Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
420 am EST Wednesday Dec 11 2013
high pressure will build from the west tonight and persist middle to
late week. A frontal system will then impact the region during the
Near term /through tonight/...
pre-dawn...much drier and cooler conditions this morning as surface
high pressure builds east from the lower Gulf Coast and Tennessee Valley
region. Some thin cirrus remains over the area while a powerful 150
knots 250 mb jet sweeps offshore of the middle Atlantic coast. Readings should
dip into the middle to upper 30s west of U.S. 17 by dawn.
High pressure will build over Georgia and the Carolinas today as the
pattern relaxes ahead of the next upper disturbance diving out
of the northern plains. Today/S forecast issues mainly with clouds
and temperatures as dry weather expected to prevail. To differing extents....
models indicate increasing isentropic ascent and moisture along
the 290-295k surfaces today across southeast Georgia. Stratocu fields
will likely expand from the Altamaha river entrance region west
and northwest later this afternoon but prefer the slower solutions
of the GFS/CMC to the robust NAM at this point with the expected
evolution of the offshore coastal trough. Otherwise sunshine is
likely to prevail with highs upper 50s north to lower 60s south.
The coastal trough will tighten up this evening as the gradient
feels the approach of the upper wave racing southeast toward Georgia the
Carolinas after midnight. The 00z NAM had all kinds of precipitation
breaking out along the coast tonight but condensation pressure deficits
seem too high initially to allow that to occur. Our only probability of precipitation
tonight brush the barrier islands of Georgia and they only result
in a slight chance pop for Georgia coastal zones. Later tonight...
a dry cold front will bump the weak coastal wave out to sea and
skies will start to clear late. Decent low level cold air advection
will get north breezes stirring nearing dawn with temperatures falling to
freezing well inland. Lows in the coastal cities closer to 40 by
Short term /Thursday through Saturday/...
Thursday...surface high pressure will move into the Tennessee Valley
region on Thursday...providing cool northeasterly winds across the
area. Maximum temperatures will range from the middle 50s along the Santee river
to the lower 60s along the Altamaha river. There should be plenty of
sunshine at the atmosphere dries out at all levels.
Thursday night...the surface high will move into the Carolinas...
allowing the winds to drop to near calm over the interior. With
clear skies and dew points in the 20s...the temperatures will also
drop that low with near ideal radiational cooling conditions.
Temperatures should drop below freezing everywhere away from the
coast...with some middle 20s possible across the northern tier of the
County Warning Area.
On Friday...the surface high moves into the Atlantic and winds turn
around to the east and southeast...modifying the air mass. We will
be a few degrees warmer on Friday...but a gradual increase in
cloudiness...especially late in the day...will keep maximum temperatures near
or just below normal.
By Friday night...a coastal trough is expected to develop just west of
the wall of the Gulf Stream...enhancing surface convergence...
leading to increasing clouds...and maybe even a brief
shower...although most of the activity should remain over the
coastal waters. High clouds will be thickening up as well as a
complex area of low pressure starts to loosely organize in the lower
Mississippi Valley...and its associated high clouds get picked up in
the southern stream jet and move quickly eastward. Temperatures will be
more than 10 degrees warmer than the night before due to the
thickening clouds and increasing southeasterly winds.
The situation gets complicated on Saturday as multiple areas of low
pressure try to develop in the Ohio Valley and southeast. There are
some significant differences in the global models with the
precipitation shields that develop...as the GFS holds most of the
significant rain west of the area...but the European model (ecmwf) has a lot more
rainfall for the area...seemingly concentrating on the coastal trough
as it moves inland. It is too early at this time to get
specific...but there is likely to be rain at some point Saturday in
Long term /Saturday night through Tuesday/...
