Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
721 PM EDT Sat Apr 18 2015
a warm front will lift north of the area Sunday morning...and an
upper level disturbance will move through Sunday afternoon and
evening. A cold front will approach the area Monday and will cross
the area Monday night. High pressure will prevail Tuesday through
Thursday. Another cold front could stall over or close to the region
late week into next weekend.
Near term /until 6 am Sunday morning/...
this evening...the risk for isolated to scattered showers will
linger into the early evening hours with weak instability in
place. Convection will be diurnal in nature and mainly driven by
mesoscale boundary collisions. The higher risk for measurable
rainfall will be primary confined to northern portions of
Dorchester...Colleton and Berkeley counties...possibly extending
as far east as Charleston County. Increased probability of precipitation to 40-50 percent
in this area with 20-30 percent chance probability of precipitation elsewhere where
convection will be more sparse. Across the far south...outflow
boundary sparking stronger convection over far southeast
Georgia/northeast Florida between Saint Simons and Jacksonville is
moving steadily north. The bulk of this activity is expected to
remain south of the Altamaha river...however the atmosphere
between Savannah and Darien has yet to be overturned by
convection. Will show 30 percent probability of precipitation across the far south for the
next few hours to take this trend into account.
Overnight...incoming guidance has backed off a bit on the amount
of shower activity that will redevelop late as a developing warm
front lifts north across the area. The latest rap and h3r are
trending with the 18z NAM in keeping the strongest low-level
jetting/isentropic assent west of the area through sunrise...which
should translate to less shower/thunderstorm activity. It is unclear
whether this trend will persist...but there is enough evidence in
the various data sets to lower probability of precipitation slightly for the overnight
period. Will limit probability of precipitation to the chance category all areas and keep
any mention of likely probability of precipitation until after sunrise Sunday. Will
continue to monitor model trends carefully and make further
adjustments with the late evening update after 00z upper air data
are received and analyzed.
Short term /6 am Sunday morning through Tuesday/...
Sunday morning...a warm front will lift north of the region.
Overrunning showers...perhaps accompanied by isolated
thunderstorms...will initially focus in the vicinity of this
boundary. Thus...morning probability of precipitation tapered from likely north/inland to
chance south. After the boundary lifts north...there may be a break
in precipitation coverage for a wide swath of the forecast area late
morning into early afternoon. Then...on the eastern periphery of an
upstream longwave trough...a shortwave trough and and associated
band of dynamic forcing for ascent/band of enhanced warm conveyor
belt moisture will eject east/northeast across the region...within
an environment featuring some degree of destabilization and 0-6 km
bulk shear 35-45 knots. This regime will support multicell
showers/thunderstorms...justifying likely/categorical probability of precipitation. While
the degree of destabilization remains uncertain due to cloud
cover...sufficient instability should develop within this
forced/sheared environment to produce a threat for at least pockets
of severe weather primarily producing damaging wind gusts Sunday
afternoon. Thus...Storm Prediction Center has upgraded to a slight risk for severe
weather Sunday. Also...since precipitable waters will approach 2 inches...enhanced
rainfall rates could produce a localized flood threat if/where
persistent and/or multiple thunderstorms occur.
The shortwave will push off the coast Sunday night...and elevated
probability of precipitation during the evening should decrease below 15 percent as somewhat
drier air/temporary subsidence develop overnight into early Monday.
The longwave trough will drive a surface cold front toward the
region Monday. Ahead of the front...warming/destabilization and
strengthening low/middle level wind fields will develop...and
thunderstorms will likely develop over the region and/or advance
into the region from upstream Monday afternoon/evening. Forecast
uncertainty regarding thunderstorm coverage arises from drying aloft
depicted by model soundings...perhaps limiting coverage of
convection. Thus...capped probability of precipitation at 30-50 percent...highest inland.
Despite the potential limiting influence of this dry air...dynamic
forcing for ascent and kinematic/thermodynamic parameters suggest a
potential for organized severe weather in the region Monday
Monday night...probability of precipitation linger but gradually decrease as the cold front
moves through. The threat for severe weather should wane after dark.
Tuesday...high pressure will push a drier airmass into the
region...accompanied by temperatures mainly 75-80f.
Long term /Tuesday night through Saturday/...
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 weakening cold front will drop into the region Thursday
night or Friday...potentially producing scattered showers. This
front will likely stall over or close to the region....then a
progressive upper shortwave will push through the area around
Saturday...bringing increasing probability of precipitation.
Aviation /00z Sunday through Thursday/...
kchs...shower activity between krbw-kdyb should diminish and
remain west of the terminal through 03z. Ceilings are expected to
lower slowly overnight as a warm front approaches from the south.
Guidance and forecast soundings support IFR ceilings developing after
midnight and remaining in place through sunrise...then slowly
rising as the front lifts north. A band of rain/thunderstorms will likely
accompany the front...so will show MVFR visibilities in -ra from 15-18z.
A risk for thunderstorms will increase late in the taf period...but
timing too uncertain this far out for a mention.
Ksav...shower activity from kssi-kjax should remain well to the
south. Will watch for possible redevelopment along mesoscale
boundary collisions...but at this time no impacts are expected at ksav. A
warm front is expected to develop and lift north overnight/early
Sunday. Best lift/risk for lower ceilings will remain north of the
Savannah River...but enough evidence is there to justify a period
of low-end MVFR ceilings from roughly 07-12z with VFR thereafter.
Could see a brief transition period 04-07z...but timing is
uncertain. While the risk for IFR ceilings is there...better
probabilities will remain north of the terminal. A risk for thunderstorms
will increase late in the taf period...but timing too uncertain
this far out for a mention.
Extended aviation outlook...flight restrictions likely Sunday afternoon
into Sunday night...especially within showers/thunderstorms. Outside
scattered showers/thunderstorms as a cold front approaches then
moves through the region...mainly VFR Monday and Monday night...then
VFR through middle week.
tonight...the setup will continue to support quiet
conditions across the local waters. A subtle warm front will lift
northward and a prevailing east-southeast to southeast flow will
gradually become more southerly late. Winds will be less than 10 knots
with seas 2-3 feet.
A warm front will lift north of the waters Sunday...followed by
intensifying south/southwest flow. The probability for Small Craft
Advisory conditions will increase especially Sunday night until a
cold front arrives Monday night...especially close to the coast
including Charleston Harbor.
The potential exists for thunderstorms with strong winds over the
waters Sunday afternoon/evening and again Monday afternoon/evening.
After the cold front passes...a quieter pattern will develop Tuesday
until late week as high pressure prevails.
have again opted against a coastal Flood Advisory. Winds are
onshore with a seabreeze setting up...but are still weak.
Astronomical influences are likely at their peak but current tidal
departures are only between 0.2-0.3 feet...and should decrease
further as high tide approaches.
There is a chance that the high tide could reach or slightly exceed
7.0 feet MLLW on Charleston Harbor Sunday and Monday evenings.