Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
325 PM EST Friday Dec 19 2014
clouds will increase tonight ahead of weak low pressure 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 Saturday morning/...
as of 3 PM Friday...a large area of high pressure centered across
the Great Lakes and Ohio Valley will drift east through tonight
and begin to wedge strongly southward across the Carolinas by Sat
morning. Meantime...a southern stream disturbance will continue to
move toward the area from the west and southwest with its most
direct impacts expected on Sat.
High and thin clouds were quickly encroaching from the west and SW and
clouds are expected to lower and thicken during the middle and late evening
hours...becoming cloudy by/around the midnight hour. The low level
flow...below 1500-2000 feet will be NE to east-northeast tonight. Above that
shallow layer...there will be a deepening and increasingly moist west-southwest
flow. Therefore...it is possible that light rain will begin to break
out before sunrise Sat as isentropic upglide begins to quickly
increase late. However...models continue to show the low levels
stubbornly hanging onto the dry air in the low levels. The magnitude
of the dry air is certainly well represented this afternoon with
dewpoints in the 20s in most locations. A seabreeze has pushed
onshore and some of the immediate coast has seen dewpoints recover
into the 30s. Mixed signals as to how soon the precipitation will
start and whether it will be measurable overnight. Therefore...will
show probability of precipitation ramping up toward morning...but remaining in the good
chance to low likely range through daybreak with the highest probability of precipitation
reserved for Sat.
Low temperatures also a challenge as increasing cloud cover will tend to
keep temperatures elevated...but dry air coupled with mainly thin cloud
cover this evening will provide a window for good radiational cooling.
Thus...we may end up with a non-diurnal temperature curve as temperatures should
tend to stabilize late evening and overnight. However...developing light
rainfall dropping through the column should allow for a period of
evaporative cooling. This may be enough to again drop temperatures at the
surface near dawn. Will forecast a sharp drop in evening temperatures with lows
mainly in the upper 30s...but middle 30s across portions of the Cape
Fear basin where cloud cover will be slowest to thicken. There is no
concern for any precipitation type other than a chilly liquid rain.
Short term /6 am Saturday 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 /20z Friday through Wednesday/...
as of 18z...weak low pressure in eastern Texas will slowly develop
and move northeast. Ahead of the system will be an assortment of
cloud layers...mainly middle and high clouds at first. High pressure
will try to ridge in at first...with northeast flow expected. Clouds
will thicken in the overnight hours with some precipitation developing
toward morning as isentropic lift kicks in. Ceilings will lower but
will likely remain VFR through the end of the forecast period. The
NAM is a little bit wetter than the previous run...but most of the
precipitation will be confined to the coastal counties.
Extended outlook...chance of showers Monday as low pressure moves up
the coast. Showers possible late Tuesday and Wednesday ahead of a strong cold
near term /through tonight/...
as of 3 PM Friday...a developing seabreeze will keep winds from
the east-southeast or southeast across the very near shore waters until near sunset.
Otherwise winds tonight will be from the NE as high pressure
begins to wedge across the Carolinas and a southern stream system
moves across the area. Wind speeds will increase to 10 to 15 knots
later this evening. Seas will be 2 feet...building to 2 to 3 feet by Sat
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.