Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
1238 am EST Sat Dec 20 2014
clouds will increase tonight ahead of weak low pressure system that
will move across the northern Gulf of Mexico. This low will pass
off the coast Saturday bringing rain and cool temperatures.
Another area of low pressure moving up the coast will bring more
clouds and rain Monday and Tuesday. A large low pressure system
will pass to the northwest Wednesday and drive a strong cold front
through the Carolinas. Cold and dry weather is expected for the
end of the week.
Near term /until 6 am this morning/...
as of 1230 am Saturday...no changes to the forecast with the
latest update. Previous discussion follows:
High clouds are thickening across the eastern Carolinas. The
leading edge of the middle-level dense overcast should arrive along
the I-95 corridor shortly after midnight. Latest high-res models
look very similar to our previous thoughts with rain chances
increasing from the SW late tonight, but with the bulk of the
precipitation holding off until after daybreak. Probability of precipitation late tonight
have been tweaked downward again across southeast North Carolina with
little change elsewhere. Hourly temperatures are running a few
degrees higher than earlier projections, especially inland where
clouds have interfered most with radiational cooling. Forecast
lows have been bumped up in response with upper 30s across southeast
North Carolina to lower 40s across the Pee Dee. Previous
discussion from 630 PM follows...
Radar composites show rain has advanced as far east as east-central
Alabama. This matches up well with analyzed & forecast lift on the
300k-305k isentropic surfaces. While virtually all model guidance
shows moist lift developing in the eastern Carolinas within this
layer (10000-15000 ft) shortly after 06z/1 am EST...there will be
significant dry air below the cloud base to overcome. Comparing
various models mean relative humidity in the 1000-700 mb layer at 06, 09, and 12z
the GFS is the most moist with the NAM the driest. 18z GFS MOS probability of precipitation
appear to be taking the dry air quite seriously...and eyeballing
forecast soundings it should take a fairly significant amount of
rain to moisten the low levels.
With this in mind I have trimmed back probability of precipitation late tonight across
coastal southeast North Carolina where measurable rain probably won't occur
until after daybreak. Probability of precipitation have actually been increased for the
Florence-Darlington-Kingstree area late. No significant changes to
temperatures have been made as increasing cloud cover will allow
most of our temperature drop to occur this evening, with fairly
steady temperatures expected late tonight.
Short term /6 am this morning through Sunday night/...
as of 3 PM Friday...cool and cloudy weekend on tap with a series of
weak surface waves passing off the southeast coast. The first
arrives early Saturday morning...associated with weakening 500 mb
shortwave. The surface wave/low is rather uninspiring but does
provide a decent amount of low level moisture. Surface high ridging
down the coast enhances isentropic lift...which should be able to
tap into the available moisture. There is a period Sat into Sat
evening where deep moisture...associated with the shortwave...is
present. Although pop remains in the likely to categorical range
total quantitative precipitation forecast will be low. Rain will be light with the bulk of the
precipitation confined to the coast. Surface and middle level features move
east Sat night followed by weak middle level subsidence. Although
drying occurs aloft the low levels remain quiet moist with The Wedge
holding strong. Skies will remain cloudy Sat night and sun with
highs running below climatology and lows near climatology. Although no
measurable precipitation is expected sun forecast soundings do suggest the
possibility of patchy drizzle...something to keep an eye on.
The next and currently best chance at significant rain in the period
will be Sun night into Monday. Another more significant surface wave
moves up the boundary that will be stalled off the coast for much of
the period. Surface high continues pushing down the coast as the low
and its associated moisture moves northeast. Increasing isentropic
lift in an area of increase deep moisture...precipitable water
values exceed 1.25 inch by the end of the period...should lead to
rain developing across much of the area by Monday morning. Cloud cover
and northerly flow will keep lows near to slightly above climatology.
Long term /Monday through Friday/...
as of 3 PM Friday...looks like rain will be ongoing at the start
of the long term period. Shallow cool air will be in place through
the morning with a northeast flow at the surface as high pressure
remains wedged in through inland Carolinas. An area of low
pressure will move up the southeast coast through the day with a
trough extending NE from the low just off shore in a more typical
wedge like scenario. As the low moves farther north by Monday
afternoon the coastal trough will get pushed inland slightly
driving warmer temperatures and more showery weather into coastal areas.
