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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
000
FXUS62 KCHS 190044
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
844 PM EDT Thu Apr 18 2019

.SYNOPSIS...
A strong cold front will approach from the west tonight, then
sweep through the region on Friday. Low pressure over the Ohio
Valley will prevail Saturday, followed by high pressure
building into the region Sunday and into early next week. 

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Atlantic moisture transport will continue to increase overnight
as southerly flow strengthens and deepens ahead of the powerful
upper trough. Initially only scattered low clouds exist, 
however stratocumulus will thicken overnight with the increase 
in low-level moisture. Most model guidance shows a surge in 
low-level dewpoints across coastal SC later tonight, along with
cooling temperatures aloft. This will yield modest surface- 
based instability. The high-res guidance continues to show a 
slug of moisture convergence moving off the Atlantic into the 
Tri-County area after 2 or 3 am, bringing scattered showers and 
perhaps a few thunderstorms into the area. Surface winds will
persist overnight, strengthening a bit toward daybreak as the
low-level jet begins to strengthen. As a result of the robust
mixing overnight, temperatures will not drop more than a few
degrees from current values.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
Friday: The focus of the short term forecast continues to be the 
potential severe weather event Friday. Overall, there has been no 
change to the overall dynamic setup. A deep trough is still forecast 
to dig across the lower Mississippi Valley and close off into an 
upper low. This anomalous setup will push impressive forcing into 
the forecast area as a strong cold front moves west to east across 
the forecast area. The ambient wind field is progged to be quite 
impressive, producing strong shear profiles and notable low level 
veering. The shear environment continues to look supportive of an 
elevated damaging wind threat and isolated tornadoes. As is almost 
always the case, the biggest question mark remains instability and 
that will likely hinge on the timing of the frontal line of 
convection. 

History favors a faster timing as the convection tends 
to race out ahead of the main front. The ECMWF and GFS continue to 
be the faster solutions, while the NAM is a few hours slower. Also 
of note, we are now getting into the window of time where the HRRR 
and RAP extend into Friday morning. It's a bit concerning that both 
the HRRR and RAP are even slower than the NAM, with the RAP showing 
the line of thunderstorms still west of our inland zones (Jenkins, 
Candler, and Tattnall counties) at 15 UTC. The slower the line is in 
getting into the area, the more the atmosphere can destabilize, 
further enhancing the severe weather threat. The forecast does slow 
arrival down a bit, but still depicts the line knocking on the 
doorstep of southeast Georgia around 12 UTC. This would make the 
main time period for severe weather for southeast Georgia roughly 
from 8am-noon, then noon-4pm for southeast South Carolina. This 
timing will likely need adjusting overnight as the line develops and 
we can make better assessments of how various models are 
initializing. As SPC notes in the Enhanced Risk severe weather 
outlook, MLCAPE values of 1000-1500 J/kg are possible. This 
instability combined with the ambient shear will favor a linear 
convective mode with embedded supercellular elements. The line is 
expected move east of the area by mid/late afternoon at the latest, 
ending the severe weather threat. 

Also of note, the strong wind field will produce gusty winds outside 
of thunderstorms on Friday. Confidence has increased to the point 
where we have issued a Wind Advisory for Beaufort, Coastal Colleton, 
and the entire Tri-County region for frequent gusts to 40-45 mph. 
Such winds will also be possible further to the southwest, primarily 
along the Georgia coast, but confidence isn't high enough at this 
time to include in the advisory. The advisory will run from late 
morning through late afternoon. 

Friday night: The front will move well to the east and mid/upper 
level moisture will strip out of the region. There doesn't appear to 
be much support for lingering shower activity so the bulk of the 
overnight will be dry. Low temperatures should dip into the low to 
mid 50s in most areas. 

Saturday through Sunday: The large and deep upper low will encompass 
much of the eastern CONUS on Saturday and then start to drift 
eastward on Sunday. The surface low will spend much of Saturday 
across the Ohio Valley, leading to a breezy and cool day with clouds 
filling in from the west and northwest. Cold temperatures aloft, 
around -20C at 500 mb, could help produce isolated showers during 
the day, but virtually every model keeps shower activity upstream of 
the forecast area. We still show a slight chance of showers but this 
area has been trimmed to a smaller region. For Sunday, high pressure 
will begin building in and skies will clear out. Look for Saturday 
highs to range from the low 60s inland to the mid/upper 60s closer 
to the coast. Then for Sunday, a nice warmup with highs rising into 
the mid and upper 70s.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Dry sfc high pressure will prevail across the region Sunday night 
through the middle of next week with temps warming each day as a 
mid/upper lvl ridge axis shifts over the Southeast United States. In 
general, high temps in the the low 80s Monday will warm into the low 
80s Tuesday, then low/mid 80s Wednesday. Overnight lows should range 
in the low mid 50s away from the coast Sunday night, then mid/upper 
50s inland to lower 60s near the coast Monday night, followed by 
low/mid 60s Tuesday night.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
Stratocumulus will thicken overnight, particularly southern SC
as convergence increases late. Fairly good chance for MVFR
ceilings moving in off the Atlantic at KCHS a few hours prior to
daybreak, along with scattered showers.

