Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 132214

Area Forecast Discussion
National Weather Service Wilmington NC
614 PM EDT Tue Mar 13 2018

Strong low pressure off the New England coast will continue to
move slowly north through tonight. Arctic high pressure across
the Northern Plains will build south through tonight and then
east across the northern Gulf Coast Wednesday and Thursday.
Temperatures will warm Thursday and Friday before another shot
of cold air reaches the area Friday night and Saturday. Warmer,
but cloudier weather will return early next week, with another
storm system approaching by Tuesday.


As of 600 PM Tuesday...After in-house met discussion including
18Z guidance, decided to go with a Freeze Warning along the
northern periphery of the ILM CWA, which includes the 6 most
northernmost counties for the time between 4 am and 9 am Wed.
Could see a brief decouple of the winds this evening but should
pick back up after midnight tonight as a mid-level s/w trof
rotates thru late. Could see a shallow stratocu or altocu cloud
deck partially affect the FA overnight into daylight Wed but
most of that should remain north of the FA. Latest MOS guidance
still wants to paint 30-32 degree lows tonight and this is
taking into account what in essence should keep min temps
slightly hier overnight. This will not be a deep freeze, that
may manifest come Thu morning.

As of 300 PM Tuesday...Downsloping westerly  mid level flow will
back to SW overnight ahead of strong approaching clipper-like
shortwave. This feature sets up an interesting if not slightly
uncertain forecast for our northern (particularly agriculturally
sensitive) forecast zones. Specifically, the need for a Freeze
Warning is unclear at this time due to some conflicting signals.
Temperature guidance depicting at least marginally freezing
temperatures for a short time tonight. However these same MOS
numbers seem to be at odds with their parent guidance. The latter
shows that clouds increase with the shortwave whereas the former MOS
bulletins hint at clear skies all night. Am leaning towards the
clear sky solution being erroneous as there is clearly a large swath
of cloudiness currently associated with the circulation and vort
center (though that could change on the leeside of the mountains).
Flurries associated with this same feature has been discussed but
they should remain to our north if they materialize at all.
Hopefully during the next shift temperature and cloud cover trends
will shed more light on the need for a Freeze Warning. The renewed
cold air advection underway by 12Z will keep highs every bit as
chilly if not colder than those observed today.


As of 300 PM Tuesday...The deep upper trough will begin to lift out
from the East Coast, but a pair of shortwaves embedded in the jet
will maintain cyclonic curvature to our upper level flow through
Thursday. A sharp upper ridge across the High Plains Wednesday night
will deamplify as it moves into the Arklatex region by Thursday
night. At the surface, Canadian high pressure will move eastward
along the Gulf Coast, crossing Florida on Thursday and moving
offshore Thursday night.

Wednesday night is likely to be the coldest night of the week across
the Eastern Carolinas as lighter winds with the surface high to our
southwest combine with a cold airmass and clear skies. Models have
stabilized over the past 36 hours with their depiction of low
temperatures in the upper 20s inland and lower 30s near the coast.
I`ve knocked off several degrees from these numbers across the
peat/pocosin soils mainly across southeastern North Carolina with
lows predicted as cold as 25 degrees in spots. This is unfortunately
where many of our local blueberry/strawberry farms are located, and
growers are hopefully preparing for this freeze event.

As a surface trough moves off the Mid-Atlantic coast Thursday
afternoon/evening, our pressure gradient will tighten substantially.
Warm advection on westerly winds should allow highs to rebound into
the 60s area-wide, with wind gusts during the afternoon
potentially reaching 30 mph. Lighter breezes will continue
through Thursday night, helping hold low temperatures in the 40s
and perhaps near 50 on the beaches. Models show a strong mid
and upper level WNW wind across the Appalachians with a
subsidence inversion sitting just above the ridge tops Thursday
night, a scenario sometimes associated with orographic cirrus
development. Should this occur, we might need to add a few more
degrees to Thursday night`s lows.


As of 300 PM Tuesday...The East Coast trough will continue to
lift to the northeast on Friday, as a low-amplitude mid/upper
ridge crosses the Mississippi River valley. This will allow a
surface high to surge southeast and push a backdoor cold front
into or through the forecast area Friday night. Will carry low
PoPs as Gulf moisture surges across the area ahead of fropa. The
cool high will settle over eastern NC Saturday ahead of a quick
moving shortwave and surface reflection which will move across
the Carolinas Saturday night, and offshore Sunday. Associated
rain chances will come to an end late Sunday, and temps will
rebound from Saturday`s unseasonable cool readings, but still
remain 3-5 below climo. The pattern will remain quite
progressive early next week, with another shortwave and
associate surface low on the doorstep by Tuesday.


As of 18Z...High confidence for VFR conditions through the forecast
period. Strong low pressure well north and east of the region will
keep moderate northwest flow throughout the day. Winds diminish
tonight with a mid cloud ceiling expected after midnight associated
with another weak impulse moving through the longwave trough. No
precip is expected with the cloud deck. Wednesday, continued gusty
northwest flow.

Extended outlook...VFR through Friday. Possible MVFR conditions
Friday night and Saturday.


As of 615 PM Tuesday...Winds and seas have dropped below SCA
thresholds and have let the SCA expire with the issuance of a
MWW. But did highlight, SCA thresholds will again be met during
Wed daylight into nightfall. This after the passage of a mid-
level s/w trof that re-tightens the sfc pg after its departure
early Wed. Winds will drop to W to WNW around 15 kt this evening
and become NW 15 to 25 kt occasional G30 kt by or just after
daybreak Wed, that continues thruout the day on Wed. Significant
seas will be on the low side due to the limited fetch given
wind directions for seas to build upon. There is a roughly 1 to
3 foot Easterly ground swell at 12+ second periods that will be
pushing across the local waters during the next few days which
will help produce a wider than normal margin for significant
seas fcsts across the local waters this period.

As of 300 PM Tuesday...Wind remains as-is or even slackens
slightly through about 06Z as very weak surface reflection of a
powerful upper level system approaches from the northwest. After
06Z this feature moves through and flow turns to NW in a
renewed surge of cold air. Wind could briefly pulse up to SCA
levels but confidence is a little too low to raise flags at this
time. Any 6 ft seas remain outside of the 20nm forecast zones
according to both WNA and locally generated NWPS.

As of 300 PM Tuesday...Canadian high pressure will take a path
eastward along the Gulf Coast, crossing Florida on Thursday and
moving offshore Thursday night. With the high so far to our south,
we`ll maintain an offshore wind through the period. Unfortunately
this is likely to be a strong offshore wind developing during the
day Thursday and continuing through Thursday night as a trough moves
off the Mid-Atlantic coast, tightening the pressure gradient
substantially and accelerating our winds to 20-25 knots with gusts
near 30 knots away from shore Thursday night. It`s a near certainty
that we`ll need a Small Craft Advisory beginning Thursday afternoon.

As of 300 PM Tuesday...Offshore flow will be the rule Friday as
surface high pressure moves off the east coast of Florida. A
cold front will drop south across the waters Friday evening,
with a quick shift to northeast winds. High pressure behind the
front will move off the east coast of NC Saturday, and result in
onshore flow through the day. The gradient will strengthen
behind the front in the NE surge, and winds may reach into a
15-20 kt range late Friday night into Saturday morning.


SC...Freeze Warning from 4 AM to 9 AM EDT Wednesday for SCZ017-023-
NC...Freeze Warning from 4 AM to 9 AM EDT Wednesday for NCZ087-096-


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