Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
744 am EDT Sat Jul 4 2015
Atlantic high pressure will prevail as a trough of low pressure
lingers inland through the middle of next week.
Near term /through tonight/...
the persistent long wave trough will remain over the eastern
United States today...as a series of upper level disturbances
round the base of the trough. However...it does appear that
Atlantic ridging may be able to expand a bit westward into the
region as the trough amplifies/deepens to the west. This may be
sufficient to keep the greatest forcing just northwest of the area
with each passing short wave. Meanwhile at the surface...the
region will remain along the western fringes of Atlantic high
pressure centered well offshore while a Piedmont trough persists
northwest of the area.
Overall...the Independence day forecast is not very clear cut in
terms of precipitation potential. Radar mosaics indicate a series
of convective complexes lined up across northern Mississippi...Alabama
and Georgia in association with each short wave disturbance. While
the building ridge should generally steer these complexes just
northwest of the area...high resolution mesoscale models show a
good potential for additional convection to develop along
outflow boundaries propagating southeast due to cold pooling.
This convection could organize into multi-cellular clusters as it
advances southeast across the area this afternoon into this
evening. In addition...convection appears to develop along the
northeast Florida/southern Georgia coast early this afternoon in
an area of enhanced low level convergence...then lift northward
into the area late in the day and this evening. All of this seems
to support increased shower/thunderstorm potential across the area
and have raised probability of precipitation accordingly. The forecast generally shows
rain chances around 50 percent...but this will likely need to be
raised in some areas as trends become better established.
There is the potential for some of the thunderstorms to become
severe...with surface based cape values rising to 2500-3000 j/kg
across much of the area. In addition...downdraft cape values of
750-1000 j/kg and inverted-v soundings support a damaging wind
threat. Given these factors...will add mention to the hazardous
As for temperatures...have lowered highs from previous forecasts
to account for the greater potential for convection and associated
cloud cover. The forecast now indicates highs of 92-94 across
much of the area...except in the upper 80s toward the immediate
Late this evening and overnight...the upper ridge may begin to
break down a bit...with the lowering heights and some passing
upper level short wave disturbances supporting low chances of
showers and thunderstorms...with the greatest coverage near the
coast. Lows should be in the lower to middle 70s across the area.
Short term /Sunday through Tuesday/...
generally a continuation of the same pattern with high pressure over
the Atlantic and low pressure inland. The main challenge is timing
of shortwave troughs aloft which will be the main forcing for
Sunday...close proximity to the upper trough along with deep
moisture will mean above normal rain chances...highest inland closer
to possible organized convection upstream. High temperatures in the
lower 90s...mainly near and west of I-95. Some showers and storms
will linger into Sunday night with lows mainly in the lower to middle
Monday...scattered to possibly numerous showers and thunderstorms
expected again with abundant clouds likely helping keep temperatures
slightly below normal closer to 90. Severe risk appears low given
Tuesday...scattered convection again although probably not quite as
much as the upper trough weakens and high pressure exerts more
influence. Highs a bit warmer closer to normal in the lower 90s away
from the locally cooler coastal areas.
Long term /Tuesday night through Friday/...
medium range guidance is in fairly good agreement through middle week
indicating more high pressure ridging. Another trough could return
late in the week. Should see low rain chances through at least
Thursday...mainly each afternoon/evening. Temperatures will
generally be near normal...with highs in the lower to middle 90s and
lows in the lower to middle 70s.
Aviation /12z Saturday through Wednesday/...
VFR conditions are generally expected to prevail through 12z
Sunday. Scattered showers and thunderstorms should develop later
today into this evening and could impact the terminals. A tempo
group has been included at both kchs and ksav from 20-24z to
indicate the potential for thunderstorms accompanied by flight
restrictions. Gusty/erratic winds are also possible in association
with any thunderstorms that develop. Although this is the most
likely timing for any thunderstorms...a few isolated storms could
occur near kchs earlier in the day and vicinity thunderstorms
is mentioned in the taf beginning at 16z.
Extended aviation outlook...brief flight restrictions are
possible in mainly afternoon/early evening showers and
thunderstorms...with the best chances likely through Tuesday.
today and tonight...the tight surface pressure found between
Atlantic high pressure centered well offshore and a trough of low
pressure northwest of the area will begin to relax some this
morning...resulting in diminishing winds over the waters. Look
for south to southwest winds to prevail with speeds upwards of
around 15 knots through tonight. Seas will may subside a bit as winds
diminish...but overall should range from 2-4 feet...highest in the
outer Georgia waters.
Sunday through Thursday...expect a persistent pattern featuring
Atlantic high pressure and an inland trough of low pressure. A
tightened pressure gradient between these features will maintain
elevated winds/seas through the period but conditions should largely
remain below advisory levels. Seas will be 2-4 feet on average...but
could reach 5 feet at times mostly beyond 20 nm.
Rip currents...the combination of lingering lunar influences...an
easterly swell of 2 feet every 9 seconds and gusty winds will
support a moderate risk for rip currents for the lower South
Carolina beaches today. It appears slightly lower winds from
south along Georgia coast will limit the rip current risk to a
high-end low category.