Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
154 PM EDT Monday Sep 1 2014
Atlantic high pressure will generally prevail through middle week. A
cold front will stall and dissipate north of the region late in
the week...followed by high pressure. A stronger cold front could
approach the region late in the weekend.
Near term /through tonight/...
today...the persistent strong upper ridge will meander over the
southeast states today...allowing temperatures to peak well above
seasonal normals. Surface high pressure will also linger over the
Atlantic waters...while an inland trough becomes more evident as
the day progresses. This pattern will support more typical
summertime conditions across the forecast area. The 12z Charleston
sounding shows precipitable water values have recovered back to
near 2 inches...evidence of increasing moisture at all levels. As
temperatures peak in the middle to upper 90s inland...dewpoints will
only fall into the low to middle 70s...producing heat index values in
the 102 to 107 degree range.
Improvements to low level moisture profiles within such warm
conditions will also allow instability to increase compared to
previous days...with cape values reaching 3000 j/kg later this
afternoon. The inland-progressing seabreeze...the Lee-side
surface trough...and evidence of a mesoscale-low to the southwest will
provide several forcing mechanisms for convective development.
The ongoing thunderstorm forecast looks on track...indicating
isolated to scattered thunderstorms along and inland of the
seabreeze...possibly becoming focused over the west/southwest
zones where the seabreeze interacts with the synoptic scale
surface features. Considering decent instability...as well as some
pockets of dry middle level air supporting downdraft convective available potential energy over 1000
j/kg...will maintain mention of isolated severe thunderstorm
potential in the hazardous weather outlook over the
southwest...west and northwest tier this afternoon/evening. The
main hazard within any storms enhanced by boundary interactions
would be damaging winds in excess of 60 miles per hour. It should also be
noted that weak wind fields in the low to middle levels will produce
slow storm motions and the potential for locally heavy rain.
Tonight...scattered convection driven by synoptic and mesoscale-scale
boundaries well inland will wane during the late evening as
convective inhibition increases and nocturnal cooling/stabilizing
influences gain influence. Activity will likely end by midnight
as this transition occurs. Areas further east will lack synoptic
forcing and be under sufficient subsidence to maintain dry
conditions during the evening and overnight time frame. There will
once again be some development of Atlantic showers and thunderstorms
in response to the favorable baroclinic warm waters. While a few
of these could attempt to move onshore...the flow veers around to
the southeast after midnight and will probably keep the bulk of
activity offshore. If convective debris clouds thin and dissipate
quickly enough...there is a risk of some late night fog and/or
stratus inland. Lows tonight will again be above climatological
Short term /Tuesday through Thursday/...
Tuesday and Tuesday night...the west/east oriented middle/upper level
ridge will remain in place across the southern Continental U.S.. meanwhile at
the surface...Atlantic high pressure will extend across the forecast
area from the east. This setup will result in a typical Summer day
with diurnal isolated to scattered showers and thunderstorms. The
best coverage will likely remain inland of the forecast area within
a Lee trough which may provide more focused convergence for
development. As such...the highest probability of precipitation are across inland zones
bordering the midlands. Given the overall thermodynamic
environment...the overall severe threat is quite low though a couple
of strong storms can/T be ruled out especially where boundary
interactions occur. Convective activity is expected to diminish
quickly with the loss of heating and the bulk of the overnight is
expected to be dry. Look for temperatures to run about 5 degrees
above normal with highs in the low/middle 90s and lows in the middle 70s.
Wednesday through Thursday...the overall setup will not appreciably
change through the middle week time frame. Aloft the upper ridge will
persist and at the surface Atlantic high pressure will be the most
prominent feature. Daily thunderstorm chances will be driven by
diurnal heating and convergence along the sea breeze...as well as an
inland trough. This surface trough should remain aligned inland of
the forecast area...keeping the best coverage outside the area. No
noteworthy changes were made to the going forecast which continues
to advertise isolated to scattered coverage and above normal
Long term /Thursday night through Sunday/...
the ever evolving upper ridge aloft will persist across the
southeast Continental U.S. As it wobbles around south of the main belt of
westerlies. There are indications that by the latter part of the
weekend the ridge will weaken and become a bit separated by a
shortwave to the north and a TUTT-like feature near the northern
Bahamas. In fact...the medium range guidance seems to be indicating
an improving chance for a more significant front to approach the
area from the northwest and then stall late in the weekend into
early next week. If this comes to fruition it would necessitate
higher rain chances late in the period. Temperatures should hold
nearly steady above normal with highs in the low 90s and lows in the
Aviation /18z Monday through Saturday/...
although isolated to scattered thunderstorm development is
possible late this afternoon into the evening...timing and
location of activity is too uncertain to include specific mention
in the tafs at this time. Will thus continue VFR conditions at
both terminals through the 18z taf period and monitor convective
trends during the next several hours. There remains a small chance
for some stratus and/or light fog during the pre-dawn hours
Tuesday morning...yet prefer to maintain an optimistic forecast
Extended aviation outlook...low chances of direct impacts from
isolated showers/thunderstorms especially each afternoon/evening
Tuesday through Friday. Also...low chances exist for early morning
fog/stratus. Otherwise...mainly VFR.
today...the coastal waters will remain under the influence of the
Bermuda-Azores high and a fairly weak anticyclonic gradient. Even
with sea breeze circulations this afternoon...winds will average
no more than 10 or 12 knots and seas will hold around 2 or 3 feet.
Tonight...little change in the synoptic pattern with the western
extension of the sub-tropical high anchored right across the area.
Light nocturnal jetting will bump winds up to 15 knots for several
hours as direction become more south to the southwest through the
night. Isolated showers and thunderstorms will again occur during
the late evening and after midnight.
Tuesday through Friday...overall the pattern will generally remain
the same as high pressure to the east results in a persistent south
to southwest flow. Winds are expected to be less than 15 knots for the
most part...with periods of slightly stronger flow possible with the
afternoon sea breeze and nocturnal surges. Seas will be 1-3
feet...highest beyond 20 nm.
The weak pressure pattern and light wind field in place each morning
could produce an environment supportive of the development of
waterspouts through middle week.