Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
659 PM EST Sat Dec 20 2014
unsettled weather is expected through Tuesday as several disturbances
move over the region. A strong cold front will approach late Tuesday
night...then shift through the area on Wednesday. Dry high pressure
will then settle over the southeast through late week...with a cold
front expected next weekend.
Near term /until 6 am Sunday morning/...
Saturday evening...satellite imagery and area radar trends
depicted another wave and associated expanding coverage of light
rain ejecting east-northeast toward the area. Accordingly...raised probability of precipitation to
high chance especially along/S of I-16 and along the immediate
coast...tapered to chance/slight chance farther N/NW. Recent high
resolution guidance...especially the hrrr...suggests that light
rain could become widespread and that higher probability of precipitation could be
required within subsequent forecasts.
Otherwise...per ongoing/expected trends slowed the rate of
temperature decrease tonight. Within The Wedge regime...stratus
could produce some drizzle and could lower toward the surface to
support at least patchy fog overnight. Persistent surface winds
should limit the potential for significant dense fog
overnight...will continue to assess.
Short term /6 am Sunday morning through Tuesday/...
Sunday...a weak southern stream middle level perturbation moving
through the lower MS valley from tonight will cross the area during
the morning hours and reinforces the pre-existing stationary front
over the Atlantic to the east/southeast of the Gulf Stream. It also keeps
the inland cool high pressure wedge anchored in place as it extends
south of the forecast area. The impulse aloft also triggers weak
cyclogenesis off the southeast coast some 250 nm east of Savannah as it
moves little throughout the day. Isentropic ascent will again be the
main factor for generating rainfall...as it increases in magnitude
during the afternoon and evening hours...and probability of precipitation accordingly will
be on the increase as well during the second half of the day. We/ll
start the period with a little light rain...drizzle and/or fog
before probability of precipitation climb into the 30-50 percentile in the afternoon.
Greatest probabilities will be over southeast Georgia and especially south of
I-16 during that time. Overcast skies and a continued feed of air
from the north/NE within the low level wedge will keep temperatures down
mainly into the 50s for maximum temperatures.
Sunday night into Monday...another and slightly stronger short wave
arrives from the Gulf of Mexico and forces the Atlantic wave to
deepen a few millibars as it lifts NE roughly along the Gulf Stream
to a position off Cape Hatteras by 00z Tuesday. This pattern
reinforces The Wedge inland...which in turn allows for a further
increase in isentropic ascent. That along with forcing from the
upper jet over the southeast corner of the country and convergence
associated with the nearby Atlantic low will cause coverage of rain
to expand into at least the 70-80 percent range Sunday
night...before coverage diminishes from west to EST on Monday as the
surface low pulls away. Precipitable waters of 1.3 to 1.4 inches and the flow
aloft opening more out of the Gulf of Mexico supports at least 1/4
to 1/2 inch amounts of rain. In addition...steeper middle level lapse
rates and showalter indices dropping to near zero could support a
little elevated convection. Low stratus decks will also support some
reduction in surface visibilities Sunday night into Monday morning
and we have added patchy fog to the forecast. There is only a small
5-10 degree temperature swing from daytime highs Sunday to nighttime lows
Sunday night. Then a fairly large north/northwest to south/southeast temperature gradient
Monday with highs near or slightly below normal.
Monday night and Tuesday...yet another quick moving southern stream
short wave arrives late Monday night and Tuesday from the Gulf of
Mexico. Further upstream a more pronounced short wave starts to dig
through the central states...which in turn amplifies the large scale
pattern across the Continental U.S.. this backs our flow aloft further to the
SW and opens the door for a feed of even deeper and tropical-like
moisture to move in. Precipitable waters climb to near 2 Standard deviations above
normal...around 1.6 to 1.7 inches so yet another round of rainfall
will occur. Probability of precipitation will be in the chance to likely range Monday night
then should peak in the categorical range Tuesday. Additional quantitative precipitation forecast
will be at least 1/2 to 3/4 inches...with potential higher amounts
in this pattern. The short wave in the plains will also trigger
cyclogenesis near the arklatex region by Tuesday...which in turn
attempts to lift our coastal warm front northward. Not sure how this
progression will actual pan out given that the inland wedge will be
reluctant to give up its hold. This will allow for warm advection to
boost temperatures at least into the lower or middle 60s...perhaps even
higher near the Altamaha. This in turn creates some surface based
instability on the order of 500-800 j/kg in sync with steep middle
level lapse rates over our coastal zones and south of I-16. It/S in
these areas where we have isolated T-storms mentioned in the latest
Long term /Tuesday night through Saturday/...
