Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 130014

Area Forecast Discussion
National Weather Service Wilmington NC
714 PM EST Tue Dec 12 2017

Arctic air will pour into the eastern Carolinas tonight into
Wednesday morning. The cold won`t last long as temperatures will
return to normal Thursday and Friday. Another cold front bring
chilly temperatures Saturday, before seasonable weather returns
on Sunday. Low pressure approaching from the Gulf Coast may
bring rain to the area Monday.


As of 715 PM Tuesday...Latest obs show that the cold front has
now moved offshore. Winds have not yet picked up to expected
levels, but looking at upstream obs expect that they will
increase and become more gusty in the next couple of hours.
Forecast good to go with no changes. Previous discussion
from this afternoon follows:

Strong cold front is progressing quickly this aftn, and is
currently aligned just west of the coast. This front will race
eastward, and although temps will drop only slowly at first,
strong CAA is accompanying this front so temps will likely seem
to fall off a precipice this evening, falling 10-15 degrees in
just a few hours. This front will also be accompanied by post-
frontal stratus at 3-5 kft, although some of this will dry up on
the downslope flow east of the Appalachians. Additionally,
gusty W/NW winds are likely, with gusts of 20-25 mph forecast
through the evening and even several hours past dark. A weak
inversion will try to setup tonight, but will be somewhat
inhibited by strong winds of 30-40 kts continuing just off the
surface, so winds will remain elevated through the night even as
gusts ease.

This strong CAA will cause mins tonight to be quite cold, mid 20s
inland to near 30 at the coast, a drop off of 30 degrees from this
aftn highs. These cold temps combined with the elevated winds will
produce wind chills colder than we have seen in quite some time -
upper teens in many spots late tonight into Wednesday morning.

After a breezy and cold morning, winds will slowly ease as high
pressure ridges into the area from the west, but a very cold day is
forecast. Despite abundant sunshine and aftn development of warm
advection, highs are forecast to be only in the low 40s across NC,
mid 40s SC, which is a few degrees below even the coolest guidance.
Despite easing from overnight, winds will remain at least somewhat
elevated through the aftn, so this combined with the cold temps will
produce wind chills in the 30s most of the aftn. Bundle up!


As of 300 PM Tuesday...Essentially a temperature forecast for
the short term period as the shortwave laden northwest flow
continues across the region at least initially. The strongest
feature will move across the area late Wednesday night with a
weaker system to follow late Thursday as the flow relaxes
somewhat. The cold air advection isn`t nearly as strong Thursday
as the system tonight and is reflected in the warming guidance
and the official high temperature forecast for Thursday. Lows
have been increased a bit for Thursday AM as well.


As of 300 PM Tuesday...Model consensus is still not great in the
long term. It`s a common problem in the winter: shortwave
energy splits as it exits the Rockies with part coming eastward
and part dropping southwestward into Baja Mexico. Each model is
slightly different with the specifics how this energy cuts off
over Mexico late this week, and then how it rejoins the
westerlies early next week.

The part of the shortwave that does make it to the East Coast on
Friday should induce a weak surface low to develop along a cold
front across the Carolinas on Friday. With so much westerly wind
throughout the atmospheric column there will be virtually no
influx of moisture, and rain chances appear very small. I have
bumped PoPs down to only 10 percent inland and 20 percent along
the coast Friday afternoon/evening. Friday night and Saturday
should see chilly temperatures with mainly clear skies in the
Canadian airmass in place. Surface high pressure will move off
the coast Saturday night, with a moderating return flow setting
up Sunday.

The difference in model handling of the Baja Mexico shortwave
energy really becomes apparent Sunday through Tuesday. The ECMWF
is easily the fastest model to kick the low back into the
westerlies, and shows rain breaking out during the day Sunday in
association with a developing surface wave of low pressure
along the Gulf Coast. Other models are at least 12 hours slower.
There are some GFS ensemble members that show an early start to
precip Sunday, but I don`t show rain developing until Sunday
evening to maintain consistency with surrounding NWS offices and
the bulk of our deterministic model guidance. Even the slower
12Z GFS and Canadian runs still show rain becoming a good bet
Sunday night into Monday, and PoPs have been increased to 40-50
percent for this period. Temperatures should remain well above
freezing during this event with no P-type concerns.


As of 00Z...There is a weak inversion setting up, enough to keep
winds fairly steady, so took the gusts out through the overnight
hours. Wednesday, time height indicates dryness at all levels, with
little to no clouds expected. West winds will become southwesterly
late in the forecast period.

Extended Outlook...VFR.


As of 715 PM Tuesday...Cold front has now passed east of the
waters with latest obs showing NW winds of 20 to 25 kts and seas
ranging from 2 to 3 ft near shore and around 6 ft further out.
Small Craft Advisory remains in effect as before with no
changes planned. Previous discussion follows:

Small Craft Advisory in effect for all waters into Wednesday
morning. A strong cold front is currently exiting the coast and
will cross the waters within the next 2-3 hours. SW winds across
the waters presently are reaching 15-25 kts, and driving seas
to as high as 7 ft at Wilmington Harbor, and 8 ft at Frying Pan
Shoals. As winds become NW post-FROPA this evening, they will
remain elevated, and may actually increase to 20-30 kts for a
time overnight before easing into Wednesday morning, and then
falling below SCA thresholds Wednesday aftn while maintaining a
W/NW direction. These strong winds will keep the dominant wave
period very short, around 5 sec, but wave heights will begin to
fall thanks to the offshore component of the wind limiting
available fetch. Seas overnight will remain 4-7 ft, but will
fall to 2-4 ft by the end of the period.

As of 300 PM Tuesday...A strong shortwave will move across the
area overnight Wednesday increasing the winds markedly. The flow
is from the southwest and in warm air advection fortunately.
Wind speeds will almost certainly increase to small craft
criteria and a gale warning cannot be ruled out. The flow
weakens quickly Thursday with a westerly flow of 10-15 knots and
even further by Friday morning to ten knots or less.
Significant seas will ramp up quickly from an initial 2-3 feet
to 5-7 feet and fall almost as quickly late Thursday into early
Friday back to 2-3 feet.

As of 300 PM Tuesday...A cold front will move across the
Carolinas early Friday. Low pressure is still expected to
develop along this front, but model consensus is that it may be
weaker than expected yesterday, or at least weaker than the GFS
model had been showing. Breezy west winds developing behind the
front Friday will continue through Saturday morning before
diminishing as high pressure moves in from the west. The next
low pressure system is now expected to move eastward and toward
the Carolinas a little faster than we thought yesterday, perhaps
reaching the area Sunday night with rain and strengthening
southerly winds.


MARINE...Small Craft Advisory until noon EST Wednesday for AMZ250-252-



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