PM Tuesday...A streak of very warm weather is expected
Friday and Saturday before a significant rain/thunderstorm event
accompanies a cold front early next week.

A low amplitude upper ridge across the Southeast will support
surface high pressure out near Bermuda on Friday and Saturday.
Southwesterly low-level flow around this high should provide two
warm days with highs easily reaching the upper 80s on Friday.
Earlier models had suggested a backdoor cold front may slip
southward into the area, but this appears to be a much lesser
possibility now. I have increased forecast highs and lows Friday
through Saturday by as much as 6 degrees in spots, and feel that I
still may be too cool, especially inland on Saturday. Capping aloft
with the ridge should keep diurnal thunderstorm activity in the
isolated range, mainly inland of each day`s seabreeze front.

Our weather pattern changes abruptly by Sunday as an impressive
upper level system (especially for late April) dives into the
southeastern quarter of the nation. Timing differences between the
GFS and ECMWF has narrowed quite a bit with this morning`s 12z model
run, and confidence is growing that a wave of thunderstorms will
progress from west to east across the area late Sunday ahead of the
surface cold front. A non-supercell severe risk may very well
develop with this system as models are showing rather strong but
unidirectional winds aloft coupled with a favorably positioned jet
stream along the Mid-Atlantic and New England coast.

As the mid and upper level system slows and almost cuts off along
the GA/SC coast on Monday, steadier rains could linger through
Monday and perhaps into Monday night along the coast. This is
excellent news for especially South Carolina as "abnormally dry" and
"moderate drought" designations are expanding on the latest
U.S. Drought Monitor summary.


As of 00Z...Good confidence cigs will hover right around 3000 ft
through at least the initial 12 hours of the TAF valid period
for all sites, with lesser confidence thereafter. So, it looks
like all sites will be in and out of MVFR criteria through the
period. Good confidence that convection will re-initiate
Wednesday afternoon area-wide.

Extended outlook...MVFR/IFR conditions possible in showers and
TSTMS Saturday through Sunday night.


As of 9 PM Tuesday...Latest obs show NE winds of 10 to 15 kts
with seas of 3 to 4 ft. This fits in well with the latest
forecast so no changes are expected with the latest update.
Previous discussion follows:

A rather weak cold front is dropping slowly south across the
waters, but will stall out near or just south over SC waters
later today. Winds will be N to NE north of the boundary and S
to SW to the south of the front. Most of the coastal waters will
see a northerly component to the winds develop through today
with the exception of AMZ256 where the front may stall producing
more variable winds. Winds will be generally light but will
increase and become gusty behind the front into this evening but
will diminish again into Wed morning. The increase in winds
with a greater on shore component will push seas up from less
than 3 ft up to 3 to 5 ft overnight into Wed morning.

As of 300 PM Tuesday...A warm front to the south will lift slowly
north across the waters late Wednesday night into Thursday. This
persists a weak gradient, and winds will be light and variable
Wednesday night before developing a SW primary direction Thursday
behind the warm front. Wind speeds increase behind the front,
becoming SW at 10-15 kts Thursday and Thursday night. There is also
likely to be a sea breeze in the near shore waters Thursday aftn,
with winds backing more to the south along with a more rapid climb
in speed. A variety of wave groups will exist Wed night, and seas
will remain low amplitude around 1-3 ft. On Thursday, a SW wind wave
will amplify and combine with long period easterly swell from a non
tropical cyclone in the middle Atlantic, to drive seas up to 3-4 ft
Thursday night.

As of 300 PM Tuesday...High pressure out near Bermuda will
control our area`s weather through the end of the week,
maintaining south- southwesterly winds and generally dry
conditions. Compared to models a day or two ago, the high is now
expected to be stronger and should deflect a backdoor cold
front that was earlier believed to make it this far south. Wind
speeds should average 15 knots on Friday and 10- 15 knots on





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