Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
1033 PM EDT Tuesday Sep 2 2014
Synopsis...a weak surface front will settle southward across central NC
on Wednesday. High pressure will build back over the area for the
remainder of the work week.
Near term /through tonight/...
as of 1030 PM Tuesday...
Convection has largely dissipated across the area this evening with
nocturnal stabilization taking hold. However... an approaching line
of showers and storms... currently moving east-southeastward through
the NC and Virginia Mountains... in advance of a weak cold front is
expected to approach/possibly move into portions of the area
overnight. With the Storm Prediction Center mesopage still showing 1000-2000 j/kg of
MUCAPE across our northwest/northern Piedmont and the latest rap and
hrrr showing the potential for some of this activity to move into
this area (nw/north piedmont) will leave a chance of showers and storms
in the forecast for this area. However.. with the lack of good deep
shear and daytime heating think this line will continue to show a
weakening trend as it moves into northwest/north portions of central NC around
midnight or shortly after. Thus... would expect any of this
weakening line to remain below severe limits.
Where heavy rainfall occurred this past afternoon-evening (generally
along and south of a line from Asheboro to Raleigh to wilson) and
skies partially clear overnight... we could see patches of fog
develop during the pre-dawn hours of Wednesday. Min temperatures near 70-
Short term /Wednesday through Wednesday night/...
as of 335 PM Tuesday...
Shear axis and attendant surface boundary will lie across the
southern counties of central NC. Isolated showers and storms along
this feature Wednesday morning will grow in coverage as the
atmosphere heats up...leading to greater low level instability.
Relatively weak steering flow suggest that the storms will be slow
moving. If storms occur over urban locations...this may lead to
minor flooding of streets and poor drainage areas.
Plan to cap probability of precipitation at 50 percent across the south...dwindling to 25-35
percent across the northwest.
Maximum temperatures Wednesday dependent on when convective coverage increases
and whether any periods of partial sun occur. Betting on some
partial sun int he late morning-early afternoon prior to convection
increasing after 2-3 PM. If skies remain overcast or convection
becomes active by lunchtime...maximum temperatures may end up being 5-6 degrees
cooler. Continued warm and muggy Wednesday night with scattered
convection slowly diminishing/dissipating. Min temperatures near 70-lower
Long term /Thursday through Tuesday/...
as of 255 PM Tuesday...
For Thursday/Fri: continued warm/humid with daily scattered showers and
storms. At the surface... the weak frontal zone will likely have
washed out by Thursday morning... and the surface high centered off the
southeast coast will introduce a moist confluent low level flow off
the Atlantic into NC. A positively tilted trough will shift from
Montana/Dakotas/scentral Canada early Thursday across ont/Quebec by Friday night...
propelling a cold front through the Great Lakes/middle miss valley. As
the Flat Ridge holds across the southeast/midsouth/Gulf states
however... we'll stay within an environment characterized by weak
shear and moderate instability (tempered by warm middle levels). Precipitable water
values should stay above normal at 1... not unexpected given
the tropical nature of the incoming air mass... and sufficient to
support scattered convection along the sea breeze... dying frontal
zone... or any other subtle boundary. Will retain afternoon and
evening storm chances both days... with temperatures holding about a
category above normal as thicknesses stay around 10 M above normal.
For Sat-Tue: still expect a transition from the warm/humid weather
to more seasonable conditions late in the weekend. The initial middle
level trough over eastern Quebec will continue shifting east over the
Maritimes... while a strong slug of strong vorticity emanating from
a strong cold vortex over the Beaufort Sea will push eastward close
on its heels... crossing Hudson Bay on Sunday. This should push the
surface cold front to just northwest of NC by sun... and well into the
state by early Monday. Both the GFS and European model (ecmwf) track their cool parent
highs over the St Lawrence Valley/southern Quebec Mon/Tue... with the
cold front pushing to eastern NC and across northwestern SC. This model
agreement leads to growing confidence that the parent high will be
dense enough to push this cooler air well into central NC...
although it may not happen until late Sun night into Monday morning.
Will follow this trend with temperatures holding above normal Sat/sun (but
not as warm in the north sun due to more clouds) with highs in the
middle 80s to near 90... but then expect temperatures to plunge for Monday/Tuesday
with thicknesses dropping to 10-20 M below normal... yielding highs
near 80 to lower 80s. Regarding precipitation chances... we're likely to
see scattered to perhaps numerous showers and storms late Sat
through at least Sun night... both along the prefrontal Piedmont
trough and ahead of and with the front itself. Shower/storm chances
are expected to peak Sun afternoon. With the middle level trough axis
swinging through the region Monday... we may see lingering chances for
rain especially south and east. The surface ridge nosing in from the
north late Monday into Tuesday accompanied by lowering precipitable water values should
mean mostly dry weather Tuesday... as the middle level trough axis lifts NE
away from the region followed by weak middle level shortwave ridging
over the Carolinas. -Gih
Aviation /18z Tuesday through Sunday/...
as of 110 PM Tuesday...
Scattered showers and storms continue across the area... but are not
currently affecting any terminals. A few storms are possible through
the overnight hours as a weak boundary approaches the area from the
northwest. Sub-VFR conditions with low clouds/fog are possible early
Wednesday at kfay and krwi where low level moisture remains high...
particularly at kfay where 1-2 inches of rain fell this evening.
Scattered convection will be more prominent Wednesday afternoon-
evening...mainly across the southern counties including in vicinity
of the kfay terminal. Otherwise VFR conditions are expected
Typical summertime weather conditions anticipated Thursday through
Saturday with areas of early morning fog and low clouds giving way
to VFR conditions aside from sub-VFR conditions in vicinity of
scattered afternoon-early evening convection. On Sunday...a front
will be approaching from the northwest. This weather system will likely
enhance the probability of convection across central NC.