Area Forecast Discussion
Issued by NWS Wilmington, NC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KILM 291651
Area Forecast Discussion
National Weather Service Wilmington NC
1251 PM EDT Thu Sep 29 2016
A cold front approaching from the west will slow down on Friday and
linger near or along the Carolina coast through this weekend.
Periods of showers and thunderstorms are possible along this
front. High pressure will build in behind the front through early
.NEAR TERM /THROUGH TONIGHT/...
As of 1000 AM Thursday...Very active morning is expected to
transition to a busy afternoon as thunderstorms continue to develop
and expand across the CWA. A Flash Flood Watch has been issued for
the ILM NC counties, and is in effect until 10 PM.
Deep cutoff upper low across IN/OH continues to spin mostly in
place, while pieces of vort energy, "spokes," rotate around the main
cutoff. At the same time, much drier air is evident on WV imagery
rotating through GA and into SC, but this will be impeded in much
eastern progress, so the eastern half of the CWA will remain quite
wet today. Thunderstorms with very heavy rainfall have been ongoing
since daybreak, and have expanded back to the SW through late
morning while motion remains quick to the NE. While individual cells
move quickly NE, backbuilding and training is forecast, hence the
recently issued FFA. Freezing levels near 14 kft combined with
moderate MUCape is creating efficient warm rain processes, and with
PWATS around 1.7 inches, this is coalescing as very heavy rainfall.
SPC Mesoanalysis shows good wind/moisture convergence at 850mb,
which combined with moderate diffluence aloft on the lee side of the
upper low, is allowing strong lift, which will only increase this
aftn with heating and increasing instability.
While periods of heavy rainfall leading to flash flooding is likely,
and is supported by recent ARW and RAP guidance, the exact location
for any significant rain may be difficult to pinpoint. Expect a rain
axis to continue where it currently exists along the best low-level
wind convergence and along the moisture axis from east to west, but
otherwise areas of thunderstorms with heavy rainfall will develop
across eastern portions of the CWA through this evening. The "fly in
the ointment" to this is a clearly evident hybrid outflow/cold front
sinking southward across the Pee Dee this morning. While this
boundary may initially force updrafts in the warm sector due to
surface convergence, air behind it is cooler and more stable than to
the SE of it, and may be limiting both to temperatures and rainfall
potential. However, deep southerly flow through the column should
eventually slow and stall the progress of this boundary, causing it
to lift back to the north this aftn. A very complex forecast
however, and bust potential even in the near term is high for temps
and precip near this wavering boundary.
Highs today are expected to rise into the low to mid 80s, but may
fluctuate quite a bit in heavy rainfall and depending on where that
front wavers this aftn. While convection is forecast to wane after
dark, warm southerly flow will persist, keeping mins from falling
out of the low 70s near the coast, upper 60s inland in the drier air.
.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 345 AM Thursday...To no surprise, the cutoff upper low and
mid-level vorts, like spokes on a bicycle tire, rotating around
it will be the driver of the sensible weather across the ILM CWA
this period. The good news, models have this cutoff/closed low
slowly opening up and lifting northward by the end of this period
as a result of the strong westerlies now able to move this upper
feature. At the sfc, the cold front will slowly make some progress
to the coast on Friday and on Saturday it basically stalls either
along the immediate coast or just offshore due to it becoming
parallel with the flow aloft. Therefore POPs will be confined
along the eastern 1/3rd of the ILM CWA and the adjacent Atlantic
waters thruout this period. The dry slot or tongue will affect
mainly the western 2/3 of the ILM CWA helping to eliminate POPs
in that area. Could see hier POPs during the pre- dawn hrs along
the immediate coast during the pre- dawn Sat and Sun hours due to
the combined nocturnal development over the waters and any mid-
level vorts having either rotated around the cutoff low or picked
up off the Florida west coast or the northern Bahamas. Max/Min
temps to remain above normal, 1 to 2 categories, thruout this
.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 3 AM Thursday...Large upper low finally lifting out to the
northeast early in the period. In its wake will be a weak pressure
gradient and a dissipating frontal boundary just offshore. A few
showers along this front may affect coastal locales but suspect that
most areas stay rain-free and seasonable. As the upper low begins to
interact with troughiness east of Canada rising surface pressures
advect into the eastern U.S. Tuesday and Wednesday. Should this flow
be NE then some lower dewpoints may creep into the Carolinas but
current progs show more ENE and likely preclude that. Building
heights aloft argue for not much in the way of meaningful precip
though a few sprinkles tough to rule out in the onshore low level
flow. This gradient could become pinched by Matthew and it may
become breezy locally by Wednesday if the quick GFS is correct.
.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 18Z...SCT +SHRA/TSRA from KILM to KLBT, isolated elsewhere
through 21Z. AFT 21Z mainly VCSH all terminals, light south to
southwest wind overnight. Within strong isolated SHRA this TAF
cycle, expect localized surface wind gusts to 30 KT. Mainly VFR
this period with occasional MVFR ceiling in and near convection
and patch VSBY restrictions of 1-3 SM between 9z-12z Friday. 12z-
18z Z VCSH continues across all terminals.
EXTENDED OUTLOOK...Primarily VFR. Isolated to scattered afternoon
showers through much the period, although limited in strength and
coverage due to dry air aloft.
-- End Changed Discussion --
NEAR TERM /THROUGH TONIGHT/...
As of 1000 AM Thursday...Surface cold front will remain inland
today, keeping predominantly S/SW winds across the waters. Wind
speeds have been 10-15 kts this morning, and little change in speed
is forecast, although occasional gusts up to 20kts are possible,
especially this evening and tonight. Seas will remain a nearly
uniform 3 ft, thanks to a 10-11 sec SE swell and 5-6 sec southerly
SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 400 AM Thursday...The movement of the cold front at a
snail`s pace will push it to the coast late Friday and possibly
either across the coastal waters or just offshore from Carolina
coastlines. The sfc pressure pattern will result in a S to SW wind
direction thruout this period. The sfc pg will remain
semi-tightened east of the cold front, with 10 to 15 kt speeds,
and somewhat relaxed in the vicinity of the cold front, with 10 kt
or less speeds. Significant seas to continue at 2 to 3 ft thruout
this period. The ese ground swell at 7 to 9 second periods will
mainly dominate the seas with local wind driven waves at 3 to 4
seconds on top. Expect convection thruout this period, with the
pre-dawn hrs thru daytime morning hrs of each day exhibiting the
hier pops during this period.
LONG TERM/SUNDAY THROUGH MONDAY/...
As of 3 AM Thursday...Weakening front along the coast Sunday will
keep winds very light though there is some uncertainty regarding
direction-though primarily onshore. Small wind chop combine w ESE
swell for a 2 to 3 ft dominant wave height. No significant changes
noted heading into Monday.
NC...Flash Flood Watch until 10 PM EDT this evening for NCZ087-096-