Area Forecast Discussion
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FXUS62 KILM 292318
Area Forecast Discussion
National Weather Service Wilmington NC
718 PM EDT Thu Sep 29 2016
A stationary front will waver along the coast through this
weekend. Periods of showers with an isolated thunderstorm are
possible along this front. Weak high pressure will build in
early next week. Hurricane Matthew may bring increasing waves
by the middle of next week.
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
As of 730 PM Thursday...Radar loops show a marked decrease in
coverage and intensity of showers/t-storms across the area over
the past couple of hours. We have dropped the Flash Flood Watch
since widespread heavy rainfall no longer appears likely.
Scattered showers and t-storms will continue for the next couple
of hours but should reach a minimum in coverage sometime around
midnight, with the bulk of the remaining activity likely offshore
then. It remains to be seen if we see a rebound in coverage late
tonight as a weak vorticity lobe pushes northward across the
coastal waters. Latest RAP model suggests it`s wise to show a
secondary peak in forecast coverage focused around 3-6 AM. Only
small changes made to temperatures. Discussion from 330 PM
WV imagery paints the picture this aftn with a massive bowling
ball of an upper low spinning across IN/KY, and our sensible
weather is almost exclusively developed in response to this
feature. The southern side of this low features very dry air
moving across the Gulf Coast states and pushing into SC/NC, while
on the eastern side moisture remains deep, and lift is being aided
by diffluence at 500mb and above, creating the tremendous rain
seen across portions of eastern NC today. The dividing line
between these two features is a nearly stationary cold front,
recognized more by a dewpoint gradient than any significant
temperature advection, and while this front will try to push
slowly east, it is aligned nearly parallel to the mid- level flow
and will struggle to make much longitudinal gain through tonight.
The combination of this cold front, significant diffluence aloft,
and many mesoscale outflow boundaries has allowed convection to
become deep and heavy, and a flash flood watch remains in place for
all of SE NC until 10pm. A line of storms has developed along a 925-
850mb confluence boundary which is somewhat being aided by a weak
sea breeze as well. Convergence along this line combined with
diffluence at 300mb is driving updrafts upward, but with only
marginal MLCape in an environment of high freezing levels and high
PWATs, warm rain processes are likely dominating any Bergeron
processes, and hence rain rates are very impressive - 2 to 3 inches
per hour at times. While the mean flow is from the SW, driving
storms to the NE, they continue to redevelop to the SW, so more
flash flooding is possible through this evening, especially in
places that haven`t been worked over already. High res ARW and RAP
continue to model this pretty well, and this suggests convection
will wane with loss of heating this evening. Another round of
showers/storms may approach the coast overnight as another vort lobe
rotates northward near the area, but most of this precip is forecast
to remain just offshore.
Another thing to note is that, while SPC has removed the MRGL risk
locally, a hybrid outflow/warm front exists across NE SC. As cells
move across this boundary they are ingesting enhanced helicity and
latest SPC mesoanalysis has 100-150 m2/s2 of 0-1 km SRH. This is not
abnormally strong, but have noticed several weakly rotating cells,
and this will likely continue for several more hours. Local
hodographs are not very supportive for any tornadoes due to
primarily unidirectional flow, but local backing along this boundary
is enough to keep cells rotating, with just a very minimal chance
for a tornado this aftn.
After convection wanes tonight, southerly winds will persist, mainly
near the coast, with some light W/SW winds developing far western
zones near/behind the cold front. This creates a challenging
temperature forecast with a significant gradient from west to east,
but expect lows near the coast to remain warm, low 70s, with some
mid 60s possible well west of i-95.
.SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/...
