Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
1047 PM EDT Wednesday Sep 2 2015
above normal temperatures are expected through Friday. A weak
trough will linger in the area through early Friday. A ridge of
high pressure will build in from the north behind a cold front by
this weekend...bringing unsettled weather and cooler temperatures
through early next week.
Near term /until 6 am Thursday morning/...
as of 1030 PM Wednesday...storms in northeastern North Carolina
has developed a massed outflow boundary pushing south through
Raleigh and Greenville at this time. A second cluster of storms
from Asheboro to Lexington is moving southeastward. This outflow
boundary is surprisingly deep...the Raleigh radar shows the
boundary extending up to around 7000 feet near Greenville which is
deep enough to lift parcels at the level of free convection. Odds are still about
1-in-4 to 1-in-5 convection will survive all the way down to
Lumberton...Elizabethtown and Burgaw. A small (20%) pop is being
maintained here after midnight. Otherwise mostly clear skies will
continue overnight with lows 70-75. Discussion from 730 PM
Closely watching scattered convective showers and storms across
the northwestern half of NC sinking southeastward. While the 500
mb cold pool centered over Virginia should not dive this far
south...a surprising number of models still show showers making it
into southeast North Carolina starting around 3-4 am. Forecast soundings
show this would not be surface- based convection but instead
rooted in a layer around 7000 feet up. Lapse rates between 800-400
mb are steep enough that should anything survive that late it
could still have thunder. To blend with National Weather Service rah and mhx I have
added a 20 pop late tonight from Lumberton to Elizabethtown to
Burgaw...otherwise very few changes were needed to the forecast
with this early evening update.
Short term /6 am Thursday morning through Friday night/...
as of 300 PM Wednesday...upper ridge builds strongly across middle
America and this will result in a trough digging down the East
Coast Thursday into Friday. Developing northwest flow aloft will allow weak middle-
level disturbances...embedded in flow...to impact the eastern
Carolinas. The strongest of these features during this time frame
will reach the area later Thursday afternoon and Thursday night. At the
surface...high pressure will assert a strong enough push to bring
a backdoor cold front across the area Friday night. The Piedmont
trough and seabreeze boundaries will strengthen Thursday afternoon and
these mesoscale boundaries should provide some modest lift...thus
allowing some convection to develop as strong heating increases
The highest precipitable water values and the greatest moisture
depth are still expected Thursday/Thursday night...although not that
impressive even for early September. At this time...will keep
probability of precipitation in the slight chance/chance category. However...higher probability of precipitation
may be warranted as we progress through late week and are better
able to nail down the timing of the various features previously
The warmest day will be Thursday when highs are expected mainly in the
middle 90s with lower 90s very near the coast and upper 80s at the
beaches. Highs on Friday will be mainly in the lower 90s...still above
normal...upper 80s at the beaches. Lows will be in the lower 70s Thursday
night and upper 60s to lower 70s Friday night.
Long term /Saturday through Wednesday/...
as of 300 PM Wednesday...complex middle-level pattern will lead to a
challenging forecast with likely an extended period of unsettled
weather across the southeast. GFS and European model (ecmwf) are in pretty good
agreement through the extended which creates higher confidence this
period...and it appears much of the extended will be unsettled with
below normal highs and seasonable lows.
Large upper ridge will build across the northeast from the Gulf
Coast...driving a surface ridge down the eastern Seaboard. As this
occurs...a back door type front will stall south of the area...as a
weakness in the pressure field strengthens and expands from the
NE...south of the ridge. This upper trough will sharpen and cut off
across the southeast...and then sit basically in place through
Tuesday as it is blocked by ridging on all sides. Beneath this
trough...the remnants of Erika will meander near the southeast coast
maintaining clouds...showers...and cool temperatures...especially
along the coast. Still some uncertainty into how much quantitative precipitation forecast is
expected since forcing is weak...but tropical moisture in the
vicinity of erikas remnants will create the potential for
showers...some of which may be heavy at times...especially Sunday
through Tuesday. Wpc quantitative precipitation forecast shows 1-2 inches through the
period...heaviest along the coast. Guidance suggests erikas remnants
will finally dissipate entirely by Wednesday and lift to the north
as the blocking ridge weakens...this will lead to improving and more
seasonable weather at the end of the extended.
Aviation /00z Thursday through Monday/...
as of 00z...the light southeast resultant will give way to a
nearly calm southwest flow in a couple of hours. A vorticity maximum
in Virginia will drop southward overnight and will have scattered
convection with it. This convection may get close enough to
our northern terminals to warrant a vcsh mention. The convection
will likely dissipate by 09z. This is mainly leaning on the NAM
model as the GFS shifts the precipitation offshore before it gets here.
Thursday...another hot day with west northwest flow. Some scattered
convection may develop around maximum heating.
Extended outlook...predominately VFR with brief morning fog and
isolated/scattered afternoon showers and thunderstorms.
near term /through tonight/...
as of 1030 PM Wednesday...virtually no changes were needed to the
forecast with this late evening update. Discussion from 300 PM
A trough will be strung out from west to east south of our local
waters but weak high pressure will hold locally through today.
Basically flow will be dominated by the sea breeze this afternoon
with on shore gustier winds...basically se-S. With winds generally
10 knots or less...seas will remain 2 feet or less.
Short term /Thursday through Friday night/...
as of 300 PM Wednesday...the slack pressure gradient...which has
been in place for several days...will finally begin to tighten...
and with that we will see wind speeds increasing. The highest
winds this period are expected in the wake of a backdoor cold
front Friday night. Wind speeds Friday night are expected to be up to 15
to 20 knots as a NE surge expands southward across the waters as high
pressure builds from the north.
A land breeze will allow for a light offshore wind Thursday morning. The
seabreeze and Piedmont trough will increase SW winds to around 10 knots
Thursday afternoon and evening. Seas will be 2 feet or less with 3 feet seas
expanding southward across the waters Friday night.
Long term /Saturday through Monday/...
as of 300 PM Wednesday...high pressure will ridge down the coast
strongly Sat/sun before slowly weakening early next week. As this
high builds SW from New England...it will be accompanied by a NE
surge...driving winds up to around 15 knots Saturday and Sunday.
Winds will veer slowly on Monday to become more easterly as the
ridge weakens...with speeds dropping to around 10 knots at the end of
the period. The sea spectrum will be significantly dominated by a
NE wind wave through the period...which will mask a low-amplitude
southeast swell. Wave heights will be 2-3 feet early Saturday...rising to
3-4 feet late Saturday and Sunday as the NE fetch persists. Wave
heights will then ease a bit Monday on the slackening winds.