Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Wilmington NC
1020 am EDT Sat Aug 30 2014
strengthening Bermuda high pressure offshore will bring warm weather
through the Labor Day weekend with a few afternoon showers and
thunderstorms possible. A weak cold front will approach from the
north late in the period but is likely to stall before reaching the
area. Bermuda high pressure and a Piedmont trough will remain the
dominant features into next weekend.
Near term /through tonight/...
as of 645 am Saturday...low clouds and fog have filled in across the
SC Pee Dee corridor from Georgetown and Kingstree northward across
Florence to Darlington and Bennettsville. The overnight runs of the
hrrr have been nearly perfect predicting the area affected by these
clouds. Surface visibilities should improve shortly now that the sun
is rising...but it take another 2-3 hours for the low stratus to
burn off into a cumulus deck. No significant changes are needed to
the forecast. Previous discussion from 300 am follows...
Middle and upper level high pressure will be almost directly overhead
today. This will keep the flow of tropical moisture originating from
the western Gulf routed up the Mississippi River valley into the
Great Lakes with no impact this far east. At the surface a pesky
little backdoor cold front appears to have sunk as far south as
northern Pender and Bladen counties this morning. This boundary may
actually linger across southeastern North Carolina for much of the
morning before returning north early this afternoon...pushed by a
combination of the local seabreeze circulation plus a slow veering
of the synoptic wind as high pressure moves farther east off the New
Although there is no synoptic forcing present today...an unstable
airmass will develop with surface-based cape rising to 1500 j/kg at
the coast and near 1000 j/kg inland. Only isolated showers are
expected...mainly east of I-95. In terms of timing...the better
shower chances may actually occur during the morning and early
afternoon hours with deeper mixing during the afternoon dragging
down dry air from aloft and diluting the surface-based instability.
Plenty of dry air aloft and very weak winds throughout the
troposphere favor only small thunder chances and no storm-cell
organization. Highs today should rise into the lower 90s inland with
upper 80s near the coast.
Mostly clear to partly cloudy skies and light winds will again favor
areas of low clouds and/or fog late tonight. Lows should fall to
Short term /Sunday through Monday night/...
as of 3 am Saturday...weakening middle level ridge on sun will still be
working to limit afternoon convection. Forecast soundings show an
abundance of middle level dry air and a weak subsidence inversion.
Elongated Bermuda high will ensure plenty of low level moisture and
warmth with the weak gradient allowing sea breeze development.
However the sea breeze is likely to be the only forcing mechanism
and it may not be enough to overcome the subsidence. A weak Piedmont
trough may develop in the afternoon but it probably will not be of
enough significance to generate convection.
Continued deep southwest flow Monday combined with weakening middle level
ridge will lead to an increase in precipitation chances. Precipitable water
values climb to 2 inches Monday afternoon with forecast soundings
showing no evidence of the previous days inversion. Weak surface
gradient will again allow for sea breeze development...which should
be more active than on sun. Piedmont trough will be much more
defined and should also act as a focus for convection. Will continue
to carry afternoon low chance pop. Temperatures will run above climatology
through the period.
Long term /Tuesday through Friday/...
as of 3 am Saturday...Bermuda high and Piedmont trough will be the
dominant surface features while aloft weak elongated 500 mb ridge
extends across much of the southern Continental U.S.. this pattern is fairly
typical of Summer and combined with precipitable water values around
2 inches...also typical for this time of year...will keep precipitation
chances near climatology through the period...30 to 40 percent. Lack of
any forcing aloft will leave the sea breeze and Piedmont trough as
triggers for convection. Outflow from storms as well as lingering
boundaries from previous days convection will also play potential
roles in convective initiation. Late in the period a cold front may
drop in from the north but the strength of this feature when it
arrives...if it arrives...is questionable. Temperatures will be near
to above climatology through the period.
Aviation /12z Saturday through Wednesday/...
as of 12z...11u-3.9u satellite images show stratus/fog along and SW
of a kmyr to klbt line...with MVFR clouds to the NE of that line.
Isolated showers are moving north onshore between kmyr and kilm...with
other isolated showers north of kflo to west of klbt. Winds are light NE at
kilm in the wake of a weak front that extends northwest from kilm to near
LIFR/IFR conditions appear most likely at kflo/klbt over the next
couple of hours...with MVFR elsewhere through middle-late morning.
IFR could occur at kmyr/kcre early this morning but confidence is
lower. Vcsh will be most likely near kcre and kilm. Expect best
chance of showers through middle afternoon...with activity decreasing
VFR expected this evening except for scattered MVFR level clouds at the
coastal terminals. Fog will develop overnight with IFR likely
inland terminals with MVFR at the coastal terminals.
Extended outlook...chance IFR/MVFR Sunday morning. Increasing
chances for afternoon showers/thunderstorms Monday through Wednesday with brief
and localized periods of MVFR possible.
near term /through tonight/...
as of 645 am Saturday...a pesky little backdoor front located near
Jacksonville NC may actually sneak down as far south as Carolina
Beach before stalling later this morning and returning north this
afternoon. It's a pretty insignificant feature in the weather-world
but could turn wind directions east to northeasterly for a few
hours. Otherwise a light southeasterly wind should prevail through
the day...enhanced by the afternoon seabreeze.
Isolated showers out across the offshore waters will move toward the
coast shortly after sunrise. Rainfall coverage should average only
10-20 percent of the area...although unidirectional wind through the
lowest 6000 feet of the atmosphere raises the potential that
waterspouts could develop even in a weak shower cell. Shower chances
should diminish rapidly this afternoon behind the developing
seabreeze front. Winds will turn more southerly with time tonight
with no increase in wind speed expected.
Seas currently around 2 feet may build to 3 feet away from shore as
a swell set produced by the Bermuda high arrives at the coast. National oceanic and atmospheric administration
buoy 41002 located about 200 miles offshore started picking up this
swell about 6 PM yesterday. This southeasterly 7 to 8 second swell
should be our dominant wave through tonight.
Short term /Sunday through Monday night/...
as of 3 am Saturday...surface gradient will remain weak through the
period with elongated Bermuda high off the coast. This will maintain
light south to southwest flow through Monday night. Speeds will run
around 10 knots overnight with diurnal increases to a solid 15 knots each
day. Seas will run 2 to 3 feet through the period.
Long term /Tuesday through Wednesday/...
as of 3 am Saturday...elongated Bermuda high remains in place Tuesday
and Wednesday with Piedmont trough becoming a little more defined. Slight
increase in gradient each afternoon will increase southwest flow to
a solid 15 knots into the early evening hours. Otherwise southwest flow
will be 10 knots or less. Seas run 2 to 3 feet.