As of 345 PM Thursday...Little change in the synoptic setup into the
weekend as bowling ball of the upper low spins slowly across IN/KY
before lifting into OH late in the period. At the surface, a very
slow moving cold front will push eastward, likely finally crossing
offshore Friday night before stalling once again. With diffluence
remaining east of the upper low, and low level confluence likely
persisting along and east of the boundary, more convection with
periods of heavy rain is forecast Friday. The best chance for
rainfall will again be the eastern third of the CWA where mid-level
RH remains elevated despite a very dry punch within the dry slot
working into the Pee Dee. This creates 2 distinct air-masses across
just the local CWA, with PWATS forecast to drop below 1 inch well
inland, while remaining at 1.5-1.75 near the coast, and inland
counties may finally have a dry day Friday. By Saturday, most of the
CWA is forecast to get into the drier air, but once again the
immediate coast may still see no relief from this humid airmass with
more showers/tstms possible along the coast.
Highs both Friday and Saturday are forecast to be slightly above
climo, low 80s, but will be heavily impacted by clouds and rainfall.
Mins inland will drop into the upper 50s/low 60s both nights within
that drier air mass, but will remain near 70 at the coast.
.LONG TERM /SUNDAY THROUGH THURSDAY/...
As of 3 PM Thursday...Incredibly the relatively sharp west to east
precipitable water gradient remains in place across NE SC/SE NC much
of the extended period before the potential of tropical moisture
late next week. In essence this brings a sensible weather forecast
of chances for showers near the coast and very little along the far
interior zones. This in part due to the dry mid-level air wrapped
around the large upper low to the west, and the remnant presence of
dry air aloft across the area even as the low pulls away from the
region on Sunday. Even so a chance of showers will prevail along
the eastern 1/3 of the forecast zones days 4-7 with isolated TSTMS
favored nocturnally over the coastal waters, or a sea breeze pop up
in the middle afternoon. Temperatures to run near to a bit above
normal, especially for the minimums this period. Tempering of
maximum temperatures days 6 and 7 is attributed to increasing NE flow
across the area, but about normal max temp readings for early OCT.
.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
As of 00Z...Upper low still nearly stationary, with a weak
surface front over our CWA. Look for dying convection over the
next several hours. Showers could briefly affect the coastal
terminals, with VCSH sufficing for now. Overnight, some light fog
is possible, briefly IFR at LBT. A coastal shower cannot be ruled
out along the coast toward morning. Friday, more of the same with
convection becoming scattered, most numerous around or just after
EXTENDED OUTLOOK...Primarily VFR. Isolated to scattered afternoon
showers through much the period, although limited in strength and
coverage due to dry air aloft.
NEAR TERM /THROUGH TONIGHT/...
As of 730 PM Thursday...No substantial changes are needed to the
forecast this evening. Scattered showers and storms remain the
largest weather concern to mariners. Shower activity should reach
a minimum in coverage around midnight but should grow in coverage
late tonight as an upper level disturbance approaches from the
south. Discussion from 330 PM follows...
Surface cold front will remain well inland through tonight, and
winds over the waters of 10-15 kts will persist from the south
through the near term. With these winds creating a 2ft/4-5 sec
southerly wave, and a low-amplitude 10-11 sec SE swell also
existing, seas will remain 3-4 ft through tonight. Showers and
thunderstorms may develop late in the period as well, most likely
affecting the outer portions of the NC waters.
SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 345 PM Thursday...A cold front will be west of the waters
Friday before wavering across or potentially still just inland on
Saturday. This creates predominantly S/SW winds of 10-15 kts on
Friday, backing to the SE around 10 kts on Saturday, again dependent
on where the front stalls. A period of variable winds is possible
late Friday into Saturday as the front drifts into the vicinity, but
speeds will be light during that time. Seas will hover around 3 ft
both Friday and Saturday with a low amplitude 9-10 sec SE swell and
2-3 ft 5 sec southerly wind wave comprising the spectrum.
LONG TERM/SUNDAY THROUGH TUESDAY/...
As of 3 PM Thursday...This period will be marked by 3-5 foot seas
and highest seas offshore, but elevated wave heights may
gradually arrive after Tuesday due to increasing swell from
Hurricane Matthew. No advisories expected through Tuesday.
Isolated showers and TSTMS can be expected this period and radar
updates are encouraged.