Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Charleston SC
707 am EDT Tuesday Apr 21 2015

high pressure will build in today through Wednesday. A weak cold front
will drop through the area Thursday night...then stall to the south.
A storm system will affect the region Friday night through Saturday
night. High pressure will build in from the northwest Sunday into
Monday. A cold front may affect the area next Tuesday.


Near term /through tonight/...
the surface cold front was moving offshore at daybreak. Surface
high pressure centered over the Gulf Coast states will gradually
build eastward and extend over the forecast area later today and
tonight with a weak low level pressure pattern. Cirrus/cirrostratus
clouds could expand a bit around middle morning and then become few
to scattered this afternoon per forecast alignment of the fast west-east
upper jet across Georgia and South Carolina. A much drier airmass
will take hold while plenty of late morning and afternoon sunshine
is on tap. Weak low level cold air advection expected to abate and
with a bit of downslope component...temperatures should warm into the middle
and upper 70s.

Tonight...perhaps some high clouds will increase late tonight.
A setup for decent radiational cooling with near calm winds and
dry air in the lowest layers. A few of our normally cooler
locations such as Allendale could see readings dip to the upper
40s with lows in the lower to middle 50s many areas west of U.S. 17.


Short term /Wednesday through Friday/...
dry high pressure will slide off the East Coast on Wednesday while a
zonal flow persists aloft. Aside from some increasing late day
cirrus we expect sunny skies. Low-level flow will back to a more
southerly trajectory late in the day...allowing surface dewpoints to
rise into the middle/upper 50s. Temperatures will top out in the lower

Wednesday night into Thursday...some increase in upper shortwave
energy will help push a back door cold front toward the area. There
are some differences in model timing of the frontal passage. The GFS
has continued to be faster and also wetter with the front...showing
stronger shortwave energy and better moisture advection ahead of it.
However the GFS remains the outlier with the remainder of the model
suite only showing isolated showers or thunderstorms dropping into the area
on Thursday...dissipating as it moves south. We therefore are only
showing 20 probability of precipitation over our northern two-thirds and dry farther south.
Significant cloud cover and some late-day cold advection in northern
areas will give a range in high temperatures of low/middle 80s near the
Altamaha river and middle 70s far north.

Shortwave ridging on Friday will support dry high pressure at the
surface...though middle-level moisture will be on the increase so cloud
cover will increase during the day. Weak cold advection and
increasing cloudiness will keep highs in the middle/upper 70s.


Long term /Friday night through Monday/...
a potent shortwave will move through the area Friday night and the coast Saturday night. A warm front will develop
over the area and slowly lift north during the day Saturday. Backing
low-level flow ahead of the shortwave will support overrunning
precipitation Friday night and Saturday morning...then some
convective rains possible Saturday afternoon and evening as the
front lifts north. There is increasing model consistency with this
scenario so we bumped probability of precipitation up to 50% on Saturday.

Dry high pressure builds in from the north Sunday into Monday with
temperatures slightly below normal. There are some hints that another
system will affect the area next Tuesday but due to large model
differences we maintained a dry forecast and persistence temperatures.


Aviation /12z Tuesday through Saturday/...
VFR conditions will prevail at kchs/ksav through 12z Wednesday.

Extended aviation outlook...low stratus or fog possible Thursday
night into Friday morning as a front drops through. Ceiling and/or
visibility restrictions possible on Saturday with showers and thunderstorms.


the cold front will move through the waters prior to middle morning
with SW flow veering west and northwest. The stronger low level jetting
will exit the Georgia waters beyond 20 nm prior to dawn and we
recent lowered our Small Craft Advisory on the outer Georgia
waters as the frontal trough has displaced the southwesterly lower
level jet offshore. Otherwise...high pressure will build over the
waters with speeds falling below 15 knots offshore and below 10 knots
near shore. Seas within 20 nm will subside to 1-2 feet by tonight.

Fairly quiet conditions will persist Wednesday through the weekend.
Southerly flow strengthens Wednesday and Wednesday night ahead of a
cold front...then turns to the north-northeast briefly late Thursday night into
Friday behind the front. Increasing westerly flow anticipated next
weekend as a storm system passes through...during which advisories
are possible.


Chs watches/warnings/advisories...


near term...
short term...jrl
long term...jrl

National Weather Service Glossary of Abbreviations