Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
100 am EDT Sat Aug 30 2014
Atlantic high pressure will expand westward through the weekend
and prevail through at least the middle of next week. A cold
front could approach or advance through the region late next week.
Near term /until 6 am this morning/...
mainly clear skies overnight with a few patches of stratocu...
The air mass has certainly moistened since yesterday and lows
tonight won't be as low as recent nights as a result. Look for
upper 60s and lower 70s inland...and middle 70s near the intra-
coastal and on the barrier islands.
Confidence on fog development remains far from ideal given the
recent spell of dry conditions...the warm ground and many areas
struggling to achieve their cross-over temperature. But favorable
condensation pressure deficits...fog stability indices down in the
teens in many locales...persistence from Friday morning and the
increase in low level moisture supports at least patchy fog inland
from US-17. We can/T rule out a little dense pockets of fog along
the northwest tier...but areal extent is not great enough to include at
Short term /6 am this morning through Monday/...
Saturday through Monday...the region will reside on the periphery of
surface high pressure centered over the Atlantic and beneath deep
layered high pressure aloft. Moisture will slowly nut steadily
increase through the period...beginning with low levels but
eventually through the lower troposphere. As a result...the
potential for diurnal thunderstorms will increase through the
Holiday weekend...and heat index values will return to more typical
values for late August/early September.
Saturday morning...a repeat of the Friday morning scenario is
possible...with enhanced low level/marine moisture possibly
supporting a few shallow showers over coastal counties/adjacent
coastal waters. Then...the focus for afternoon showers/thunderstorms
will shift into areas along/west of I-95. Given the cap still
evident in model soundings...capped probability of precipitation at slight chance in this
Saturday night...the focus for isolated showers/thunderstorms will
shift over coastal waters...perhaps brushing coastal counties. Of
interest...the 29/12z NAM depicts inland convection persisting well
into the overnight hours. While we cannot completely discount this
solution...per typical diurnal trends probability of precipitation taper to less than 15
percent inland with no mention of precipitation during the overnight
hours. Also of note...light winds/low level moisture could support
patchy fog late Saturday night into early Sunday...but forecast
confidence is insufficient to warrant a mention within public
Sunday through Monday...precipitable waters increase to around 2 inches and maximum
MLCAPES could potentially surge to at least 2500-3500 j/kg...
supported by high temperatures in the lower to middle 90s and dewpoints in the
lower to middle 70s. This regime should support diurnal showers/
thunderstorms perhaps accompanied by a threat for locally heavy rain
and a low-end threat for isolated damaging wind gusts. As the upper
ridge begins to flatten Monday...slightly stronger low-middle level
winds could support multicell thunderstorms and a slightly greater
chance for damaging wind gusts.
Sunday night...the focus for showers/thunderstorms should again
shift over coastal waters.
Due to the subtle cap produced by the upper ridge/depicted by model
soundings...capped highest probability of precipitation at 30-40 percent inland Sunday and
Monday afternoons. Eventually...locally greater probability of precipitation could be
Long term /Monday night through Friday/...
through the long term period...the large upper ridge will continue
to influence the area. The ridge will undergo a transition in the
middle/late week time period as it retrogrades back to the west across
the lower Mississippi vally in response to a trough moving across
central and eastern Canada. The pattern in the low levels will still
feature the influence of the Atlantic high...though there are
indications that an inland trough of low pressure will develop in
the Tuesday/Wednesday time frame. As this occurs...the coverage of
afternoon/evening showers and thunderstorms may tick upward a bit.
The forecast features slight chance to chance probability of precipitation each afternoon
and evening. Temperatures will remain above normal with the ridge
Aviation /06z Saturday through Wednesday/...
the vast majority of the valid taf cycle at kchs and ksav will be
VFR through 06z Sunday. Could see a few hours of low stratus
and/or fog with sub-VFR conditions at either or both terminals
from about 08-13z Saturday. But the worst conditions look to stay
inland from the airstrips with a southeast and south synoptic flow.
Isolated rain showers/thunderstorms and rain may pose a brief concern this afternoon in
association with the inland penetrating sea breeze. But there is
no justification to include anything at this time.
Extended aviation outlook...low chances of direct impacts from
isolated showers/thunderstorms this weekend through early next
week. Also...low chances for early morning fog/stratus through
early next week. Otherwise...mainly VFR.
overnight...a ridge of Atlantic high pressure stretched across the
coastal waters will become absorbed by a stronger 1027 mb high
moving off New England late. This will allow for some fluctuations
in southeast and south winds through the night...but on average no higher
than about 10 or 15 knots. Seas will be no more than 2 or 3 feet...mainly
in easterly swells.
Saturday through Wednesday...deep layered high pressure will prevail.
Southeast winds Saturday will veer toward the S/SW Saturday night through
Wednesday. Winds will generally remain less than 15 knots.
However...local winds could approach 20 knots due to the daily sea
breeze circulation and a nocturnal surge depicted by models Sunday
night. Seas will range from 1-2 feet within 20 nm and 2-3 feet beyond.
Sunday night...3 feet seas could push into nearshore waters and seas
as high as 4-5 feet could briefly develop beyond 20 nm. However...odds
are high that small craft advisories will not be required through