Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
748 am EDT Sat Apr 18 2015
a weak pressure pattern will blanket the region today. A warm
front will develop and lift north of the area tonight. An upper
level disturbance will move through on Sunday...then a cold front
will cross the area Monday night. High pressure will prevail
Tuesday through Thursday...then a cold front will sweep through
Thursday night or Friday. Another storm system may affect the
southeast United States next weekend.
Near term /through tonight/...
today a rather diffuse pattern is setting up with the offshore
low pressure out of the picture and the residual surface wedge over
the Carolinas a much more ill-defined feature. The middle level short
wave responsible for the early morning rains is forecast to dampen
and shift offshore to the NE. A weak middle level vorticity axis is
expected to linger across northern South Carolina today while a
subtle short wave ridge at 500 mb builds into the region later
today. Models indicate a very moist air mass will blanket the
forecast area this morning with deeper moisture moving offshore
after early this afternoon. Models generate scattered shallow
convective rains today as the sea breeze develops but confidence
low on both coverage and timing as a slightly drier deep layered
regime works in later today. We kept chance probability of precipitation mainly in the 30
percent range for the afternoon hours.
Temperatures are tricky as low level thickness values again very warm in
the middle 80s and big questions on cloud cover trends this afternoon.
We think enough breaks in the clouds will send temperatures into at least
the lower 80s over southeast Georgia and near 80 into southeast South Carolina.
The remnant wedge may pin temperatures back in the middle 70s over our far
inland tier bordering the South Carolina midlands where clouds
tend to hold in steady under the vorticity axis in the remnant surface
wedge. There could be an isolated thunderstorm somewhere to the south of
I-16 today but that would only occur if more breaks in the clouds
develop. Middle level instability is quite poor.
A developing warm front with some classic looking model features
takes shape tonight and showery weather will redevelop overnight
as higher precipitable waters shift in from the Gulf and middle and upper flow
backs significantly to our west as an amplified upper trough ejects
into the plains states. Probability of precipitation are tricky given timing uncertainties
and varying model quantitative precipitation forecast depictions but the highest rain chances will
occur after midnight and especially nearing daybreak on Sunday.
Middle level instability also increases late and there could be a few
thunderstorms that develop across southeast Georgia late. Very warm and moist
conditions overnight may lead to some fog formation but more
definitely very low stratus clouds.
Short term /Sunday through Tuesday/...
a longwave trough over the central United States will deepen as it
shifts to the east on Sunday...ejecting several pronounced
shortwaves into the local area. A warm front will be lifting north
during the morning...with deepening and strengthening southerly flow
advecting precipitable waters in excess of 1.90" into southeast GA/SC. In the
morning the precipitation will be primarily driven by overrunning as warm
conveyor belt moisture pushes across the high pressure wedge over
inland SC. Later in the day the Mode will switch to a more
convective regime as the entire forecast area resides in the warm
sector and moderate surface-based instability /1000-1500 j/kg cape/
develops. Given the extensive cloud cover and weak lapse rates...
severe weather does not look like a significant concern on Sunday.
However we maintained categorical probability of precipitation for the day...with probability of precipitation
trending down from west to east Sunday night as the best dynamical
forcing shifts east.
Monday looks interesting...mainly in regard to thunderstorm
potential. The longwave trough axis remains to our west with another
series of weak shortwaves lifting northeast into the area. The main
change from Sunday will be significantly drier air moving in between
400 and 850 mb. This middle-level dry advection will allow for
prodigious breaks in the clouds...helping push temperatures into the middle
80s. Despite the middle-level dry air...the boundary layer will remain
quite moist due to continued southerly flow. With surface dewpoints
remaining in the middle 60s...surface-based convective available potential energy are expected to
increase to at least 1500-2000 j/kg during the afternoon with lifted
indices ranging from -6 to -9c. Low and middle-level lapse rates are
also pretty impressive and 0-6 km bulk shear increases to 40-50 knots
during the afternoon. The latest model consensus has the main cold
front moving through Monday evening though a pre-frontal trough will
likely trigger initial convection during the day. Although shear
values will be quite high...the unidirectional nature of the wind
fields suggest the potential for multi-cell thunderstorms though
some supercells cannot be ruled out. The advent of the dry air aloft
will yield primarily a damaging wind threat though the lapse rates
will also support some hail potential. It is not surprising that the
latest day 3 outlook from the Storm Prediction Center places our
entire forecast area in the slight risk. We plan to include mention
of severe weather potential in our local hazardous weather outlook.
The front should be offshore by late Monday night with much drier
low-level air filtering in. On Tuesday we expect mostly sunny skies
and dewpoints dropping into the low/middle 50s. A downslope wind
component will help offset the cold air advection and produce high
temperatures of 79-82f.
Long term /Tuesday night through Friday/...
a relatively dry and seasonable pattern will be in place Tuesday
night through Thursday with zonal flow aloft and weak high pressure
at the surface. Highs will climb into the lower 80s with lows in the
50s/60s. A weak cold front will drop through Thursday night or
Friday...potentially producing scattered showers. Then Saturday a
progressive upper shortwave will push through the area...bringing
better rain chances and increasing cloud cover.
Aviation /12z Saturday through Wednesday/...
ksav...areas of IFR/LIFR ceilings intermingled with occasional foggy
conditions in the wake of overnight rains. Soundings indicate
MVFR ceilings may linger into late morning with a chance of showers
but the afternoon should be VFR for the most part but still not
totally ruling out a shower. Tonight...confidence is low with
increasing potential for low clouds and fog...then a chance of
Kchs...a short wave trough in the middle levels tapped deep moisture
advection overnight producing light rains. Scattered light rains
could linger through the morning under a region of broad lift
aloft. This afternoon...conditions should become VFR per forecast
soundings. Tonight...deteriorating conditions once again with a
warm front developing to the south...deep moisture...rain chances
and increasing potential for IFR ceilings after midnight.
Extended aviation outlook...MVFR ceilings and/or visibilities expected
Sunday and Sunday night with showers and isolated thunderstorms associated
with an upper disturbance. Occasional IFR conditions possible. Mainly
VFR Monday and Monday night...except for occasional reductions in
ceiling and visibility with scattered showers/storms with a cold
front. VFR then expected Tuesday into Wednesday.
no significant concerns with winds and seas through tonight as a
weak pressure pattern gives way to mostly light onshore flow by tonight.
Seas 2 to 3 feet on average. Visibility problems are not expected this
morning but will have to watch for low ceilings causing some visibility
obscurations in some local intra-coastal waterways later tonight.
A warm front lifts north of the waters Sunday with an increasing
southerly flow until a cold front sweeps through Monday night. Winds
will approach 20 knots late in the day Monday as a low-level jet moves
through ahead of the cold front. A quieter pattern develops Tuesday
through late week as high pressure prevails.
although we are in a relatively high tide cycle...surface winds now
shifting to the south do not support much in the way of positive
tidal anomaly. Since tides did not reach 7.0 feet MLLW at Charleston
on Friday evening...the chances for tidal flooding this evening
is relatively low.