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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
000
FXUS62 KCHS 171131
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
731 AM EDT Tue Sep 17 2019

.SYNOPSIS...
Hurricane Humberto will continue to move away from the coast
through mid week. A cold front will advance through the area
Wednesday, followed by high pressure into early next week. 

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: Aloft, the primary feature of interest is the high 
amplitude ridge that extends from the Lower Mississippi Valley 
northward into central Canada. At the surface, Humberto will 
begin to accelerate to the east while a cold front approaches 
the the forecast area from the north late in the day. The 
forecast is dry through the day ahead of the front. The main 
story will be the continued well above normal temperatures, and 
at least a low end chance of record highs. The forecast depicts 
highs ranging from the low to mid 90s across southeast South 
Carolina and from the mid to even upper 90s for southeast 
Georgia. Such values would be on the order of 7-10 degrees above
normal. Based on daily high temperatures records (see Climate 
section below), Savannah will be very close while Charleston is 
expected to fall a few degrees short. Nonetheless, a very hot 
day for mid September. 

Tonight: The cold front will steadily drop southward through 
the forecast area, likely reaching the Altamaha by sunrise 
Wednesday. The biggest forecast challenge will be if any showers
can develop along and immediately ahead of the front, and what 
will the coverage look like through the night. Given that the 
front is moving in from the north with a relatively limited 
moisture field at a climatologically unfavorable time for 
convection, conventional wisdom would favor minimal coverage. 
Fortunately, most model guidance agrees with this with a limited
precipitation response. Therefore, the forecast features rain 
chances around 20 percent in most areas late tonight with a 
small tier of 30 percents over an area from Beaufort County 
south including the adjacent coastal waters. There isn't much 
support for thunder, so we have handled it with just showers. 
Overnight lows should range from around 70 across Berkeley 
County to the low 70s southward to the Altamaha River.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
Wednesday: The timing of a backdoor cold front through the 
forecast area has generally remained consistent on recent model 
runs and we maintained good persistence with the mid week 
forecast. The temporary increase in moisture will likely result 
in scattered showers across parts of the area around mid 
morning, then chances will shift to the south of I-16 by the 
afternoon as dew points fall from the north in the wake of the 
front. Our only changes were to bump up POPs to modest chance 
ranges closer to the Altamaha River area. Temps will not be 
nearly as warm with highs ranging from low to mid 80s north of 
I-16 with some close to 90 degree readings possible south of a 
line from Metter to Ludowici GA. 

Thursday: Cooler high pressure to the north at the surface will
wedge to the lee of the mountains, providing temps that should 
run a bit below climo with the upper ridge displaced to the 
west. It should also be a bit breezy along the immediate coast 
with a tight pressure gradient across the area. Sunshine should 
prevail as temps reach the upper 70s far north to the lower 80s 
much of southeast GA.

Friday: Deeper ridging will develop as mid-level heights build 
over the region. In the lower levels, high pressure will reside 
across the Mid Atlantic states. We did introduce a slight chance
for a few light showers coming in off the ocean into coastal 
GA. Since 850-500 MB condensation pressure deficits are still 
pretty high, we don't expect much measurable pcpn if at all in 
this area. Temps will remain constant for the most part with 
highs reaching 80 to 85 degrees from north to south after 
morning lows that may dip into the mid-upper 50s in cooler spots
along our northwest tier of zones.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
Deep layered ridging will persist over the weekend and then 
tend to break down during the first part of next week. Models 
continue to show dry weather prevailing through the entire 
period. High temps are expected to warm slightly above climo 
during the period, however overnight temps should continue to 
fall into the 60s each night away from the ocean as the 
atmosphere remains quite dry.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
VFR conditions are expected to prevail at KCHS and KSAV through
12z Wednesday. A cold front will move through the terminals late
tonight, and could bring isolated showers to the area. Chances
are too low to mention in the forecast at this time. Also, there
is a low end chance for MVFR ceilings at KSAV. 

Extended Aviation Outlook: There are chances for at least 
patchy MVFR cigs through the southern portion of the coastal 
corridor Wednesday morning. Otherwise, VFR conditions will 
prevail.

&&

.MARINE...
Today through Tonight: Winds will continue to improve across 
the local waters, with north to northeast flow at 15 knots or 
less. Overnight, winds will start to pick back up as a cold 
front pushes southward. By sunrise Wednesday expect to see 15-20
knots for the South Carolina waters, with 10-15 further south. 
Seas during the day will run 4-6 feet in the Charleston County 
waters and 4-7 feet in the outer Georgia waters. Elsewhere 
expect 3-5 feet. Overnight seas will begin to tick upward again 
late as winds pick up behind the front. The Small Craft Advisory
remains in effect for the Charleston County waters and the 
outer Georgia waters. A new Small Craft Advisory for the 
southern South Carolina waters has been hoisted starting after 
midnight tonight.

Northeast flow will dominate throughout the week in the wake of
the backdoor cold front on Wednesday. The initial surge of 
winds will develop southward over the waters and SCA conditions 
will likely reach the GA near shore waters by afternoon at the 
latest as seas build to 4-6 ft. SCA conditions are expected all 
waters into late week as winds and waves remain elevated with 
gradients tightened by high pressure to the north. Winds are 
expected to lighten by the weekend with seas coming down a bit 
more slowly than wind speeds. 

Rip Currents: Moderate Risk at all beaches today due mainly to 
earlier rip current reports and continued elevated/long period 
swell emanating from Hurricane Humberto. The rip current risk 
will then remain elevated through the end of the week as 
northeast winds and the longshore currents strengthen behind the
mid-week backdoor cold front.

&&

.TIDES/COASTAL FLOODING...
Low chances for minor coastal flooding around the times of high
tide through late week, especially along the South Carolina 
coast.

&&

.CLIMATE...
Daily record high temperatures for Tuesday, 9/17.

Charleston International Airport (KCHS)
96...1972

Savannah/Hilton Head International Airport (KSAV)
97...1972, 1933, 1896

Downtown Charleston (KCXM)
94...1896

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 2 AM Wednesday to 8 PM EDT Friday 
     for AMZ352.
     Small Craft Advisory until 4 AM EDT Sunday for AMZ374.
     Small Craft Advisory until 6 PM EDT Friday for AMZ350.

&&

$$

NEAR TERM...BSH
SHORT TERM...
LONG TERM...
AVIATION...BSH
MARINE...BSH
TIDES/COASTAL FLOODING...
CLIMATE...