Its always tough to time when this will occur but for now will
show a trend of warming along the coast. Pcp may be more
intermittent as the low tracks north and coastal Carolinas end up
in a northwest flow on the back end initially followed by a deeper warmer
and moister southerly flow on Tuesday ahead of next system. Therefore
shallow cool air in place Monday morning will be replaced by deeper
warmer air briefly along the coast and then cooler air through Monday
Deeper southerly flow will set up on Tuesday ahead of next storm system
shifting east from the Mississippi Valley. Unsettled weather
will remain ahead of this system through Tuesday into Wednesday before cold
front moves through late Wednesday. The digging middle to upper trough will
finally move off shore by Thursday morning leaving drier and cooler
weather for Christmas day. Overall expect cloudy and damp to wet
weather through the first half of the period with cooler more
seasonable weather Thursday into Friday as high pressure builds across the
Temperatures will start out cool but will warm as coastal trough/warm front
pushes inland late Monday into Tuesday. Tight gradient will exist on
Monday between cooler temperatures inland...in the 40s most places...to
possible 60s along the coast. By Tuesday the warmer air should make it
into the western reaches of local forecast area leaving temperatures closer
to 60 most places for Tuesday and Wednesday until cold front moves through by
Aviation /06z Saturday through Wednesday/...
as of 06z...expect VFR conditions overnight to deteriorate to MVFR
during the morning hours as rain moves over the eastern Carolinas.
Moist isentropic lift will bring -ra to our area beginning around
daybreak...first at kflo and kmyr/kcre followed by the northern
terminals shortly after 12z. Middle level VFR ceilings will gradually lower
through the morning as precipitation saturates the low levels. Guidance
continues to favor MVFR ceilings during the day...although tempo IFR
cannot be ruled out at the coastal sites where heavier precipitation is
possible. Northeast winds at or below 10 knots will prevail during the day.
Rain should taper off by 21z inland and before 00z along the coast.
But with low pressure lingering offshore...low clouds 2-4kft will
likely persist through the evening hours.
Extended outlook...MVFR ceilings possible on sun. Rain likely Monday with
MVFR/IFR conditions as a coastal trough develops offshore. More
showers expected Tuesday/Wednesday ahead of a strong cold front. Gusty winds
will develop during the day Wednesday.
near term /through tonight/...
as of 1230 am Saturday...no changes are necessary to the forecast.
Previous discussion follows:
Adjusted initial sea heights down slightly in the gridded forecast
database given current observations of nothing higher than 1 foot
within 20 miles of shore. NE winds should gradually build seas to 2
feet overnight with 3 footers starting to show up by daybreak.
Short term /Saturday through Sunday night/...
as of 3 PM Friday...high pressure continues to build down the coast
through the period while a series of weak surface waves/lows pass
east of the waters. The result will be a slightly pinched gradient
and northeast flow of 10 to 15 kts...with potential for brief
periods of 15 to 20 knots. Extended period of northeast flow will build
seas to 2 to 4 feet for much of the waters. Exception will be
locations sheltered to northeast flow or with an offshore component.
Here seas would be 1 to 2 feet.
Long term /Monday through Wednesday/...
as of 330 PM Friday...low pressure will move up the southeast coast
on Monday. An elongated trough will extend up through the waters
with northeast flow on the west side and southerly flow on the east
side. As the low moves north there may be a period of southerly
winds across all of the waters but overall expect more variable
winds at 10 to 15 kts. May see winds come back around to off shore
as the low moves up the coast but a deeper southerly flow will set
up ahead of next system by Tuesday.
As this large and potent storm system pushes a cold front east the
gradient will tighten Tuesday into Wednesday with winds up to 15 to 20 kts.
This will push seas up from 2 to 4 feet to 3 to 5 feet by late Tuesday. Wna
shows seas ramping up above 6 feet by Wednesday morning and possible up to 7
to 8 feet in outer waters in stronger SW to west flow as front moves
through the waters on Wednesday. Expect small craft conditions mainly on
Wednesday and lasting into Thursday.