Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
729 PM EDT Wednesday Mar 12 2014
a strong cold front will cross the area tonight. Much colder
temperatures are expected Thursday and Friday as Canadian high
pressure builds across the area. A gradual warming trend will occur
over the weekend. A low pressure system may bring unsettled weather
by early next week. High pressure builds into the area Tuesday
Near term /through Thursday/...
as of 730 PM Wednesday...a well defined cold front is slamming
eastward across the two Carolinas this evening. Deep convection
including severe thunderstorms is confined to central and northern
North Carolina and points north where mesoanalysis fields indicate a
stronger degree of surface convergence ongoing. Actual surface (or
elevated) instability is minimal (less than 500 j/kg) anywhere...but
upper level dynamics increase the farther north you travel making
convection more likely and more significant.
For our portion of the Carolinas the significant convection should
remain north of here. A trailing line of shallow convective showers
will shoot overhead...reaching Florence and the Interstate 95
corridor around 00z/8 PM EDT...and the coast only 2-3 hours later.
Radar composites show wide breaks in the line currently although the
20z RUC in particular showed a more solid line developing toward the
coast in a few hours. Forecast probability of precipitation range from 20-40 percent.
Southwest surface winds ahead of the line 15-25 miles per hour will shift to
the northwest as the front crosses. Forecast soundings from
virtually any model show strong cold advection developing in the
01-03z time frame with low-level thermal profiles becoming dry
adiabatic despite the time of day. Winds just 1000 feet up should
increase to 35 knots with 40 knot winds lurking just above 2000
feet. These winds are technically within the surface-based mixed
layer and are therefore fair game to mix down in gusts. This is the
continuing justification for the Wind Advisory.
Skies will become clear overnight with cold advection dropping 850
mb temperatures down to -8c to -9c by sunrise. Steep low-level thermal
profiles will continue overnight due to the cold advection and
strong winds...and lows should "only" fall into the middle to upper 30s
despite the bitterly cold conditions aloft.
Short term /Thursday night through Friday night/...
as of 230 PM Wednesday...big changes at the onset of the short term
period in the wake of the passage of a strong cold front.
Temperatures will once again drop below climatology. In fact...Thu/S
high temperatures will range several categories below normal. Slightly
warmer temperatures as low-level thickness nudges upward and the air-mass
modifies Friday. Time-height cross sections show a very dry column
in place with strong subsidence through the day on Thursday. Some
moisture increases aloft late Friday but overall skies will be clear
to mostly clear through the bulk of the short term period. Other-
wise...momentum Transfer will result in windy west to northwest flow
Thursday...especially across the NC zones. Winds are expected to
decouple Thursday evening with clear skies supporting radiational
cooling. Thus mins Thursday night will be in the upper 20s with the
cool spots down into the middle 20s. Much warmer overnight temperatures are
expected by Friday night with weak southerly flow and increasing
Long term /Saturday through Wednesday/...
as of 230 PM Wednesday...a weak surface pressure pattern on Saturday
with high pressure well off the coast paired with zonal middle level
flow on Saturday. Expect seasonable temperatures but also perhaps
increasing cloud cover in the low level warm air advection. Upper pattern becomes
quite complex by Sunday with three jet stream branches. One will be
around polar vortex over Hudson Bay...the second around a cutoff
over Mexico...and the third diving across the plains and Ohio
Valley. The median jet stream will be the most instrumental locally
as it generates low pressure along a cold front Sunday night into
Monday. There does appear to be an appreciable influx of Atlantic
and Gomex moisture preceding this setup leading to some fairly good
rainfall chances and perhaps amounts. One thing that does look
different in the extended is the cleanliness of the frontal passage. The
boundary was previously prognosticated to linger and keep the weather
unsettled whereas now it may blow through and allow good drying by
Tuesday after bringing a Monday and Monday night that will be colder
than previously portrayed.
Aviation /00z Thursday through Monday/...
as of 00z...expect VFR to prevail through the valid taf
period...though cannot entirely rule out a brief period this
evening of MVFR due to low visibilities in heavy rainfall.
VFR prevails across the area this evening with middle to high clouds
and southwest winds at or below 15 kts with gusts near 25 kts. An
approaching cold front continues to push towards the area...with a
line of rain showers/thunderstorms and rain and gusty winds just ahead of it. Expect only
showers to affect our terminals while thunderstorms and rain remain north of our area.
With the line moving through...could see gusts up to 40 kts. Behind
the front...few/sct clouds will linger before skies become clear by
the late overnight hours. Strong west-northwest winds will
infiltrate in behind the front as strong cold air advection takes
place. On Thursday...VFR prevails with nearly clear skies and
persistent gusty northwest winds...with gusts subsiding towards the
end of the valid period.
Extended outlook...VFR expected Friday. Chance of showers on
Saturday. Unsettle weather Sunday into Monday with intermittent rain
and possible MVFR/IFR conditions.
near term /through tonight/...
as of 730 PM Wednesday...a powerful cold front will move offshore
between 9 PM and midnight...accompanied by a broken line of showers.
Southwest winds 15-25 knots ahead of the front will veer to the
northwest and increase to 20-30 knots with gale-force gusts expected
to develop for at least several hours late this evening into the
early morning hours Thursday. Seas currently measured at 8 feet at
The Frying Pan Shoals buoy and 6 feet at the Cape Fear Harbor buoy
will probably increase by another 1-3 feet by midnight...but should
begin to subside late tonight as fetch lengths shorten with the
increasingly offshore wind direction.
Short term /Thursday through Friday night/...
as of 230 PM Wednesday...strong offshore flow Thursday morning will
gradually diminish across the coastal waters Thursday night as the
flow veers from the west-northwest to a north-northeast direction. Light/variable flow with
surface ridge in the vicinity Friday morning will become southerly
by Friday afternoon as the ridge axis shifts offshore. SW winds will
increase to 10-15 knots (closer to 15 knots cape fear) Friday night. A
large range of seas will exist Thursday with a strong offshore
trajectory. Seas will be highest off Cape Fear where the fetch is a
little longer. Seas will quickly subside late Thursday night into
Friday morning as the aforementioned ridge takes control. A nice
boating day Friday with atypical ocean conditions for March under
ridge of high pressure.
Long term /Saturday through Monday/...
as of 230 PM Wednesday...southwesterly return flow more typical of
the warm season on Saturday as high pressure sits well off the
coast. Changes expected however as cold front approaching from the
northwest and a complex flow pattern aloft evolves. Surface low approaches
from the west as it cuts across the Gulf states on Sunday...possibly
hanging up the front this far east right over the region leading to
some uncertainty as to how sundays winds will pan out. Frontal passage is
slated for Sunday night leading to a moderate northerly wind on
Monday as the high builds in. No advisories expected through the
period. The best chance for some 5 feet seas looks like Monday but the
offshore flow will keep them relatively confined to outer waters
while wave shadowing leading to smaller seas near shore precludes
SC...Wind Advisory until 6 am EDT Thursday for scz017-023-024-032-
NC...Wind Advisory until 6 am EDT Thursday for ncz087-096-099-
Fire Weather Watch from Thursday morning through Thursday
afternoon for ncz087-096-099-105>110.
Marine...Gale Warning until 6 PM EDT Thursday for amz250-252-254-256.