Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
1154 am EST Wednesday Dec 11 2013
high pressure will settle over the area today. A dry cold front
will move through tonight with cold and dry high pressure building
in behind it Thursday into Friday. The weekend is looking more
and more unsettled as a storm system tracks across the southeast.
Cooler and drier conditions are expected early next week.
Near term /through tonight/...
as of 1100 am Wednesday...high pressure almost directly overhead
will weaken through today before a reinforcing shot of cold air
will arrive overnight as clipper system moves across the northeast.
The center of the high is oriented from SW to NE and therefore
winds will come around becoming more on shore through this
afternoon into this evening. The only real effect this will have
on our weather is a slight push of higher dewpoint air over mainly
coastal areas but overall sunny and cool weather will continue
through today. Temperature advection will remain fairly neutral with
temperatures just making it into the 50s for a few hours this afternoon
under full December sun...running over 5 degrees below normal.
By tonight a strong area of high pressure will build east from the
central states as broad middle to upper trough extending down from the
Great Lakes pushes a dry cold front east. This will move through
around midnight tonight with surge of cool air behind it. The surface
boundary will provide a shallow push of cooler air at first but as
the middle to upper trough axis reaches the coastal Carolinas toward
morning...a deep north-northwest flow of cold and dry air will set up behind
it. Therefore initially there will be more radiational cooling as
atmosphere decouples under calm winds and clear skies this
evening. Expect temperatures to drop out after sunset...quickly dropping
into the 30s by midnight. As middle to upper trough pushes dry cold
front through after midnight the winds will pick up with cold air advection to
follow. This will help temperatures stabilize at first before deeper
cooler air makes its way in toward morning. As middle to upper trough
approaches may see a few middle to high clouds around through the
early morning hours but overall expect mostly clear skies and cold air advection
through the early morning hours with low temperatures similar to this
morning...30 to 35 most places with cooler readings well inland
and in traditionally cooler places.
Short term /Thursday through Friday night/...
as of 3 am Wednesday...surface high continues to build in from the
west on Thursday with cold advection continuing into the afternoon.
Little change in partial thickness values during Thursday indicates the
strength of the cold air which will be able to offset any air mass
modification due to sunny skies. Temperatures will remain below climatology with
highs struggling to reach 50 across much of the area. Surface ridge
axis will be basically overhead Thursday night into Friday setting up what
should be a good radiational cooling event. Boundary layer flow
pronged to quite week and with deep dry air lows well into the 20s
Surface high is shifted off the coast by progressive flow
aloft...ending up east of Hatteras Friday evening. Return flow will
develop as the high exits east but temperatures will be slow to
respond. Highs will remain below climatology Friday. Another potential
sticking point is a coastal trough expected to develop late Friday or
Friday night. As this feature develops return flow will back to
easterly...mainly Friday night. Abundant sunshine will be present Friday
but do expect an increase in high cloud late in the day as moisture
ahead of a system for the weekend spreads over the southeast.
Clouds lower late Friday night as coastal trough moves to the coast. At
the same time weak isentropic lift in the presence of increasing low
level moisture may generate some light rain across the area. Given
that very dry air will be in place before the event unfolds will
maintain inherited slight chance pop for most of the area. Not planning
on an increase in pop as guidance is sometimes is too quick to erode
the dry low level air. Cloud cover and increased boundary layer flow
will keep lows near to slightly above climatology Friday night.
Long term /Saturday through Tuesday/...
as of 3 am Wednesday...period begins with southern stream system
moving across the Gulf states. The shortwave remains positively
tilted keeping the surface low weaker and fast moving. Coastal
trough/warm front lifts north of the area Sat as the surface low
passes west of the area. Several factors suggest Sat afternoon
through Sat night could be quite wet. Deep southwest flow will pump
plenty of moisture into the region. Precipitable waters reach 1.5
inch Sat and exceed 1.7 inch Sat night. Flow aloft is strongly
divergent and forecast soundings depict some elevated instability.
Nocturnal timing is less than ideal but convergence along the strong
cold front that will move off the coast Sun morning may be able to
offset the lack of surface based instability.
Confidence is increasing for Sat and Sat night so an increase in pop
is warranted. However...there remains some difference between the
medium range guidance. The GFS/CMC are in fairly good agreement but
the European model (ecmwf) depicts less amplification of the middle level pattern Sat
night and sun. This would keep the cold front stalled closer to the
coast a bit longer. The differences are not huge but enough to put
sun/sun evening forecast in question. For now will maintain inherited
low chance/slight chance pop sun and dry forecast Sun night.
Dry high pressure builds in behind the cold front for the start of
next week. A weak front moves across the area Tuesday but its passage
will be mainly a wind shift. Zonal middle level flow will keep coldest
air north of the area with temperatures a little below climatology as the
Aviation /12 Wednesday through Sunday/...
as of 12z...VFR will prevail through the valid period. Expect plenty
of sunshine today as dry high pressure builds into the mid-Atlantic.
Northeast winds at or below 8 kts today will become light and variable later
this afternoon. Soundings indicate the air mass will be too dry for
any fog concerns tonight. Look for another northerly surge after 09z
Thursday...with calm winds becoming north-northwest around 5 kts by daybreak.
Extended outlook...VFR through Friday. Increasing clouds Saturday
with showers/MVFR Saturday into Sunday.
near term /through tonight/...
as of 1100 am Wednesday...high pressure will weaken over the
waters through today before a stronger area of high pressure
builds in from the west by Thursday. Expect northeasterly winds
becoming more on shore through this afternoon as center of high
orients itself from SW to NE across the Carolinas. Winds will
remain generally in the 10 to 15 knots range....with a slight
weakening through this evening. As winds diminish...seas will
subside to 2 to 3 feet through this evening.
A middle to upper trough will push a dry cold front through just
after midnight. This will produce an increase in north-northwest winds up to
20 kts into the early morning hours. Seas will begin a rising
trend at that time making their way back up to to 3 to 4 feet in
surge through the morning. Seas south of Cape Fear will remain
lower due to the northerly flow.
Short term /Thursday through Friday night/...
as of 3 am Wednesday...northwest flow will gradually decrease Thursday
into Thursday night...dropping from 15 to 20 knots Thursday to 10 to 15 knots Thursday
night. Winds will veer to north and then northeast Thursday night into
Friday as surface high moves to the NC coast. Gradient remains weak Friday
and Friday night with north to northeast flow 10 knots or less through the
end of the period. Seas 2 to 4 feet Thursday will drop to 2 to 3 feet late
Thursday night and fall to 2 feet Friday and Friday night.
Long term /Saturday through Sunday/...
as of 3 am Wednesday...anticipate poor conditions for much of the
period as slowly intensifying low pressure passes to the west.
Southeast flow of 15 to 20 knots Sat will approach 20 to 25 knots Sat
night and veer to southwest as gradient tightens. Cold front crosses
the waters sun with 15 to 20 knots winds veering to west and eventually
northwest. Cold advection is initially slow to develop so winds may
drop under 15 knots for a period Sun night before returning to 15 to 20
knots as the period ends. Seas will build on Sat as southerly flow
increases...peaking around 5 to 8 feet late Sat and Sat night.
Development of offshore winds on sun will help knock seas below 6 feet
before the end of the period...but Small Craft Advisory headlines are likely to be
required into Sun afternoon.