Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
1113 am EDT Thursday Jul 24 2014
an Atlantic ridge of high pressure will remain south of the area
into early next week...with a weak trough of low pressure
prevailing inland. A cold front will approach late Monday and
become stationary south and southeast of the area during the
middle of next week.
Near term /through tonight/...
today...the forecast area is currently located between an upper
ridge off the Florida Atlantic coast and an approaching trough
just west of the appalachian corridor. The axis of the upper trough
will make subtle movements east as the day progresses...with a
surface trough also shifting toward the coastline through the late
afternoon time frame. The orientation of these synoptic features
will support a solid west flow in the low to middle levels that will
effectively pin the seabreeze to coastline for much of the
afternoon. Temperatures could approach 90 degrees in coastal
communities as a result...while peaking in the middle 90s across
inland areas. Moderate boundary layer flow in the 20 to 25 knots
range suggests that decent mixing this afternoon could support
Latest high resolution model solutions are in fairly good
agreement that convective development will be delayed due to the
pinned seabreeze and a persistent middle level capping inversion.
Isolated thunderstorms could initially develop along the seabreeze
in South Carolina coastal counties...however the bulk of the
activity could arrive later today and this evening. Shortwave
energy embedded within and tracking just downstream of the trough
axis will steadily progress east/southeast later today. Thunderstorms
associated with this approaching energy will likely begin to shift
into the far western forecast area by late afternoon...steadily
spreading east into the evening hours. Ongoing forecast rain
chances represent this scenario well...and little change was
needed with the late morning update. Middle layer flow appears a bit
enhanced for middle Summer...suggesting that if boundary interactions
can enhance updraft strength within any late day
thunderstorms...isolated damaging wind gusts could result.
However...lapse rates are quite marginal and strongest energy will
be confined well to the north. Thus organized severe potential
appears low at this time.
Tonight...the surface trough will approach the area before
stalling toward daybreak Friday. Evening convection will
dissipate late as instability/convergence weaken...although a few
showers could linger mostly over South Carolina which will be
closer to the diminishing trough. Lows should range from the lower
to middle 70s inland to upper 70s at the coast.
Short term /Friday through Sunday/...
Friday...the local region will be positioned near the southern apex
of a long wave trough covering the east/southeast states...snuggled in
between one anticyclone over the northwest Bahamas and another covering
much of the southern tier of the nation. At the same time at the
surface...the east/NE to west/SW aligned trough will become more
ill-defined over or just inland from the County warning forecast area...keeping ample
moisture and instability in place. Lapse rates are poor
overall...but there is some forcing for ascent in association with
the right entrance region to the upper jet and a potential
perturbation that moves in aloft during peak heating. These features
along with the sea breeze and surface trough will trigger scattered
showers/T-storms. A warm west/SW synoptic flow will negate some of
the 50-70 percent cloud cover...boosting temperatures into the lower 90s
before rain-cooled convection sets in.
Saturday through Sunday...the long wave trough lifts out later
Saturday and in its place there is a merger of the the two
anticyclones by Sunday...with ridging to encompass the area from the
Bahamas to the Southern Plains and much of The Rockies. There are
still hints of the remnants of the surface trough which appears to
morph into the typical Piedmont trough. There does appear to be a
subtle short wave that moves through Saturday...but none are
apparent at this juncture for Sunday. With less availability of
moisture given that precipitable waters fall to between the 25th and 50th
percentile over the weekend our rain chances would be no more than
isolated to scattered coverage Saturday and isolated coverage
Sunday. With less coverage of showers/T-storms and an expansion of
low level thickness our maximum temperatures will climb into the lower and
middle 90s away from the beaches. While heat indices won't be
anywhere near the heat advisory criteria of 110 degrees or
greater...by Sunday some places will experience a heat index of
about 104-108 degrees. We mention this because we have only issued
one heat advisory so far this year and values such as these have be
uncommon of late.
Long term /Sunday night through Wednesday/...
a deep layered gyre over or near southern Hudson Bay in Canada will
circulate occasional short waves through the Great Lakes and Ohio
Valley...which in turn will carve out a broad and fairly well
amplified trough across the eastern half of the country during the
early and middle parts of next week. This is definitely not a
Summer-like pattern...and it will send an unusual late July cold
front into the area from the northwest later Monday and early Tuesday.
Surprisingly it looks like there is enough northwest push behind the
front to send it through the area...before it stalls out not far
to the southeast and south for the middle of the week. Rain chances will
climb Monday and Tuesday in conjunction with the front...mainly in
the 20-40 percent range. Provided the front clears the area rain
chances are no more than 20 percent Wednesday...then 20-30 percent
next Thursday as the front starts to trek back north.
Compressional heating out ahead of the front allows for above
normal temperatures Monday...with only very minimal cool advection to
drop readings slightly below normal Tuesday through Thursday.
Aviation /14z Thursday through Monday/...
solid VFR into middle afternoon at the airfields...with southwest/west
winds becoming gusty at times within a well mixed atmosphere.
Attention then shifts to the development of rain showers/thunderstorms and rain in
association with the sea breeze and a nearby trough off to the
west/northwest. Flight restrictions are possible from about 20z to
03z...most notably at kchs where the best interaction of
boundaries will occur in sync with the passage of energy aloft.
Low-end chance of some late night fog and/or stratus if sufficient
rain occurs and skies clear.
Extended aviation outlook...most of the time through Sunday will be
VFR...with only small probabilities of brief flight restrictions in
isolated to scattered afternoon/evening convection. Better chances
for sub-VFR weather Monday in advance of a cold front.
today and tonight...a trough of low pressure inland will progress
toward the coast this evening before stalling. South to southwest
winds will be breezy today ahead of the trough...then diminish and
shift west late tonight. Have maintained winds no more than 20
knots...just below advisory levels...but there could be a few gusts
near 25 knots...mainly along the Charleston County coast and offshore
waters. Seas should peak near 4 feet...highest east.
Friday through Saturday...nothing out of the Ordinary in regards to
summertime marine conditions...with a west and SW flow at less than
15 knots as the coastal waters will lie between a poorly defined inland
trough and the Bermuda-Azores high across Florida.
Sunday through Tuesday...low pressure moving through the NE quadrant
of the country early next week while simultaneously the sub-
tropical Atlantic ridge is shunted a little further south. This
allows for a cold front to approach early in the week...probably
stalling out over or near the coastal waters late in the forecast
period. There is a fairly decent tightening of the gradient that
occurs...a good 2-3 mb spread from north to south over the local
marine zones. It does not look to be strong enough for Small Craft
Advisory conditions...but we could be pushing 20 knots at times and
seas will be building to 4 or 5 feet. Will also need to remain
alert for organized thunderstorms in association with the cold
front...mainly late Monday into Tuesday.