Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
419 am EDT Friday Aug 22 2014
today will bring yet another day of unseasonable heat and humidity.
Some relief may come with a cold front on Saturday but the return
of true seasonable weather probably won't be until Sunday. Much
of the upcoming week looks seasonable and rain-free.
Near term /through tonight/...
as of 330 am Friday...plenty of middle and upper level clouds across
the forecast area at the moment. This will likely keep minimal fog development
and coverage for early this morning.
Amplifying middle-upper ridge to occur this period...with its axis
extending from the central Gulf Coast states to the western Great
Lakes...basically along the Mississippi River valley. At the same
time...a middle-level short wave trough will drop nearly due south from the NE
states as the flow aloft amplifies. This will help drive a surface cold
front...Aka backdoor cold front...to the doorsteps of the ilm County Warning Area
by daybreak Saturday.
Will be dealing with plenty of debris clouds from convection north
of the forecast area overnight and this morning. As the flow aloft
amplifies...those middle-level impulses ie. Vorts...will take aim at
the ilm County Warning Area especially tonight. As far as probability of precipitation this period...will
indicate a 20-30 pop from sea breeze induced convection this
afternoon. For later tonight associated with an upper vorticity and the
surface cold front dropping south...will indicate a low chance pop.
Have stayed closer to a higher maximum temperature forecast for today...with
the GFS MOS guidance heavily leaned on. With low to middle 70s
dewpoints...heat indices will reach 105+ this afternoon across a
select ilm SC counties. Will issue an afternoon heat advisory for
those counties. Elsewhere...a Special Weather Statement will be utilized to convey the
heat that does not meet heat advection criteria but nevertheless could
still be extremely dangerous if one does not take the proper
precautions or steps when venturing outside.
Short term /Saturday through Sunday night/...
as of 3 am Friday...backdoor cold front pushes through on Saturday.
MOS numbers are generally spread by 5 degrees with the mav warmer of
the two. Have basically split the difference...the actual cold air advection lags
and the 'cooling' may be more a function of fairly extensive cloud
cover associated with frontal passage. MOS probability of precipitation are surprisingly low
area-wide...generally slight chance to lowest chance range. Given
the antecedent warm and humid airmass in place prefer the 40-ish
probability of precipitation close to previous forecast. More seasonable and less humid air
starts creeping in Saturday night into Sunday. The moisture however
is slower to advect off to the south. Clouds may thus hang in on
Sunday and some slight chance probability of precipitation will remain in the
forecast...slightly favoring coastal and southern locales which are
closer to the boundary.
Long term /Monday through Thursday/...
as of 3 am Friday...drying becomes pronounced on Monday as high
pressure wedge pushes front and its associated moisture even further to
the south. Meanwhile building upper ridge expands across MS and Ohio
Valley regions bringing a northerly flow locally. This will be in
place through Wednesday and some very pleasant weather is expected.
Both ridges weaken on Wednesday which may allow some low level
moisture return in the form of noticeably more humid air...but still
too shallow to support any meaningful precipitation chances.
Aviation /08z Friday through Tuesday/...
as of 06z...VFR conditions may give way to patches of MVFR visibilities
overnight as calm and humid conditions support some patchy MVFR br.
Elsewhere...expect thickening middle/high clouds to keep visibilities from
dropping below MVFR. Visibilities will improve to VFR soon after 12z with
west-northwest winds around 5 kts this morning. As the sea breeze moves
onshore...winds along the coast will become S-SW around 10 knots. Not
expecting much in the way of afternoon convection other than some
isolated sea breeze storms possibly developing after 18z. Will
include thunderstorms in the vicinity for the coastal sites at this time...eventhough confidence
is not as high like previous days. This mainly due to the
influence of the upper high pressure ridge which tends to suppress
Extended outlook...chance afternoon/evening rain showers/thunderstorms and rain through Sunday.
Expect VFR Monday/Tuesday with high pressure over the East Coast.
near term /through tonight/...
as of 330 am Friday...a surface trough will extend north-S across the
eastern Carolinas into tonight. Ridging from high pressure
centered well offshore will extend to the southeast states...south of
the area waters. The 2 combined will produce SW winds across the
local waters today. For tonight...a backdoor cold front will drop
south reaching the doorsteps at Surf City toward daybreak Sat.
This will result in winds veering to the west across the ilm NC
waters late tonight. The surface pg will yield a 10 to occasionally 15
knots wind speed. Significant seas will range between 2 and 3 feet.
Short period wind driven waves from the sea breeze circulation
will drive the significant seas. An underlying 1 foot east-southeast ground
swell at 9 second periods will remain identifiable.
Short term /Saturday through Sunday night/...
as of 3 am Friday...the combination of a surface trough right along
the coast early Saturday and a cold front sliding north to south
across the region will lead to a light west wind that should start
veering first over northern zones in the evening and then farther
south overnight. This sense of timing may need tweaking however as
models are having a little trouble resolving the two boundaries. At
least a turn to the north looks certain by Saturday night and a
gradual increase in wind speed and thus wave heights should get
underway. Sunday will bring a rather atypical northeasterly flow
with a little bit of gustiness as high pressure builds in from the
north. Scec tough to rule out over northern zones.
Long term /Monday through Tuesday/...
as of 3 am Friday...rather unusual northeasterly surge of high
pressure to continue through the period with little change. The
persistence of the fetch in both time and space could lead to some
advisory-worthy 6 feet seas but wind speeds will tend to stay
sub-advisory. Also interesting but highly uncertain is the
possibility of tropical system well offshore. No direct impacts
expected at this point but its related swells could be radiating
this way by the end of the period or just beyond.
SC...heat advisory from noon today to 6 PM EDT this evening for