Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 201342
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
942 AM EDT FRI MAY 20 2016

.SYNOPSIS...
A slow moving storm system and associated cold front will bring
showers and thunderstorms to the area through Saturday. Conditions
slowly dry out Sunday and Monday as the system moves away slowly.
High pressure will build in and dominate the weather Tuesday
through Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 940 AM Friday...No significant modification were needed to
the ongoing forecast for the morning updates...but did raise
minimums just a degree or 2 across the deeper interior tonight as
recent near term model trends are suggesting. The previous
forecaster discussion follows.

A stalled front across Georgia will soon be on the move, drawn
northward into a region of lowering surface pressures anticipated
to develop late this afternoon and tonight across the Carolinas.
The reason for this is the approach of a reasonably strong upper
level disturbance from the west. The passage of this upper feature
and the surface warm front will be the focus for showers and
thunderstorms, some of which could put down briefly heavy rainfall
tonight.

Until this system arrives, low clouds continue to cover a good
portion of interior South Carolina with a veil of mid level clouds
elsewhere. Isentropic lift along the 305K theta surface
(approximately 8000 feet AGL) is increasing and should redevelop
showers with coverage increasing to 60-80 percent this afternoon.
Temperatures should remain below normal again today with highs
only reaching 66-68 near and west of I-95, 70-72 east of I-95 to
the coast.

The best rainfall rates should occur tonight as the upper
disturbance passes overhead and isentropic lift is maximized.
Advection of warm air from the south should also lead to enough
destabilization of the airmass for elevated thunderstorms to occur.
QPF tonight is 0.5 to 0.7 inches with typical convection variation
expected.

In terms of model preference, the 00z GFS appears quite reasonable
with its handling of low pressure developing near the front tonight.
However the 00Z NAM develops the low this afternoon well before the
upper disturbance arrives, then moves it northward right into the
heart of the in-situ CAD airmass. This solution isn`t supported by
climatological movement of these systems.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 315 AM Friday...Surface low pressure will exit the NC coast
late Saturday morning, moving northeastward off the Mid-Atlantic
coast for the rest of the weekend. Lingering showers and
thunderstorms during the morning should end as a dry slug of air
arrives in the mid-levels. Given the very high sun angles this time
of year, this dry slot should allow enough sunshine in to create
significant destabilization of the airmass Saturday afternoon, and a
second round of scattered showers and t-storms is expected to
develop.

Wind profiles Saturday afternoon should be largely unidirectional
with some modest speed shear indicated by model soundings. Bulk
shear across the 0-6 km layer should remain less than 30 knots.
SBCAPE of 1000-2000 J/kg (assuming high temps approaching 80
degrees) will have at least 500 J/kg of DCAPE to work with given the
dry air aloft. This dry air aloft is the largest concern with any
thunderstorms Saturday afternoon as enhanced downdrafts and strong,
gusty winds may occur. SPC`s latest day 2 outlook maintains a 5%
marginal risk for severe thunderstorms across the area.

The upper level low trailing behind this system is not expected to
hurry off the east coast, and in fact our mid-level heights and
temperatures should drop substantially Sunday as building heights
north of the Great Lakes force the low southward. I cannot in good
conscience put a dry forecast out for Sunday given the steep lapse
rates and residual low-level moisture expected to linger.
Temperatures should again run below normal Sunday, only reaching the
lower to middle 70s.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 315 AM Friday...Mid level low pressure will finally be on the
move via a kicker system out west Monday. In its wake a broad
southwest flow will develop. This flow will keep the eastern
Carolinas dry for the most part as high pressure moves overhead
and offshore. Guidance is still showing a weak shortwave moving
across the Ohio valley Thursday representing the only chance of
rain in the extended. Confidence is not high as thermal profiles
are still very dry. Temperatures will increase through the period
from highs in the 70s Monday to well into the middle 80s by
Thursday as 850MB readings increase from about seven degrees
Celsius to 15. Overnight lows will follow a similar trend.

&&

.AVIATION /13Z FRIDAY THROUGH TUESDAY/...
As of 12Z...Light showers have broken out over some of the region in
response to isentropic lift. High pressure to our north will
continue to give us northeast flow at the surface with southerly
flow from 2k feet and above. Overrunning type precip is expected
through much of the day. Low pressure will form along a near
stationary boundary later today...increasing our chances for
precip as well as lowering ceilings.

Extended Outlook...Showers/MVFR likely with periods of IFR through
Saturday. Showers decrease with improvement to VFR on Sunday through
the end of the extended period.

&&

.MARINE...
NEAR TERM /THROUGH TONIGHT/...
As of 940 AM Friday...No changes to the marine forecast at this
time...a exercise caution headline will remain hoisted through the
afternoon. The previous forecaster discussion follows.

A stalled front lying across Georgia will
begin to move northward later today, reaching the Grand Strand
beaches this evening and Cape Fear after midnight. The reason for
this movement is the approach of a rather strong upper level
disturbance from the west that will help lower surface pressures
across the coastal Carolinas. This area of gradually lowering
pressure should eventually develop a closed surface low after
midnight near or just north of Cape Fear. A period of rather
strong NE winds is developing now and should last through early
this evening in a tight pressure gradient just north of the warm
front. Small craft should exercise caution through this evening
due to winds near 20 knots and seas up to 5 feet.

In terms of model preference, the 00z GFS is preferred over the 00z
NAM as the NAM develops the surface low a good 6-9 hours earlier,
well before the upper disturbance approaches, and then moves the low
across the warm front into the relatively cold airmass inland.

SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 315 AM Friday...As the surface low moves northeastward and
away from the Carolinas, west to southwesterly winds are expected
Saturday. These winds will turn northwesterly Saturday night, then
northerly Sunday as much cooler and drier air builds southward. Wind
speeds around 15 knots are expected Saturday morning, then again
Sunday afternoon and evening.

LONG TERM /MONDAY THROUGH TUESDAY/...
As of 315 AM Friday...A very modest northwest flow will be in place
early next week across the waters. Speeds will be limited to ten
knots or less as high pressure is slow to move across the area. This
pattern will continue until later Tuesday when the high slips
offshore and a weak return flow develops. Benign winds lead to lower
significant seas of mostly 2-3 feet.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...None.
NC...None.
MARINE...None.

&&

$$

NEAR TERM...TRA/8
SHORT TERM...TRA
LONG TERM...SHK
AVIATION...DL
MARINE...TRA/8/SHK



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