Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Charleston SC
354 am EDT Friday Jul 25 2014

Synopsis...
an Atlantic ridge of high pressure will remain south of the area
into early next week...with a weak trough of low pressure prevailing
inland. A cold front will approach and cross the region late Monday
and become stationary south and southeast of the area through the
middle of next week.

&&

Near term /through tonight/...
early this morning...convection continues to develop in
association with surface trough and short wave near a the
northernmost fringes of the forecast area. A few of these will
sneak into northern Berkeley and northern Charleston
counties...but by and large it/ll be rainfree everywhere else.
Conditions will remain very warm and extremely muggy with dew
points as high as the middle and upper 70s.

For today...we/ll most likely get some of the active weather we
were looking for yesterday...that which never materialized due to
strong capping. But the cap is weaker today in many areas...and
boundaries and convergence will be more plentiful.

The large scale shows that the local area will lie near the apex
of a somewhat flat trough in the east/se...anchored on each side
by anticyclonic flow. In addition a TUTT low is situated off the
NE tip of Cuba in the Gulf of Mexico. Simultaneously at the
surface there is the persistent but slightly less defined trough
not far to our north/NW...while the sub-tropical Atlantic ridge is
positioned across southern and central Florida.

Nearby convection early this morning will eventually wane...but
this will leave behind a well defined differential heating and low
level convergence boundary. A reformation of convection will occur
in association with this feature by late morning/early
afternoon across our northern zones. At the same time additional
activity will develop along the resultant sea breeze east of the
I-95 corridor...as well as it response to the nearby surface
trough. Throw a little channeled vorticity into the mix and ample
moisture and instability and we look for scattered coverage of
showers/T-storms to occur. Lapse rates are poor...but dew points
will average in the lower and middle 70s...generating sbcapes of
at least 1500-2500 j/kg. That along with some 20-25 knots middle level
winds and boundary interactions could be enough to spur a few
stronger storms with wet microbursts possible. Alignment of
convection will initially be greatest across the Charleston quadrant-
County area...before encompassing much of the central and northern
zones for the middle-late afternoon. Finally activity will arrive
from sea breeze activity south of the Altamaha river late in the
day. Overall 40-50 probability of precipitation are in order...and this will likely be too
conservative and adjustments may become necessary once trends
become better defined.

Low level thickness is a bit lower than yesterday and cloud cover
is higher. This will result in temperatures not quite as extreme as the
middle 90s of yesterday...but given a westerly flow throughout much
of the vertical...we still anticipate lower 90s in many locations
prior to convective development. The diurnal temperature curve will also
need some modifications once this transpires...but we have painted
some lower temperatures for the middle/late afternoon.

Tonight...activity will die off with nocturnal stabilization...and
convection looks to be confined to the Atlantic waters by
midnight. The noticeable slot of dry air shown on water vapor
images will make a push into the forecast area from the northwest and
skies will show a clearing trend with time. This causes temperatures to
drop to the lower and middle 70s...and it may provide a window for
some radiation fog to develop as the atmosphere decouples and
ample low level moisture is available from where it rained. Since
this is not shown in any of the guidance we will not yet add to
the forecast grids.

&&

Short term /Saturday through Monday/...
Saturday and Saturday night...the forecast area will remain
downstream of an expansive upper ridge and anticyclone centered over
the central and southwestern Continental U.S.. at the surface...the pattern
features subtropical high pressure over the open Atlantic and an
inland trough of low pressure. There will be plenty of middle/upper
level dry air...which will keep precipitable waters below 2 inches and down around
1.75 inches. But with the sea breeze and the inland
trough...isolated showers and storms will be possible in the
afternoon/evening hours. It will be a warm day with highs rising
into the middle 90s away from the beaches. Showers and storms should
diminish quickly with the loss of heating and a dry forecast is in
place through the overnight.

