Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
114 am EDT Friday Sep 4 2015
a weak pressure pattern will persist today. A cold front will
cross the region from north to south Saturday...and high pressure
will build into the area from the north. The front will become
stationary south of the region...and low pressure could develop
off the southeast coast. The region will then remain situated
between inland high pressure and offshore low pressure until the
middle of next week.
Near term /until 6 am this morning/...
overnight...dry weather is expected to prevail with temperatures only
falling to the middle 70s most areas. The main concern will be the
possibility of some patchy fog and/or shallow ground fog late
tonight as winds become calm and some breaks in cloud cover occur.
Have maintained patchy fog for most areas away from the coast
late tonight into the early morning hours where surface temperatures are
cooler and dewpoints remain in the middle 70s. However...dense fog is
not anticipated at this time.
Short term /6 am this morning through Sunday/...
Friday...a weak pressure pattern will exist across the local area in
advance of a backdoor cold front from off the Delaware-Maryland-Virginia to Minnesota.
While aloft we find a northwest flow sandwiched between a broad and deep
trough in the western Atlantic and an amplified ridge from the lower
MS valley to the upper Midwest. With sufficient insolation and 850
mb temperatures that are forecast to reach 19-20c...or around 2 Standard
deviations above normal...will lead to an unseasonably warm day. Not
anywhere near record highs...but still peaking in the lower and
middle 90s inland from the intra-coastal. Associated heat indices
will climb to 100-105f...but below our heat advisory criteria. A
strong cap and the lack of any significant forcing suggest that the
chances for convection isn/T more than 20-30 percent...driven mainly
by a weak afternoon sea breeze and in close proximity to a subtle
trough inland. Low level lapse rates are rather high for early
September and there is a fairly dry middle level environment which
could spur a couple of isolated strong T-storms.
Friday night...convection will wane in the evening as nocturnal
influences takes over. Then the upstream backdoor cold front
approaches from the north toward the Santee river close to daybreak
Saturday. This may allow for a limited overnight risk for showers
and storms...mainly northern zones. Temperatures will remain above normal
in advance of this feature. If cloud cover thins out enough...given
favorable condensation suggests an inland fog risk. We will re-
evaluate in later forecast packages.
Saturday through Sunday...a Rex block pattern will develop
aloft...with an unusual east/southeast to west/northwest aligned trough developing
aloft nearby Saturday...then into the area by Sunday. To the north
we find a deep anticyclone over the NE states. This pattern will
send the backdoor cold front into and through our area
Saturday...before it stalls not far to the south on Sunday. A high
pressure wedge will develop in wake of the cold front as it builds
south/SW from the NE and New England. Slightly cooler air will
arrive in response to this transition...with middle and upper 80s more
common for highs and even some upper 60s sneaking into the northwest tier
for lows Saturday night. Short waves embedded within the cyclonic
flow aloft will ride into and through the region periodically
through the weekend. That will combine with decent moisture and the
forcing from the cold front and a backdoor sea breeze ion Saturday
to trigger scattered probability of precipitation. While we lose the forcing from the cold
front on Sunday...we gain better low level convergence from off the
Atlantic within a deep easterly flow from off the Atlantic. This
will lead to mostly low topped and scattered convection areawide.
There are indications of a little better shear over southern zones
Saturday...and if that comes to fruition and the front is slower in
arriving this could allow for a risk of a few strong or marginally
Meanwhile in the SW Atlantic...short wave energy associated with
the remnants of Erika may allow for a broad and weak region of low
pressure to form the NE of the Bahamas Saturday...before it lifts
northward on Sunday.
Long term /Sunday night through Thursday/...
a blocking pattern will maintain surface high pressure to the
north...a stationary front weakening to a trough of low pressure
south/southeast of the area...a broad area of surface low pressure
off the southeast coast and broad low pressure aloft over the
southeast. Labor Day into Tuesday...long fetch onshore flow enhanced
by surface features and the upper low/pool of vorticity aloft should
provide sufficient moisture/instability to support at least
scattered showers/isolated thunderstorms during the daytime hours
across the region...and probability of precipitation are expected to refocus closer to the
coast and across adjacent coastal waters during the nighttime hours.
Capped maximum probability of precipitation at 50 percent through the period...although
locally wetter periods/higher probability of precipitation are possible especially near the
coast. Temperatures in the lower 80s on the beaches and middle 80s
inland are expected Labor Day...then temperatures should moderate
into the middle/upper 80s inland Tuesday. By the middle of next week
through the end of this forecast period...inland high pressure and
offshore low pressure will weaken...the upper low/trough axis will
retrograde west of the region and Atlantic high pressure will expend
into the region from the east. This scenario should support a trend
toward warmer/above normal temperatures and slightly lower probability of precipitation.
Aviation /06z Friday through Tuesday/...
VFR conditions are expected at the kchs and ksav terminals
through 06z Saturday. However...there is a slight chance of late
day or evening showers or thunderstorms. Probs too low for any inclusions
on the 06z taf cycle.
Extended aviation outlook...low to moderate potential for periodic
flight restrictions in mainly diurnal rain showers/thunderstorms and rain and/or late
night/early morning stratus into early next week. Otherwise
generally VFR conditions will prevail.
overnight...light winds will remain at or below 10 kts. Seas will
range between 1-2 feet.
Friday...a lax pressure pattern will prevail...allowing a weak
afternoon sea breeze to kick in as winds clock around from west/northwest
in the morning to east/southeast in the afternoon. Seas won't be any higher
than 1 or 2 feet.
Friday night and Saturday...broad and weak low pressure begins to
form to the north/NE of the Bahamas as a backdoor cold front arrives
from the north late Friday night...pushing into and through the area
Saturday. Behind the front an inland high pressure wedge will
develop...and that along with a backdoor type sea breeze will cause
modest NE/east pinching to occur. Not enough for any advisory
conditions...but enough to boost winds up to around 15 knots and
gusty with seas as high as 3 or perhaps 4 feet.
Sunday through Tuesday...decent pinching will persist at the start
of the period between broad low pressure to the east/southeast and an
inland wedge reluctant to relax it/S hold over the southeast. Gradually
though the gradient starts to slacken as the Atlantic low heads
northward and the inland high weakens. Winds and seas look to stay
below any advisory thresholds...unless the pinching is stronger than
now anticipated for Sunday. We/ll need to maintain a close watch for
back swell arriving in response to the broad low pressure system in
Rip currents...an elevated risk for rip currents could develop over
the upcoming Holiday weekend as a moderate NE to east wind develops
and small swells could start impacting the beaches beginning
Waterspouts...light wind fields and sufficient atmospheric
conditions may support a risk for waterspouts Friday and Saturday
mornings along any convergence lines that develop.