Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
1010 PM EST Friday Jan 30 2015
high pressure will build into the region from the northwest
through Saturday. As the high moves into the Atlantic by Sunday a
weak coastal trough will develop offshore and lift north ahead of
a cold front passing over the area Monday. High pressure will then
return to the region by Tuesday...before shifting offshore ahead
of another storm system approaching from the Gulf of Mexico by
late in the week.
Near term /until 6 am Saturday morning/...
high pressure will build in from the northwest overnight. Middle
evening surface observations indicate our colder temperatures to this
point across inland southeast Georgia where it decoupled very quickly
and winds were calm. Low level cold air advection underway across
eastern South Carolina and light breezes noted across the Charleston
tri-County area at this hour where boundary layer temperatures were falling
at a good clip. We tweaked some hourly temperatures on the update package
but did not make any significant low temperature changes per latest trends.
Skies will remain mainly clear with some high level thin clouds
passing through...especially across S Georgia.
Short term /6 am Saturday morning through Monday/...
Saturday...a split flow regime will prevail aloft across the
Continental U.S....with deep layered ridging to develop atop the local area.
Cool Canadian high pressure covering much of the eastern half of
the nation and centered in the Ohio Valley will shift toward the
Atlantic coast toward nightfall. Aside from some cirrus clouds
arriving within the northwest flow aloft in the afternoon...the air mass
is void of moisture. Considerable insolation...rising heights
aloft and a modifying air mass will negate some of the cooler air
coming in off the Atlantic as an onshore synoptic flow develops.
We stayed close to the low level thickness forecast and will carry
lower 50s north and middle 50s south. Parts of the SC coast will
struggle to hit 50 with the onshore trajectories moving across
nearby waters that are around 50.
Saturday night...middle level impulses to the northwest and west will
flatten the ridging aloft and force the surface ridge into the
nearby Atlantic. Cirriform clouds will continue to stream overhead
and will limit the amount of radiational cooling that is able to
occur. As a result we won't be as cold as tonight...but still could
see some freezing temperatures along the northwest tier if clouds remain thin
Sunday...the split flow persists aloft as the northern stream starts
to undergo some amplification as a strengthening short wave dives
east/southeast through the Central Plains and middle Mississippi Valley. This
feature sends a deepening surface cyclone through Missouri into
Kentucky...with an associated cold front to trail south/SW to the
Texas Gulf Coast by late. In the local area the surface high slides
further into the Atlantic...reaching near 30n/70w by 00z
Monday...and an ill defined coastal trough attempts to form out near
the Gulf Stream in response to a subtle perturbation aloft.
Isolated to perhaps scattered showers will form in proximity to
the inverted Atlantic trough...but most if not all of these look
to stay offshore through the day. Instead any rain chances won't
move in until late over far western sections as a south/SW low
level flow develops in conjunction with some late day isentropic
ascent. It/ll be a battle between increasing and thickening clouds
against impressive warm advection in regards to temperatures. We
anticipate a huge diurnal swing in temperatures...some 30-35 degrees as
we reach the middle and upper 60s. Due to sea breeze circulations the
barrier islands will be limited to the upper 50s and lower 60s.
Sunday night and Monday...the northern stream short wave heads
east/southeast and into the Ohio Valley by early Monday...maintaining it/S
positive or neutral tilt as it skirts by the north on Monday.
It/S associated surface low continues to drop several millibars
Sunday night as it travels through the virginias and then
blossoms further as it hits the Atlantic off the Delaware-Maryland-Virginia Monday.
Trailing south/SW from the low will be a fairly potent cold front
that. Consensus is for this front to reach into or near our northwest
tier by wake-up time Monday...before it slides quickly through the
rest of the County warning forecast area and into the Atlantic by early to mid-afternoon.
In regards to sensible weather...there is a low end risk for some
sea fog to develop and move ashore Sunday evening...but the main
weather will be for an increase in showers as they increase rapidly
during the late evening and overnight period in response to the
approaching cold front...a 135 knots upper jet and a 40-50 knots low level
jet. Precipitable waters will surge to 1.25 inches or greater and this along with
the large scale forcing for ascent will support high end likely or
categorical probability of precipitation. Once timing is better defined we/ll most probably
require 100 probability of precipitation at a later forecast time. There is some instability
that attempts to form...but since there is ample cinh to negate
this...we have held off on adding thunder to the forecast. But it
does remain a low end probability. Rain chances will fade from
west to east on Monday with the passage of the cold front...and
most if not all sections will see the rain chance end by early
afternoon...during the middle afternoon at the latest over the coast.
