Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 271340

Area Forecast Discussion
National Weather Service Wilmington NC
940 AM EDT Sat May 27 2017

Increasingly hot temperatures are expected over the weekend
as high pressure over Florida moves slowly eastward. By Tuesday
the high should be far enough away to allow a weak cold front
to stall across the Carolinas. This front will linger through
Friday, enhancing the potential for scattered thunderstorms.


As of 940 AM Saturday...No significant changes to the forecast
at this time. A batch of convection will pass north of the area
between 5pm and 10 pm. A bit later in the evening a few storms
will approach our inland I-95 zones between 10pm and midnight
but diurnal cooling will cause them to weaken on approach at far
western doorstep.

As of 300 AM Saturday...A hot and dry start to the Memorial Day
weekend as flat elongated ridge across the Gulf Coast exerts its
influence locally. However, the weather will become potentially more
active late tonight as a vorticity impulse embedded in a band of
increased westerlies aloft rotates just north of the area.

Satellite imagery early this morning shows mostly clear sky
conditions minus the odd patch of cirrus moving overhead. Broad high
pressure centered across FL remains the dominant feature, while a
weak baroclinic boundary oriented E-to-W lies across VA and back
into the OH Vly. It is the confluence between the upper features
driving this surface setup that will create the enhanced westerlies
and potential convection late. Before this occurs, a thermal plume
of very warm air will advect from W to E across the region causing
850mb temps to climb towards +16C, and 700mb temps up over +11C.
This warming of the column combined with dry W/NW flow aloft will
drive temps into the low 90s away from the beaches this aftn, while
also serving as a cap to any diurnal convection. In fact, even as
PWATs climb towards 1.5 inches, diurnal CU will struggle to gain
much depth thanks to this dry and warm air aloft. In other words, it
should be a great start weather-wise for the holiday wknd, with S/SW
wind gusts of 20-25+ mph being the only non-ideal parameter this

Winds will ease after dark, and the focus will shift to an area of
convection likely to be ongoing to the west this evening. A potent
vorticity impulse will race eastward tonight, likely shifting just
north of the area before exiting offshore early Sunday morning. A
very warm and increasingly unstable airmass ahead of this feature
suggests that tstms will develop beneath it, and may form into an
MCS as is typically seen in these "Ring of Fire" type setups. The
question is how much of this will persist as we lose instability
tonight, as well as how far south will the convection sink as it
becomes impeded by the ridge. Most high res guidance suggests that
tstms will just graze the northern fringe of the CWA after midnight,
and even this will be in a weakening state. It is interesting to
note that both the NSSL WRF and latest ARW suggest convection
falling apart as a cold pool surges ahead of the tstm complex. Not
sure how much to trust that exact scenario, but weakening of tstms
seems likely as they zoom past the area despite strong ML lapse
rates. SPC has placed the far NW tier of counties into a MRGL risk
for tonight, and with increasing mid-level winds and steep lapse
rates, if the cap is broken and convection can sustain itself
tonight, a strong wind risk does exist. However, expect most of the
strong storms to be north and west of the CWA, and have carried CHC
POP along and north of a line from Hartsville, Maple Hill,
NC, with barely any chance at all south of there.

Continued warm SW winds much of the night will keep mins from
falling much  below 70 most locations tonight. Lower rain cooled
mins are possible in a few locations if tstms develop, but this will
be a very near term forecast challenge if it occurs.


As of 300 AM Saturday...500 mb high pressure will strengthen as
it slowly moves eastward across Florida early next week. Surface
high pressure typically sets up around 30 degrees north latitude
this time of year, but will be located much farther south this
time given the position of the upper center. Deep westerly flow
is expected across the Carolinas both days bringing hot
temperatures and a chance of thunderstorms.

A series of 500 mb vort maxes will move across North Carolina
and northernmost South Carolina Sunday afternoon through Monday
night. Each model has a different timing and path for the individual
disturbances, so it`s not possible to have much confidence about
the timing and location of the best risk for storms. Despite
steep lapse rates aloft and relatively good low-level moisture,
there is a good deal of dry air noted in all models at and above
the 800 mb level which should reduce the number of storms able
to develop. Even if only a couple storm cells do develop, strong
westerly winds throughout the depth of the troposphere should
push these cells eastward at 30 mph. Given even limited storm
organization, dry air aloft would favor storms propagating
rapidly eastward on "running" outflow boundaries that could push
the effective storm motion up over 40 mph. Forecast PoPs each
12-hour period range from 20-30 percent, generally higher north
of Florence, Myrtle Beach, and Southport.