Saturday night looks wet...although the models differ on the
evolution of the main storm systems. The GFS brings a distinct cold
frontal passage through the area...while the European model (ecmwf) hangs more energy
back...even indicating a weak surface wave possible in southern
Georgia by Sunday morning. This trend creates some significant
differences in rain threat on Sunday...as the GFS dries the area out
quickly...when the European model (ecmwf) indicates another slug of rain moving
through the area during the day...especially over the Georgia
counties where its quantitative precipitation forecast is in excess of 1/2 inch.
In looking at the models...the main difference between them appears
to be associated with severe weather potential statement they handle the upper level jet and its
associated divergence. By 12z Sunday...the GFS has a classic looking
baroclinic Leaf shape with the jet having a strong meridional
component...while the European model (ecmwf) is noticeably flatter with the jet as its
northern extent terminates in the middle Atlantic rather than New
The GFS ensemble MOS spread is not huge like last nights
run...indicating the GFS is at least more consistent with its own
ensemble members. Therefore...I am somewhat discounting the idea that
another large area of rain will move through on Sunday per the
European model (ecmwf). This will obviously need to be monitored with future
By Monday...the models are back into better agreement with the front
slowing over the western Atlantic and an elongated area of high
pressure extending from Ohio back into south Texas...putting the
area squarely in northeasterly low level flow once again. These
highs will be weaker than the previous few as they will contain
mainly modified Continental air...so temperatures should be near normal.
These conditions should continue into middle week.
Aviation /09z Wednesday through Sunday/...
VFR but will be watching for an increase in stratocu around ksav
late afternoon or this evening. Current thinking keeps MVFR ceilings
either offshore or to the south of the ksav terminal through 06z
Extended aviation outlook...mainly VFR conditions through Friday.
There could be periods of IFR due to low clouds and rain during the
weekend...improving again for early next week.
high pressure will build in from the west and northwest today but a pinched
low level pressure gradient expected to linger off the Georgia coast
where a developing coastal trough keeps moisture gradient fairly
impressive throughout today. Over Georgia waters we maintained
15 to 20 knots on average today while letting building high pressure quell
winds and seas over our near shore South Carolina marine zones. The
best pinching through this evening likely to setup from Grays Reef
seaward and we have extended the Small Craft Advisory for our outer
waters to account for these elevated conditions.
A solid northerly surge will develop late tonight as colder and
drier air punches in. Winds should increase to 15 to 20 knots with
some higher gusts all waters nearing daybreak Thursday and seas
remain 4 to 6 feet over waters beyond 20 nm off the Georgia coast.
Northerly winds will stay fresh during Thursday as moderately strong
high pressure sits over the Ohio and Tennessee valleys to the
northwest of the County Warning Area. Winds should generally stay a little below Small Craft Advisory
criteria...but a combination of wind wave and swell will keep seas
above 6 feet out beyond 40 nm in the offshore Georgia
waters...necessitating scas out there.
Thursday night the area of high pressure moves into the
Carolinas...causing winds to veer to the northeast and weaken with
time. Seas should also come down during the night to below 6 feet in
the outer Georgia waters. Winds will continue to veer to the east on
Friday...but remain below any Small Craft Advisory criteria.
Conditions will deteriorate on Saturday as winds turn southerly and
increase in response to developing low pressure over the southeast.
There is still some uncertainty on how strong the winds will be as a
coastal trough may develop and work towards the coast. This would tend
to keep winds near the coast a little lighter than otherwise would
be expected with a low over the southeast. These details will need
to be worked out with future forecast packages.
By Saturday night...winds should turn to the southwest and increase
further as a cold front approaches from the west. There are still
some model differences on the timing and strength of the frontal
passage Saturday night into Sunday morning...but advisories will
likely be needed over most of the waters at some point.
Sunday and Sunday night...winds will turn northwest behind the
front. Current thinking is that the winds will not be strong enough
to require scas except maybe in the offshore Georgia waters yet
again. Winds will then be northerly on Monday in a scenario similar
to this Thursday with high pressure building in to the northwest of
the coastal waters.
Marine...Small Craft Advisory until 5 am EST Friday for amz374.