Area Forecast Discussion
Issued by NWS Wilmington, NC

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359
FXUS62 KILM 232302
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
702 PM EDT SAT JUL 23 2016

.SYNOPSIS...
A ridge of high pressure will expand across the Carolinas bringing
increasingly hot and humid conditions through the weekend and
much of next week. Isolated showers and thunderstorms are possible
each afternoon, and heat advisories will likely be needed each day
beginning Sunday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
As of 700 PM Saturday...PoPs have been raised to 60-80 percent
across a corridor from the Cape Fear River in SE Bladen County
across Whiteville to near Fair Bluff and Nichols. Colliding
outflow boundaries are producing an impressive pulse of deep
convection and heavy rain now, but with no upper level support and
a decreasing number of future boundary collisions this activity
should wind down over the next 2 hours. By 11 PM I`ve got PoPs
below 15 percent with clearing skies. The only other change to the
forecast is to introduce some 20-30 percent PoPs late tonight
south of Cape Fear where the landbreeze may have enough low-level
convergence associated with it to ignite storms near or just off
the beaches. The 20z and 21z HRRR runs both suggest this is
possible, and this does fit the synoptic pattern for this time of
year. Discussion from 300 PM follows...

Hot and humid day continues as broad upper ridging begins to
expand to the east. The combination of Bermuda high pressure and
this expanding upper ridge has produced temps into the 90s away
from the coast, which combined with low 70s dewpoints has pushed
heat index values up over 100 degrees most areas. While this is
hot, it is likely the coolest of the next several days...

This heat and humidity is also leading to a highly unstable airmass.
Latest analysis from SPC shows 1000-2000 J/kg of MLCAPE with LI`s to
-6 and steep low level lapse rates. There is minimal mid level
forcing however, but diffluence remains at jet-level in a somewhat
weakened state from this morning as the RRQ of a southward sinking
jet moves off the GA/FL coast. This in addition to residual outflow
from this morning`s tstms has allowed convection to develop across
SC, and should expand at least somewhat west this evening, with new
development also possible in southern NC. No severe is expected, but
very slow storm motion thanks to anti-parallel winds from the
surface to the mid-levels could produce an isolated flooding threat
through this evening. Convection will wane after dark with no mid-
level energy expected to tap residual elevated instability.

Once tstms wane, expect a clear to partly cloudy night with some
stratocu possible late near the coast. Guidance was far too cool
with mins last night, but the LLJ is forecast to be 15-20 kts weaker
than last night, so a blend of the guidance is preferred, and lows
are forecast to be a bit cooler than last night, 74 well inland to
78 near the coast. While fog is not forecast to be widespread or
significant, some patchy fog is possible where rain has been
heaviest. Local UPS Fog procedure develops just some spotty fog, so
have left any mention out of the Wx grids as most areas will keep
unrestricted visibility.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/...
AS OF 330 PM SATURDAY...Hot and dry will be the rule beginning
Sunday as a massive ridge of high pressure envelops much of the
southern tier of the CONUS. Although the ridge itself does not have
tremendous latitudinal amplitude, it is quite strong and 1000-500mb
thicknesses rise towards 580dm Sunday and Monday, with heights
climbing towards 595dm directly overhead the Carolinas by Monday
aftn. At the same time, 1000-850mb thicknesses approach 1435m,
fueling 850mb temps around 20C.

This ridge suppresses a weak mid-level low to the SW, moving overhead
Florida late Sunday, with easterly flow developing through all but
the lowest 100-150mb of the column. Typically this is a moistening
flow direction, but in this case trajectories originate from a dry
air mass offshore, and PWATs drop towards the bottom quartile for
late July by Monday. At the surface, return flow prevails however, so
ample sunshine combined with warm/moist flow and bulging thicknesses
aloft will create hot and humid conditions but with very few tstms.
Highs Sunday will rise well into the 90s, with mid 90s expected
inland and around 90 at the beaches. On Monday, temps will be just a
bit cooler thanks to the aforementioned upper low causing slightly
reduced thicknesses, but will still rise into the low 90s. These
hot temps combined with dewpoints climbing back into the low/mid
70s, will necessitate a heat advisory for Sunday, and one has been
issued, valid from 11am to 8pm, for all zones. Slightly cooler
temps and and lower dewpoints Monday will keep heat index values
just below advisory thresholds. Mins both nights will be well above
climo, mid and upr 70s.

While the airmass will be very unstable this period, convection will
be isolated at best, thanks to increasingly dry air aloft and
warming mid-level temps. Although 500mb and 850mb temps drop a bit
on Monday, the airmass dries even further, so will cap POP at SCHC
Sunday and Monday, with the highest values along the sea breeze in
SC.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
AS OF 330 PM SATURDAY...Overall medium range guidance is pointing
to strong mid level ridging, on the order of 594DM Tuesday into
Wednesday. Heights slowly fall from that point onward through the
remainder of the period. Other than the Piedmont trough, still not
much in the way of forcing for convection. Certainly subtle mid
level features will help to kick off activity but all but
impossible to pinpoint at this time. The forecst of slowly
increasing pops still looks good with the best chance Thursday
when PWs jump to just over two inches. No changes to the
temperature forecast.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
As of 00Z...Synoptically, little has changed over the past week.
Convection will wind down this evening. The HRRR has some weak
convection toward morning near the coast. Do not really see the
impetus for this. Sunday, any isolated convection will form first on
the sea breeze resultant, with a secondary region along the thermal
trough. Predominately southerly flow.

EXTENDED OUTLOOK...Isolated to scattered afternoon convection
Tuesday through Thursday may create periods of MVFR/IFR.
Otherwise expect VFR.

&&

.MARINE...
NEAR TERM /THROUGH TONIGHT/...
As of 700 PM Saturday...Winds at Wrightsville Beach have reached
18 knots in gusts over the past hour with the seabreeze
circulation. These stronger nearshore winds are peaking about now
and should gradually veer more southwesterly and diminish in speed
overnight. Discussion from 300 PM follows...

SW winds of 5-10 kts this aftn will slowly increase tonight to 10
to as much as 15 kts around offshore Bermuda high pressure. Near
shore, a sea breeze has backed winds to the south with a subtle
increase in speed, but this will gradually return to the synoptic
SW flow this evening. Seas will remain 2-3 ft, with an 8 sec SE
swell and 4 sec SW wind wave comprising the spectrum.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
AS OF 330 PM SATURDAY...Bermuda ridging will remain the most
influential feature through the short term, creating consistent wind
and wave conditions. SW winds will prevail both days, with speeds
increasing from around 10 kts Sunday, to 10-15 kts on Monday. A
persistent but weak 2ft/8sec SE swell will exist in the spectrum,
but avg wave periods will gradually shorten as a SW 4-5 sec wave
amplifies. Wave heights will be 2-3 ft through Monday, becoming more
uniformly 3 ft late.

LONG TERM /TUESDAY THROUGH THURSDAY/...
AS OF 330 PM SATURDAY...Expect south to southwest winds of 10-15 knots
through the period as the Bermuda Azores high pressure system and
the Piedmont trough are the main drivers of winds. Significant seas
will be 1-3 feet with wave periods mostly confined to shorter period
wind waves.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...Heat Advisory from 11 AM to 8 PM EDT Sunday for SCZ017-023-024-
     032-033-039-053>056.
NC...Heat Advisory from 11 AM to 8 PM EDT Sunday for NCZ087-096-099-
     105>110.
MARINE...None.

&&

$$

SYNOPSIS...SHK
NEAR TERM...TRA/JDW
SHORT TERM...JDW
LONG TERM...SHK
AVIATION...DL



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