With high angle sunshine and 850 mb temps of +18C to +19C, high
temperatures should soar into the 90s both days away from the
beaches. Monday looks like the warmest day when temps in the
mid 90s could occur inland with heat indices eclipsing 100
degrees during the afternoon hours. There is only one record
high that might be in jeopardy: Monday`s record in Florence is
only 95 (set in 1991) versus our latest forecast of 93.

Moderate to strong seabreeze circulations are expected both
days with afternoon wind gusts near the beaches likely reaching
25 mph. Synoptic westerly wind should keep the seabreeze front
very close to the coast with only limited penetration (~10
miles) inland expected. Very warm nighttime lows 70-75 degrees
are expected both nights.


As of 300 PM Friday...Primary headline for the extended forecast
period is `unsettled`, as a low-amplitude but broad upper trough
translates slowly east across New England and the Mid-Atlantic
seaboard. Near the base of the trough, phasing impulses from the
southern jet stream over top the Gulf ridge will allow a series of
whisking disturbances to bring TSTM chances much of the upcoming
week when coupled with late May and early June heating and an
active afternoon sea breeze boundary. Temperatures however will
remain slightly above normal as the thermal influence is
exerted more by the Gulf and SE states upper ridge. No shortage
of column mositure or dry mid level caps to impede convection
with PWAT values of 1.5 to 1.75 inches next week, so what
convection does form will be capable of dumping good rain
amounts. Storm motion generally will be from west to east, so
all locations will have a good chance of rainfall and TSTMS at
one point during the upcoming week.


As of 12Z...VFR conditions will prevail as a typical summer
pattern develops across the Carolinas today. West-southwest
winds 10 knots or less will become stronger from the south at
the MYR and CRE airports with this afternoon`s seabreeze. A
disturbance approaching from the west may help develop
cumulus/altocumulus clouds with bases in the 6000-foot range
tonight with isolated showers possible.

Extended Outlook...Brief MVFR/IFR conditions are possible in
scattered showers and thunderstorms Sunday Evening, and again
Monday Night through Tuesday.


As of 300 AM Saturday...Elongated high pressure across the Gulf
Coast will exert its influence on the local waters today. SW winds
in the vicinity of this high will increase steadily this aftn,
becoming 15-20 kts across all waters. A local backing and increase
in speed is forecast within the near-shore sea breeze circulation as
well where wind gusts of up to 25 kts are expected through this
evening. Winds will remain primarily SW through tonight with only a
slight decrease in speeds. However, a complex of tstms moving to the
north may create some variable wind directions due to boundary
interactions overnight. Seas will be 3-4 ft most of this period,
with the SW wind chop of 5 sec masking the lower amplitude 8-9 sec
SE swell.

As of 300 AM Saturday...Surface high pressure that typically
sets up around 30 degrees north latitude (North Florida) this
time of year will instead be displaced a few hundred miles to
the south. West-southwesterly winds are expected both Sunday and
Monday along the northern periphery of the high. Given the
unusual southern position of the high, a backdoor cold front may
get close to our portion of the coast Sunday afternoon and
evening. Model consensus drops the front no farther south than
the southern Outer Banks currently.

Inland temperatures should soar into the 90s both Sunday and
Monday, which should generate a moderate to strong seabreeze
both days. Wind gusts could reach 20-25 knots near the beaches
between 3-7 PM. Also of concern, there is a chance of fast-
moving thunderstorms Sunday evening and again Monday
afternoon/evening. Winds aloft are such that any storms could be
moving eastward at 30-35 knots, making it imperative that
offshore mariners keep a close watch on weather conditions well
off to the west.

As of 300 PM Friday...SW winds this period 15 KT or less except
higher afternoon gusts associated with an active sea breeze.
Sea heights generally a 2-3 foot range this period, along with
isolated to scattered inshore TSTMS mainly in the late
afternoons and evening as storms push off land over the 0-20NM
waters. Troughing inland may bring the highest winds of 15-20 KT
sustained Wednesday. Mariners should get a radar update before
heading out this period as there will be a chance of storms each





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