Tuesday night temperatures will remain rather mild ahead of a cold front to
the west. We could see a few thunderstorms before the front shifts
offshore Wednesday afternoon. Gusty winds should also accompany the
front as strong cold air advection occurs behind it. A much
quieter/dry period of weather is then anticipated as high pressure
builds over the region behind the front starting Wednesday night.
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 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. Another
cold front could approach next weekend...bringing the next chance of
precipitation to the area.
Aviation /00z Sunday through Thursday/...
do not expected improvement beyond MVFR ceilings tonight and
Sunday...and IFR/LIFR ceilings will prevail much of the time this
evening into Sunday morning. Prevalence of light rain remains
somewhat uncertain as additional waves push east/NE into the region
overnight and again Sunday. Highest probability of precipitation should occur at ksav
tonight...although rain could expand into the kchs area. Even if a
break in precipitation develops late tonight into Sunday morning...
another round of rain should commence at both sites Sunday
Extended aviation outlook...periods of MVFR or lower conditions
are likely with rain/showers and low stratus through early Tuesday
while a wedge of high pressure persists inland and waves of low
pressure lift northeast across the nearby Atlantic. Additional
flight restrictions appear likely Tuesday afternoon through
Wednesday afternoon as a strong cold front approaches from the
west with showers and possibly thunderstorms. Windy conditions
should also occur near the time of frontal passage on Wednesday.
the tight northeast gradient will persist through tonight as high
pressure continues to wedge down the eastern Seaboard and low
pressure spins off the SC coast. A steady 15-20 knots wind expected
over most of the marine area overnight with seas 3-5 feet...highest
beyond 20 nm.
Sunday...a pinched pressure gradient between developing low pressure
off the Georgia coast near the Gulf Stream and a strong inland wedge of
high pressure will persist. This will result in north/NE winds of at
least 15-20 knots...with gusts at times to near 25 knots. If the gusts
become frequent enough we/d need a Small Craft Advisory...which
we/ll defer to future shifts to see if we indeed should hoist the
advisory headline. The fetch is favorable enough for seas to be as
high as 3-5 feet...greatest over amz374 and the easternmost parts of
Sunday night and Monday...the Atlantic wave deepens some as it lifts
NE along the Gulf Stream...while the inland wedge holds firm. The
gradient though does slacken some of the low shifts north of the
local waters...and winds and seas will be less and below any
Monday night and Tuesday...the inland wedge is still in place during
much of the period...but it does attempt to erode by Tuesday night
as the coastal warm front starts to lift north/NW. Until this
transition though our winds will struggle to veer around...as speeds
hold near or below 15 knots and seas are limited to 3 or 4 feet.
Tuesday night and Wednesday...finally the coastal warm front will
shift north of the local waters Tuesday night as a strong cold front
moves through Wednesday. Warm advection and the resulting
marine-layering effects will limit the strength of winds ahead of
the front...but small craft advisories are certainly possible.
Behind the front the combination of cold advection and strong
isallobaric pressure rises may support gales...or at the least small
craft advisories. Mariners are also advised that some stronger
T-storms are possible in advance of the cold front...capable of
producing gusty winds and lightning strikes.
Thursday...conditions will improve over all waters as high pressure
becomes centered just south of the area. Winds/seas should
eventually lower below Small Craft Advisory levels for all marine
zones as cold advection wanes and the gradient relaxes.
Sea fog...low stratus decks could equate to reduced visibilities
into Tuesday. Actual sea fog may have a window to develop as
slightly warmer air arrives atop middle/upper 50 shelf water temperatures
within a south and southwest flow late Tuesday into Tuesday night
prior to winds increasing enough to scour it out Wednesday. Night.
although there are no immediate concerns in regards to tides...given
the perigean Spring tides early next week...there is an increasing
risk for shallow coastal flooding Monday and Tuesday around the time
of the morning high tide. This situation might be further exacerbated
due to steady rains prior to and during the high tidal cycle.