Winds will rapidly increase during the morning at both
terminals, peaking in the late morning through early afternoon 
hours. The latest timing on the line of strong to severe 
thunderstorms has it pushing toward KSAV after 18Z and KCHS 
after 20Z. Damaging straight line winds will be the primary 
threat with this line, though isolated tornadoes are also 
possible. The convection should be off the Atlantic coast by 00Z
Saturday. Flight restrictions are a near certainty with the main
band of storms but given the timing uncertainty this far out we
have yet to include specific mention of restricted vsby/cigs.

Although winds at the 1 kft and 2 kft levels will be very strong
on Friday, ample mixing down to the surface should preclude
low-level wind shear concerns.

Extended Aviation Outlook: VFR conditions should return 
overnight Friday.

&&

.MARINE...
Tonight: Conditions will deteriorate with time tonight. As a 
strong cold front approaches from the west and south winds 
strengthen, Small Craft Advisory winds/seas will spread across 
the coastal waters, including Charleston Harbor overnight, and 
gale force gusts are expected to spread into AMZ374 overnight. 
Seas 2-3 feet nearshore and 3-4 feet beyond 20 nm to start will 
build to as high as 4-8 feet nearshore waters and 7-9 feet 
beyond 20 nm by late tonight. 

Friday through Tuesday: A strong cold front and associated line
of thunderstorms is expected to move across the local waters on
Friday. Southerly winds will increase significantly Friday 
morning ahead of the cold front and confidence has increased 
such that Gale Warnings are now in effect for all waters 
including Charleston Harbor. Expect frequent gusts into the 
35-40 knot range beginning in the morning and continuing into 
the afternoon. The likelihood of gales will end from west to 
east as the cold front moves through and the warnings will come 
down at various times. However, all the warnings should be down 
by late Saturday night. Seas will increase significantly, and 
could rise into the 6-10 ft range out to 20 nm on Friday, and 
9-12 ft beyond. Conditions will improve Friday night, but we 
will certainly need Small Craft Advisories through Saturday 
evening to account for elevated west-southwesterly winds and 
seas in an enhanced pressure gradient around low pressure well 
to the north. Much more tranquil conditions can be expected 
starting Sunday and continuing into early next week as high 
pressure builds in across the Southeast. 

As thunderstorms move across the waters on Friday, strong 
damaging winds and isolated waterspouts will be possible through
the day, first in the Georgia waters and then in the South 
Carolina waters. 

Surf Zone Hazards: Strong winds could produce high surf and a 
High Surf Advisory could be needed for breakers of 5 ft or 
greater. A Moderate Risk of Rip Currents is in effect Friday at 
all beaches.

&&

.TIDES/COASTAL FLOODING...
Issued a Coastal Flood Advisory for Charleston/coastal Colleton
county this evening for expected minor salt water flooding
around the time of the high tide. Also, we will remain alert 
for the potential for locally heavy rain around the time of the 
Friday morning high tide in Charleston and surrounding coastal 
communities. 

Blow-out tides are possible with Saturday morning's low tide.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Wind Advisory from 10 AM to 5 PM EDT Friday for SCZ044-045-
     048>050-052.
     Coastal Flood Advisory until 11 PM EDT this evening for SCZ049-
     050.
MARINE...Small Craft Advisory from midnight tonight to 10 AM EDT Friday 
     for AMZ352-354.
     Gale Warning from 10 AM to 8 PM EDT Friday for AMZ330-352.
     Small Craft Advisory from midnight tonight to 7 AM EDT Friday 
     for AMZ350.
     Gale Warning from 7 AM to 11 PM EDT Friday for AMZ350.
     Gale Warning from 10 AM to 5 PM EDT Friday for AMZ354.
     Small Craft Advisory until 4 AM EDT Friday for AMZ374.
     Gale Warning from 4 AM to 11 PM EDT Friday for AMZ374.
     Small Craft Advisory from 2 AM to 10 AM EDT Friday for AMZ330.

&&

$$
NEAR TERM...JRL
SHORT TERM...BSH
LONG TERM...
AVIATION...BSH/JRL
MARINE...BSH/SPR
TIDES/COASTAL FLOODING...