Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
712 am EST Friday Dec 19 2014
high pressure will prevail today. Unsettled weather is expected later
tonight through Tuesday night as several disturbances shift over the
region. A cold front will eventually shift through the area on Wednesday
with dryer conditions to follow through late week.
Near term /through tonight/...
weak middle level ridging ahead and general subsidence is on tap
today across the forecast area...along with the arrival of a weak
surface trough the eastern Carolinas this morning. This precedes a
progressive short wave trough ejecting through the Southern Plains
which will tap some moisture from the Gulf of Mexico and spread a
mix of convective and stratiform rains across the lower Gulf Coast
region. Periods of mainly high clouds anticipated today with mild
temperatures in the 60s all areas this afternoon. 00z h3r...NAM...and Gem
cloud field forecasts indicated thicker high clouds around daybreak...
thinning late morning into middle afternoon before expanding later in
the day. We maintained strong persistence with our previous maximum
temperature forecast given the low level thickness forecasts and mainly thin
Very fast upper flow across the southeast states continues to make
for a tricky forecast introducing probability of precipitation tonight. Although models
maintain similar quantitative precipitation forecast timing from previous runs... a somewhat quicker
progression of rains is possible and we had to make only minor
adjustments from our previous thinking given consensus of condensation
deficit forecasts and initial arrival of upper forcing. Sprinkles and
patchy light rains may break out in the evening hours in a few areas
to the west of I-95 but much higher confidence rains expand after
06z with categorical probability of precipitation late as sunrise arrives Saturday. Overnight
quantitative precipitation forecast should mainly be light with vertical profiles only moistening
up significantly later in the night. Clouds and rain will help to
anchor a weak wedge pattern at the surface with lows 40-45 north
and middle 40s S of I-16.
Short term /Saturday through Monday/...
Saturday...a wet day remains in the forecast as isentropic ascent
lifts moisture over a wedge of high pressure at the surface. Activity
will likely be ongoing into at least early afternoon while forcing
persists from a 500 mb shortwave passing over the southeast and a strong
upper level jet positioned over much of the southern United States.
However...the middle/upper pattern suggests a quickly paced event that
offers less precipitation accumulation...with perhaps just a few tenths of
inch over most areas. Precipitation should end from west to east during the
evening as the deepest moisture shifts offshore. Overall temperatures will
be cooler within The Wedge and under cloudy skies. Expect temperatures to
peak in the lower 50s in southeast South Carolina and middle/upper 50s
in southeast Georgia. We could see a few locations reach 60 in our
most southern zones.
Saturday night...we should see a period of dry weather...but
moisture will begin increasing from the south ahead of another wave
of low pressure shifting/developing over the northeast Gulf of
Mexico. Low temperatures should range in the low/middle 40s...coolest over
inland southeast South Carolina. Temperatures should be slightly warmer
along the immediate coast.
Sunday...the day should start off mostly dry...but models are in
general agreement that moisture will build from the south as an area
of low pressure over the northeast Gulf of Mexico slowly shifts over
the southeastern United States Sunday afternoon and night. Have
therefore indicated 20-30 probability of precipitation in southeast Georgia spreading into
southeast South Carolina Sunday evening and then increasing upwards
to around 70 probability of precipitation Sunday night for all areas as the low tracks over
or immediately along the southeast coast. Greatest chances of precipitation
appear to be over southeast Georgia and along the South Carolina
coast at this time...especially as a series of 500 mb shortwaves shift
over the area Sunday night. Overall high temperatures will be dependent on
the track of the surface low...but should reach the middle/upper 50s over
most locations away from the coast. Temperatures could peak in the lower
60s along coastal areas in southeast Georgia. Overnight lows should
range in the middle/upper 40s away from the coast...and in the low 50s
along the coast.
Monday...showers will likely be ongoing to start off the day as an
area of low pressure continues to make way to the northeast. Most
precipitation should shift offshore by the afternoon...but chances of
showers will remain in the forecast through much of the evening
while abundant low-level moisture lags behind the low to the
northeast. Temperatures will be dependent on what is left of The Wedge
inland...but should be slightly warmer...in the upper 50s inland to
lower 60s in southeast Georgia and along the South Carolina coast.
Long term /Monday night through Thursday/...
Monday night into Tuesday night...the pattern will remain unsettled
as ahead of cold front that eventually pushes through the southeast
early Wednesday. At this time...models suggest chances of precipitation
increasing Monday night into Tuesday as a warm front tries to lift
north into a weakening wedge of high pressure at the surface. A period
of warming should also occur within a southerly flow ahead of the
approaching cold front to the west. Have therefore introduced
showers for all areas...rather than rain. We could see enough
instability to include a few thunderstorms in the forecast should
trends persist...mainly in southeast Georgia Tuesday afternoon and
evening. Overall high temperatures should range in the low/middle 60s for most
areas on Tuesday...even with cloud cover present. Upper 60s are
expected over southeast Georgia. Chances of precipitation should quickly
diminish as the cold front begins to shift through the area late
Tuesday night. Tuesday night lows should range in the upper 40s to
around 50 over most areas.
Wednesday through Friday...a much quieter/dry period of weather is
anticipated as high pressure builds over the region behind a cold
front shifting offshore early Wednesday. Temperatures will be slow to warm
through Thursday...until high pressure becomes more centered over
the western Atlantic. Overall high temperatures should range in the
middle/upper 50s Wednesday and Thursday...before southerly flow returns
to the area along the western edge of high pressure on Friday. Temperatures
should peak into the low/middle 60s by late week.
Aviation /12z Friday through Tuesday/...
shallow ground fog at kchs at dawn...dissipating by middle morning. VFR
at ksav and kchs likely through 12z Saturday. Ceilings should begin
to lower after midnight but latest forecast soundings suggest ceilings
remain VFR until 12z Saturday although some lower clouds are certainly
possible if rains develop even sooner than expected. As of the 12z
cycle...these rains are expected to break out after midnight
across the coastal corridor but mainly remain relatively light.
Extended aviation outlook...periods of MVFR or lower conditions
appear likely with rain/showers Saturday through Tuesday while a
wedge of high pressure persists inland and several areas of low
pressure shift along the southeast coast.
another day of light winds and flat seas prior to some slowly
building NE flow tonight as a secondary low pressure wave develops
along the west wall of the Gulf Stream early Saturday morning. We
will see NE winds increase closer to 15 knots by 12z Saturday off the
Charleston County coast but lesser speeds further southward given
the angle of the developing pinching gradients. 1-2 feet seas most
of today will build to 2-3 feet tonight.
Saturday through Wednesday...winds and seas are expected to remain
below Small Craft Advisory levels this weekend as a wedge of high
pressure strengthens inland and a low pressure system shifts from the
northeast Gulf of Mexico along the southeast coast. Periods of
rain/showers will continue over the waters into early next week
until a cold front shifts offshore Wednesday. Strong winds will
accompany the front as it shifts offshore early Wednesday...with at
least Small Craft Advisory conditions expected for all waters. A
gale watch could be eventually needed for a portion of the waters on
Wednesday morning and afternoon as winds gust to 30-35 knots. Seas will
also build upwards to 6-8 feet Wednesday afternoon before slowly
improving Wednesday night.