Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 211142

Area Forecast Discussion
National Weather Service Wilmington NC
742 AM EDT Wed Mar 21 2018

Cold high pressure under gusty NW winds today and again Thursday
will keep temps 10 to as much as 15 degrees below normal. If
winds subside enough during the next 3 nights and early
mornings, frost and or freeze issues will increase dramatically
across the area. High pressure will maintain cool daytime
weather this Saturday but will give way to rain spreading in
from the west Saturday night and Sunday.


As of 300 AM Wednesday...After a busy night of severe weather
including multiple reports of up to 2" diameter hail near Myrtle
Beach, all is quiet at this hour offering a welcome reprieve from
the activity of earlier.

The reason for the decrease in convection, which has now eroded to
nothing locally, is the advection of drier and cooler air both at
the surface and aloft, which has significantly changed the
atmospheric profile early this morning. Whereas RAP soundings
earlier showed significant residual elevated instability within an
EML, lapse rates have stabilized as the entire column "shifts"
colder, with the drier air aloft also squelching any convection. The
dry air is mostly aloft, as a frontal inversion is trapping moisture
in the form of the widespread stratus currently trapped across the
CWA. As low pressure strengthens offshore along the stalled front,
surface flow will back to the W/NW later this morning with dry
advection thereafter causing ceilings to lift and break through the
aftn. This will have little impact on temperatures however, and a
January-like day is forecast as 850mb temps below 0C cause highs to
climb only to around 50 far NW counties, to the mid 50s along the
South Santee river.

Additionally, another spoke of vorticity energy will rotate overhead
late this morning into the aftn as the cold core/upper low swings
into the tidewater of Virginia. Across SC, this will have little
impact other than some cold air cu development. However, forecast
soundings and high res guidance suggest a chance for showers across
SE NC as this forcing combines with increasing column saturation up
through about 700mb. Have capped POP at low-chc, with the best
chance being late morning, but QPF will be very low. Despite the
column being cold and 850mb temps remaining below 0C, all precip
today which does occur will be of the liquid variety thanks to a dry
dendritic zone and freezing levels too high for any snow in what
will be light precip rates. As this final piece of energy rotates
offshore this aftn, even drier air will advect into the column and
many locations will see sunshine before sunset this evening. This
dry advection will also be aided by westerly winds gusting above
20mph this aftn as the gradient pinches and a secondary front
crosses the region.

A cold night is forecast tonight, as this late March "winter"
continues. Forecast mins are currently in the 35-38 degree range,
which could be cold enough for some frost. However, forecast
profiles and MOS guidance keep winds elevated enough overnight that
frost should not be of concern.


As of 400 AM Wednesday...If you think this period is going to be
boring with no appreciable weather and only few clouds, think
again. Yes, skies will mainly be clear thruout this period with
occasional thin cirrus. Progged atmospheric soundings across the
local area indicate no moisture thru the atm column with PWs
less than one quarter of an inch at times. Thus, no rainfall
this period. Downslope flow during this period will scour out
any moisture that does make it across the Appalachians. Thus,
the mostly clear fcst.

Here`s where the fcst gets difficult. Farmers, local growers,
Ma and Pa`s place and any other low temperature sensitive
plants, crops, shrubs and trees will become exposed to
temperatures in the low to mid 30s Friday and again Saturday
mornings respectively. A combination of the NAM and European MOS
Guidance was used here due to their slightly lower min temp fcst
with the GFS MOS. At this point, the sfc pg remains semi-tightened
Fri morning and further relaxed Sat morning when compared to
Fri morning. Thus, winds are forecast to stay semi-active Fri
morning, NW around 5 mph, and semi-active Sat morning, NW less
than 5 mph. These speeds will be enough to prevent any dew and
frost development especially Fri Morning. The sfc dewpoint
depression Friday morning will be the hiest of the 2 mornings in
question. And if winds either potentially subside-some and/or
decouple Fri morning, more-so than what guidance is saying,
min temps could drop to and below 32 degrees for several hrs.


As of 340 PM Tuesday...this forecast period continues to show a
cool down as an upper level trough will be remain along the
east coast. In the large scale trough a shortwave is seen
dropping southward on the west side. This will bring in a colder
shot of air and a back door cold front. The 12 UTC GFS and
ECMWF are not in good agreement with the GFS being much faster
than the ECMWF. So uncertainty is high on timing of this front.

Will favor the faster GFS and its ensemble and introduce a
chance of showers late Saturday into Sunday night as the frontal
boundary shifts southward.

Temperatures will be below normal for the period with a
frost/freeze possible Friday night. Otherwise lows in the 40s
and highs in the mid to upper 50s to lower 60s.


As of 12Z...Stratus cigs 600 ft to 800 ft this morning at KLBT/KILM,
with VFR at KCRE/KMYR and MVFR at KFLO. As a impulse moves
northwest to southeast across the terminals this morning, VCSH
at worse tempo -RA can expected, but more importantly it should
signal the end of any IFR at KILM/KLBT. Gradually rising cigs
and increasingly gusty winds expected after the impulse passes.
By afternoon westerly winds with gusts of 23 kts and VFR likely
at all terminals.

Extended outlook...VFR. MVFR/IFR/Rain Saturday night and Sunday.


As of 300 AM Wednesday...Small Craft Advisory remains in effect for
all waters, and will persist at least through the near term period.
Gusty NE winds behind a cold front stalled south of the waters will
gradually shift to the west by daybreak as low pressure develops to
the east. These winds will remain 10-15 kts into the late morning
before increasing as a surge of dry air associated with a secondary
front crosses the waters. This will push winds back up to 20-25 kts
with higher gusts, and these wind speeds are expected to persist
into Thursday with slow veering back to the NW. Despite the
predominantly offshore wind, wave heights will be steady in the 5-8
ft range, falling to 4-6 ft late, with a residual SW swell and NW
wind wave comprising the spectrum.

As of 400 AM Wednesday...SCA to remain in effect for the local
waters thru 6 pm Thursday. Winds and respective seas will be on
a slow diminishing trend this period. The sfc pressure pattern
will yield a NW direction thruout this period. The sfc pg will
be on a slow relaxing phase with the hier winds at the start of
this period, 15 to 25 kt, slowly diminishing to 10 to
occasionally 15 kt at the end of this period. Significant seas
will already have a negative aspect to it for trying to build
and that is the limited fetch associated with NW offshore winds.
Looking at 3 to 6 ft at the start of this period, subsiding to
1 to 3 ft by the end of this period. Not much of an underlying
ground swell this period just 3 to 5 second period wind waves.

As of 340 PM Tuesday...high pressure will ridge down the coast
with a surge of cooler air on Sunday. The high pressure will
extend down over the local waters Friday through Saturday before
a cold front approaches from the north. Northwest flow of 10 to
15 knots will continue through Friday before veering to the
south and west on Saturday and Sunday. Winds will increase on
Sunday with a backdoor front and seas are expected to increase
to 4 to 7 feet by Sunday late.


MARINE...Small Craft Advisory until 6 PM EDT Thursday for AMZ250-252-


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