Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
400 am EDT Monday Sep 1 2014
mainly high pressure will 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...welcome to meteorological autumn! Of course it won't feel
as such with a continued Summer pattern across the forecast area.
Deep layered ridging will again prevail...with the core of the middle
level anticyclone to have migrated over southern Georgia. Concurrently
at the surface we/ll be on the western periphery of the sub-
tropical Atlantic ridge...while a Lee-side trough becomes better
defined this afternoon. Similar to yesterday the models are
painting the formation of a mesoscale-low along the inland trough over
or near our southeast Georgia counties this afternoon. So despite some cap
associated with the deep ridging...there is the synoptically
driven features of the trough and meso-low...plus the typical
mesoscale-scale boundaries that occur during the warmer months of the
year. The cap will limit both the depth and persistence of the
updraft strength...but the other aforementioned factors and a good
thermodynamic atmosphere supports scattered showers and T-storms
Any early morning morning induced showers and T-storms should stay
over the Atlantic as the synoptic flow veers around to the SW.
Thus we have maintained a rainfree forecast until isolated
activity starts popping on the sea breeze during the early
afternoon inland from the US-17 corridor. Coverage of showers and
T-storms will then climb during the middle and late afternoon hours
across inland sections as boundary interactions initiate in
response to the various surface features. Some of the severe
weather parameters this afternoon are quite good...with ml convective available potential energy
of 3000 j/kg or greater...dcapes over 1000 j/kg with pockets of
dry air aloft...lifted indices -6 to -8c and total totals that
approach 50. As a result we feel compelled to add mention of
isolated severe storms to our SW...west and northwest tier this
afternoon/evening. The main hazard would be damaging winds in
excess of 60 miles per hour. In addition...movement within the atmosphere is
rather sluggish. With precipitable waters up near 2 inches we look for locally
heavy rains in some of the persistent showers and T-storms.
Prior to convection messing up the diurnal temperature curve we
anticipate another hot one today...with maximum temperatures about 4-8
degrees above normal for September first. Mainly middle 90s inland
with upper 80s and lower 90s closer to the shoreline. These temperatures
combined with dew points mainly in the lower or middle 70s will
generate rather oppressive heat indices of 102-107 degrees for
about 3-5 hours this afternoon.
Tonight...scattered convection driven by synoptic and mesoscale-scale
boundaries well inland will wane during the late evening as cinh
increases and nocturnal cooling/stabilizing influences take over.
Activity looks to be done by midnight or so as this transition
occurs. Further east there is the lack of synoptic forcing and
enough subsidence to keep things dry. Like recent nights there
will be some development of Atlantic showers and T-storms 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 them
offshore. If convective debris clouds thin our 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 /07z Monday through Friday/...
while we have maintained VFR conditions through 06z Tuesday at the
terminals...there is a low end chance of flight restrictions due
to stratus and/or fog from about 10-14z. Since we/re only
forecasting isolated coverage of rain showers/thunderstorms and rain along the inland
penetrating sea breeze as it pushes through the terminals around
18-20z we have kept any cumulonimbus clouds and thunderstorms in the vicinity out of the 06z taf
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 lax 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. Not
much in the way of showers and T-storms...mostly isolated in
coverage and during this morning.
Waterspouts...the formation of the early morning land breeze over
the Atlantic and then the transition to the late morning sea
breeze could spur isolated waterspouts across the marine community
within a light wind regime and a decent thermodynamic environment.
If confidence rises enough a marine weather statement will be
issued with the 7 am forecast package.
Tonight...little change in the synoptic pattern with the western
extension of the sub-tropical high anchored right across the area.
Small nocturnal jetting will bump winds up as high as near 15 knots
for several hours as they clock around from the south to the SW
through the night. Isolated showers and T-storms 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.