Winds will be on the increase Sunday night in response to the
stronger low level jet...rising to as high as 15-25 miles per hour across the
SC coastal zones. A pronounced rise/fall couplet will then develop
on Monday in wake of the cold front with large isallobaric pressure
rises to occur. This along with deeper mixing and a packed
pressure gradient will support a breezy to windy day as west/northwest
winds peak at 20-30 miles per hour and gusty.
Temperatures will fall little Sunday night...then steady out overnight
given the insulating effects of the clouds and a warmer south/SW
flow. Temperatures on Monday are restricted to the upper 50s and lower
60s as the 0c 850 mb isotherm moves into at least our SC counties.
Lake winds...it/ll certainly be gusty along the Lake Shore of Lake
Moultrie Monday...but mixing will be restricted somewhat over the
bulk of the lake waters. As a result we/re not yet certain that a
lake Wind Advisory will be required.
Long term /Monday night through Friday/...
rain chances will drop below 10 percent by Monday evening. Low
temperatures Monday night will reflect the cooler air mass in
place...falling into the low 30s west and north to the middle to upper
30s coast and south. Transient high pressure will move quickly
across the eastern United States in the wake of the departing low
system on Tuesday. Numerical model solutions then indicate southern
stream shortwave energy moving into the Gulf of Mexico and
generating a low pressure system on Wednesday. Strength and timing
difference between models are still quite significant. There remains
at least a 6 hour variance between the faster and slower solutions
regarding when the deepening low will have tracked up to the
southeast coast. Will remain somewhat generic and conservative
regarding rain chances until there is better model agreement. At
this point...will reintroduce rain chances by Wednesday as a coastal
trough develops along the periphery of the surface high over the
Atlantic. Have then indicated increasing coverage/potential of rain
Wednesday night into early Thursday...when the faster model solution
tracks the Gulf low up the southeast coastline. Although coverage is
expected to decrease from west to east later Thursday into
Friday...will maintain slight chance probability of precipitation to account for a few
showers that could be initiated as an upper trough moves across the
East Coast behind the departing low system.
High temperatures each afternoon will generally be in the 50s.
Monday night will be the coolest...with low temperatures each night
thereafter generally in the middle 30s inland to low 40s near the coast.
Aviation /03z Saturday through Wednesday/...
VFR through the 00z/01 taf period.
Extended aviation outlook...VFR to prevail until Sunday night into
Monday morning when MVFR or possibly lower conditions are likely due
to a passing cold front. Breezy conditions and/or wind shear is also
likely Sunday night/Monday.
overnight...expect winds to remain elevated as the pressure
gradient remains enhanced on the eastern flank of building high
pressure. Think winds will be near advisory levels...especially
over the Charleston County nearshore waters and offshore Georgia waters
and this is where we will keep the advisory going through the
night. Seas could build up to 6 feet out near 60 nm off the Georgia coast
as a slightly more favorable fetch sets up due to the northerly
flow. The best surging across the intra-coastal will occur along
the Charleston County coast and favorable locals such as Fort
Sumter and Fort Johnson will likely see at least occasional gusts
topping 20 knots through middle morning.
Saturday...the previous late night northerly surge will linger into
the morning hours and we have extended the small craft advisories until
7 am on amz350 and until 11 am on the amz374 waters.
Otherwise...Continental high pressure will weaken a bit as it
shifts toward the east and splits into two separate centers by
evening. Winds and seas will come down as a result as they flow
veers around to the NE and east.
Saturday night and Sunday...a transient high will move overhead
Saturday night then slides east of the waters Sunday. Light to
gentle east and southeast winds Saturday night will become moderate
southerly on Sunday.
Sunday night and Monday...the pressure gradient tightens up between
the Atlantic high and an upstream low and approaching cold front
that sweeps through Monday. Warm advection will limit the amount of
mixing in advance of the front...but we could still reach Small
Craft Advisory conditions. Cold advection...a well developed
rise/fall couplet will easily support a solid small craft
advisories...with even a medium risk for possible gales across
amz350-374. Although not depicted in the latest forecast...there is
two other concerns that mariners may have to contend with. The first
is a small window for possible sea fog Sunday evening if the
increase in winds are delayed. The second is the risk for some
stronger T-storms out near the Gulf Stream in advance of the cold
front. Confidence is not yet high enough to add to the gridded
Tuesday through Wednesday...a transient high pressure wedge will
prevail as low pressure eventually develops in the west/northwest Gulf and
a coastal trough attempts to form nearby. Latest prognosis is that
conditions remain below the advisory criteria during this time.
Marine...Small Craft Advisory until 11 am EST Saturday for amz374.
Small Craft Advisory until 7 am EST Saturday for amz350.