Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
309 PM EDT Wednesday Sep 17 2014
high pressure will extend down into the Carolinas from the north
providing Fall-like weather for the remainder of the week. The next
cold front will move through on Tuesday followed by cool and dry
high pressure through the middle of next week.
Near term /through tonight/...
as of 300 PM Wednesday...a deep trough will remain aloft through
tonight while at the surface...high pressure will ridge more
strongly south across the eastern Carolinas. Weak disturbances
embedded in fast paced...up to 70 knots flow...will continue to
traverse the area. Models continue to key in on remnant mesoscale convective vortex across
the middle Mississippi/Tennessee Valley with a piece of this energy
perhaps brushing our forecast area late afternoon/this evening. Still no
organized convection of significance has developed on the leading
edge of this feature across the southeast states as of middle afternoon.
Water vapor imagery shows plenty of dry air across the breath of the
Carolinas. A few showers have managed to overcome this dry air...
developing along a portion of the convergent seabreeze in a rather
unstable airmass given modest cooling continues in the middle levels.
We do expect more in the way of showers and possibly some
thunderstorms to develop through late afternoon/early evening. The
convection should sharply wane with loss of heating thus allowing
the instability to greatly diminish. Will show scattered probability of precipitation
decreasing from west to east tonight...lastly along the coast...as the
airmass stabilizes and upper level support moves by.
While convection should continue to have a difficult time building
to a significant depth...the potential is there for showers/
thunderstorms to produce strong/gusty winds. This is reflected in
the high downdraft cape values across the area this afternoon due to
drier air above and below the nearly saturated layer...2-12 kft.
Also...low wet bulb zeros and low 50 dbz core thresholds suggest a
potential for small hail in thunderstorms that are able to attain
Fog/stratus may again fill in overnight and Thursday morning as a
strengthening subsidence inversion traps a continuous supply of
moisture below 1 kft on low-level easterly flow. Model soundings
show the inversion will be most pronounced inland.
Lows tonight should be similar to last night if not a degree or two
cooler...especially along the coast. Will forecast lows in the middle
60s with several inland communities dropping into the lower 60s
while several beach towns only reach the upper 60s to near 70.
Short term /Thursday through Friday night/...
as of 300 PM Wednesday...high pressure building down into the
Carolinas from the Great Lakes on Thursday will shift east producing
a very pronounced wedge pattern by Friday afternoon. At the same time the
middle to upper trough over the East Coast will lift off to the
northeast. Before the trough lifts off...a couple of minor
perturbations riding around the base may enhance clouds and low
end pcp on Thursday...but not counting on much. As trough lifts off
through Friday...it may provide enough of a weakness to allow
developing low pressure off the southeast coast to retrograde
back to the west-northwest through Friday night. Overall expect decent
northeast flow as gradient becomes more pinched as trough of low
pressure shifts north toward the southeast coast through late Friday
into Friday night. The NAM has the low moving farther west with a
greater on shore flow of moisture through Friday and a much wetter
forecast...while the GFS/European model (ecmwf) confine the greatest moisture off
shore and just along the coast. For now will go with brunt of clouds
and probability of precipitation off shore and just along the coast in NE flow through
Northeast flow will help keep high temperatures along the coast a
little below climatology through the period. Farther inland highs will be
near climatology with a little more sun. Low temperatures will end up a
little above climatology due to clouds and boundary layer mixing.
Long term /Saturday through Wednesday/...
as of 300 PM Wednesday...potentially unsettled to start the
period...but even by d4 there is considerable model spread. Stalled
boundary offshore will be a focus for low pressure development...but
the question remains how close to the coast this low will form. The
NAM/CMC are in one Camp with deep easterly flow ahead of a low
pressure moving right along the coast...causing widespread and
possibly heavy...rainfall. The GFS/European model (ecmwf) are further offshore...with
NE flow through the weekend as the low pressure stays well off the
coast. Wpc favoring the European model (ecmwf) and its ensembles...which have
fluctuated little from last week...and thus will continue with the
inherited forecast leaning significantly on the European model (ecmwf) with some
blending with the wetter solutions. This suggests at least
widespread cloud cover Saturday as the low pressure makes its
closest approach...with showers most likely across the eastern
zones. Improvement will occur on Sunday as the low pressure pulls
off to the northeast...and temperatures will be around seasonable through
the weekend...although Sunday will likely be a bit warmer.
A brief period of return flow develops ahead of a strong cold front
which is slated to cross Tuesday afternoon. This brings a day or two of
warmer temperatures...before Canadian high pressure envelops the
region for mid-week. Convection looks likely on Tuesday with the
cold front...and will bump pop to chance for Tuesday as surface
convergence...pva...and some jet-level diffluence combine in a rich
Theta-E air mass.
Aviation /18z Wednesday through Monday/...
as of 18z...terminals are VFR this afternoon except for MVFR ceilings at
klbt. An upper impulse will move through late day coupled with peak
heating. Isolated/scattered convection will develop...with moderate
confidence kilm will experience a thunderstorm by 21z. Precipitation
should gradually diminish this evening.
By midnight IFR/LIFR stratus/visibilities will be an increasing concern.
All MOS guidance indicates IFR/LIFR conditions. Confidence is high
MVFR conditions will develop. Confidence moderate to high LIFR
conditions will develop with lowest confidence at klbt. Ceilings rise
to MVFR by mid-morning.
Extended outlook...gusty northeast winds expected Friday/Sat as the
cool air wedge strengthens over the Carolinas. VFR on Sunday.
near term /through tonight/...
as of 300 PM Wednesday...NE winds will remain through the period as
high pressure ridges more strongly southward. A slight NE surge
tonight will increase wind speeds up to 10 to 15 knots. An easterly
swell from distant Hurricane Edouard will allow seas to be
considerably higher than the wind speeds would otherwise indicate.
Seas of 3 to 4 feet will be common with some 5 feet seas across the
outermost waters. The swell period will be on the order of 12 to 13
seconds. Seas will trend about a foot lower overnight.
Short term /Thursday through Friday night/...
as of 300 PM Wednesday...high pressure ridging down from the north will
produce light northeast flow on Thursday around 10 kts or less. Winds
will will increase significantly through Friday as distinct high
pressure wedge sets up and low pressure develops off the southeast
coast. NE winds will increase up to 20 knots with higher gusts
through Friday into Friday night and may become more easterly depending
on position and strength of developing low off the southeast
coast. Seas 2 to 4 feet on Thursday will diminish slightly through the
day as long period swells decrease. As NE winds increase through
Friday...seas may reach Small Craft Advisory thresholds into Friday evening...especially
north of Cape Fear...building to 4 to 5 feet with potential for 6
feet. Mainly in outer waters.
Long term /Saturday through Monday/...
as of 300 PM Wednesday...concern through the extended involves a low
pressure system prognosticated to develop along a stalled cold front off
the coast. Still significant uncertainty into the strength and exact
placement of this feature...but at least some impacts are expected
locally. This low pressure will makes its closest approach to the
waters on Saturday...producing enhanced NE winds of 15-20
kts...possibly higher if the low is closer/stronger than currently
forecast. Winds then become variable on Sunday as a weak ridge of
high pressure builds towards the area...before turning to the SW
wind SW on Monday...with speeds rising back to 10-15 kts. Waves will
be primarily NE-wind-wave driven on Saturday with heights 4-5
feet...but again...if the system is stronger or closer an Small Craft Advisory may be
needed. Thereafter...wave heights fall to 2-4 feet on Sunday...and
then 1-3 on Monday with a SW wind wave and weak southeast swell forming in