Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
402 PM EDT Monday Sep 1 2014
high pressure will prevail into next weekend. A cold front will
stall and dissipate north of the region by late week...then a
stronger cold front could advance into the region late in the
weekend or early next week.
Near term /until 6 am Tuesday morning/...
late afternoon convective coverage appears more similar to a
typical summertime scenario when compared to previous days...as
isolated to scattered showers/thunderstorms have initiated along
and inland of the seabreeze. Cape values currently range from 2000
j/kg north to 4000 j/kg south according to latest Storm Prediction Center
mesoanalysis...and when combined with precipitable water values
near 2 inches...expect activity to continue into the early evening
hours while generally shifting inland with the seabreeze. Several
numerical model solutions suggest that convection could linger
across far inland zones well into the evening time
frame...possibly due to non-diurnal forcing from synoptic scale
features such as the Lee-side surface trough and a weak mesoscale-low
to the southwest of the forecast area. Rain chances in the 20 to
30 percent range late this afternoon will thus diminish from east
to west...with slight chance probability of precipitation remaining across the far inland
counties as late as midnight. Subsequent forecasts can address
radar trends and end mention of precipitation early if necessary.
The best convective potential will eventually transition to the
marine zones after midnight.
Despite some decent instability...thunderstorms are struggling to
deepen...likely as a result of persistent subsidence under strong
upper ridging. However prefer to maintain mention of isolated
severe thunderstorm potential in the hazardous weather outlook
over the southwest...west and northwest tier through this evening.
Boundary interactions will likely be needed to provide sufficient
energy to overcome subsidence and capping that is currently
limiting strengthening potential by late afternoon. The main
hazard within any storms enhanced by boundary interactions will be
damaging wind gusts and heavy downpours.
Temperatures will fall into the low to middle 70s overnight...several
degrees warmer than seasonal normals. Model guidance is once
again hinting at the potential for patchy fog during the pre-dawn
hours Tuesday morning. However...sref and NAM visibility solutions
are rather conservative and thus prefer not to mention fog in the
forecast at this time. Areas that receive notable rain and where
sky cover scatters could have the best chance for some light fog
Short term /6 am Tuesday morning through Thursday/...
Tuesday...beneath an west/east oriented middle/upper level ridge and
on the western periphery of Atlantic high pressure will support
another hot/humid day featuring highs in the middle/upper 90s and
heat index values 100-105f away from the coast. Maintained slight
chance probability of precipitation close to the coast as the sea breeze pushes inland and
chance probability of precipitation inland close to a surface trough. 01/12z guidance
suggests that convective coverage could eventually require locally
greater probability of precipitation inland...but highest probability of precipitation are capped around 40
percent due to the presence/influence of the upper ridge. Precipitable waters
around 2 inches and erratic thunderstorm propagation could support
locally heavy rain. The thermodynamic environment should limit the
severe potential to nothing more than a couple of brief/pulse
damaging wind events especially where boundary interactions occur.
Tuesday evening...convective activity is expected to diminish
quickly with the loss of heating and the bulk of the overnight is
expected to be dry. Temperatures should bottom out in the middle 70s
many areas...except around 80f at the coast.
Wednesday...a cold front will approach the region from the
north/northwest. This front will eventually stall north of the
region...but the frontal trough will combine with the sea breeze to
support at least isolated/scattered showers/thunderstorms especially
during the afternoon. Precipitable waters around 2 inches will once again support
locally heavy rain...while the severe threat should remain low.
Otherwise...highs at least in the lower to middle 90s and heat index
values 100-105f will again prevail outside thunderstorms.
Wednesday night...some guidance depicts a normal diurnal dissipation
of convection while other guidance suggests that convection could
persist well into the overnight. Previous forecasts included slight
chance probability of precipitation. Without a strong reason to lower probability of precipitation maintained
without much change...but forecast confidence remains low.
Thursday...the stationary front will gradually dissipate north of
the region. 01/12z guidance suggests that ridging will strengthen at
the surface and aloft...perhaps accompanied by some degree of
subsidence/subtle drying as suggested by model soundings.
Thus...maintained slight chance/low chance probability of precipitation...but Thursday could
be somewhat less active as compared with Tuesday/Wednesday. Deep
layered onshore flow could translate to temperatures a couple of
degrees cooler than Tuesday/Wednesday...featuring highs in the upper
80s/lower 90s east of I-95 and middle 90s well inland.
Long term /Thursday night through Monday/...
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 /19z Monday through Saturday/...
isolated to scattered thunderstorms have developed along and
inland of the seabreeze by late afternoon. Will continue to
monitor convective trends during the next several hours to
determine the need to explicitly mention thunderstorms and rain impacts at either
taf site...yet prefer to remain conservative with the tafs due to
the pulse nature of development and coverage at this time.
Convection will generally push inland and diminish toward the
evening 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 VFR forecast for now.
Extended aviation outlook...low chances of direct impacts from
isolated showers/thunderstorms especially each afternoon/evening
Tuesday through Friday. Otherwise...mainly VFR.
tonight...little change expected in the overall synoptic
pattern...as the marine zones remain on the western periphery of
surface high pressure anchored across the forecast area. Nocturnal
jetting will allow wind speeds to increase up to 15 knots for several
hours as directions become more south to the southwest through the
night. Isolated showers and thunderstorms will again transition
into the waters during the late evening and after midnight.
Tuesday through Friday...overall the pattern will generally
remain the same through midweek as Atlantic high pressure produces
general/persistent southerly flow mainly less than 15 knots.
Deviations will occur mainly as the daily sea breeze turns winds
toward the south/southeast...as nocturnal surges develop and as
the land breeze pushes offshore each morning.
Thursday through Friday...a shifting of the primary surface high
position will result in prevailing east/southeast flow mainly 15 knots
or less...perhaps enhanced by the daily sea breeze circulation.
Seas this week will remain 1-3 feet...highest beyond 20 nm.
The weak pressure pattern/light wind field each morning could
support waterspout development...especially Tuesday and Wednesday.