Sunday through Monday...the middle/upper level pattern will begin to
quickly amplify especially Sunday night into Monday as a potent
shortwave moves across the Great Lakes region. Height falls Don/T
appear to start across the forecast area until Monday...and there is
actually a notable amount of middle level warming and capping that
develops on Sunday. This should be enough to keep Sunday nearly
dry...though an isolated shower/thunderstorm is possible across
southeast Georgia. The most active period of the period will be
reserved for late Monday as a strong cold front approaches from the
northwest. Showers and thunderstorms are expected to develop along
and ahead of the cold front...likely not impacting much of the area
until late in the day. These showers and storms will be developing
in an area with 500 mb temperatures falling to -8c and middle level lapse
rates as steep as about -7c. There is also increased middle level flow
with the amplifying trough which will result in deep layer shear
approaching 25-30 knots. As such...a few strong to severe storms may
be possible depending on how convection develops upstream of the
area. Probability of precipitation have been increased a bit for Monday...but still favor
inland areas into the evening based on timing the arrival of the
front. It will continue to be quite warm through Monday...especially
with some compressional warming ahead of the front. Widespread middle
90s away from the immediate coast can be expected each day.

&&

Long term /Monday night through Thursday/...
the cold front is prognosticated to move through the area Monday night and
become nearly stationary to the east and south. Behind the
front...expect a noticeable airmass difference as heights fall
overhead within the amplifying eastern Continental U.S. Trough. This setup will
keep temperatures 2-4 degrees climatology through the end of the week. The
best chance for rain...and resulting highest probability of precipitation...will be in the
Monday night time period associated with the front.
Thereafter...probability of precipitation will become more diurnal in nature as the low
level flow turns more east to northeast with time.

&&

Aviation /07z Friday through Tuesday/...
kchs...lingering convection through daybreak with a short wave
will stay pretty far north/NE of the terminal. Thus VFR weather
will prevail into this afternoon. However...convergence will
become enhanced due to the interaction of the sea breeze and
nearby proximity to the inland trough...thus we have introduced
thunderstorms in the vicinity from 19-24z. Depending upon how trends bear out...flight
restrictions are certainly possible during this time. Convection
dies off quickly in the evening and VFR conditions will again
prevail.

Ksav...no impacts at the terminal this morning nor through much
of the upcoming day. Probabilities of convection during the middle
afternoon into the early evening are too low to include any
mention of at this stage...but activity could certainly be added
in later taf issuances. Thus for now prevailing VFR conditions
through 06z Saturday.

Extended aviation outlook...primarily VFR. A cold front is expected
to move through the area late Monday. Brief ceiling and
visibility reductions may be possible with thunderstorms that
develop in the evening hours.

&&

Marine...
today...there is considerable less of a gradient and mixing than
yesterday...with a more relaxed pressure pattern between the
Azores high to the southeast and south...and a nearby trough to the
north/NW. The end result is for gentle to moderate west/SW breezes
and seas no higher than 2-3 feet.

Tonight...the inland trough is even less pronounced...so the
nocturnal surge will be subtle at best. This allows for south/SW
winds less than 15 knots early on to veer around to the SW/west late
as land breeze circulations arrive. Little change in seas.

Of note...some of the storms that either advect offshore or develop
today and tonight could be on the strong side. Thus mariners should
maintain a weather watch and monitor National oceanic and atmospheric administration weather radio all hazards
of appropriate vhf channels.

Saturday through Tuesday...the first portion of the forecast period
will feature high pressure to the east and an inland trough of low
pressure. This will promote a modest southwest flow through Sunday.
Then Sunday night Onward the pressure gradient will tighten in
response to an approaching cold front. This will result in winds
increasing into the 15-20 knots range...and perhaps even marginally
supportive of small craft advisories for portions of the waters
Monday and Monday night. The gradient and resulting winds will then
decrease on Tuesday.

&&

Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...none.

&&

$$

Near term...
short term...bsh
long term...bsh
aviation...
marine...

National Weather Service Glossary of Abbreviations