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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 190813

Area Forecast Discussion
National Weather Service Charleston SC
413 AM EDT Fri Apr 19 2019


A strong cold front will move through the region today. Low
pressure over the Ohio Valley will prevail Saturday, followed by
high pressure Sunday through the middle next week.


Through Sunrise: Low-level convergence and moisture transport
continue to rapidly increase across the coastal areas this 
morning as warm air advection/isentropic lift spreads in from 
the south. Early morning radars already show shower and some 
tstm activity beginning to percolate along the Georgia and
northeast Florida coast along the nose of the strongest 
isentropic assent. Expect convection to gradually increase in 
both coverage and intensity over the next several hours with the
bulk of the activity poised to impact the lower South Carolina 
coast into the Charleston Tri-County area through daybreak. The 
latest RAP and H3R with support from various other HREF members 
suggest convection could become rather widespread as the initial
impulse of isentropic assent peaks, so categorical pops will be
placed in the Moncks Corner-Charleston-Beaufort-Walterboro 
corridor through 12z. Instability is building so thunderstorms 
are possible. Could see a few strong convectively induced wind 
gusts with the deeper updrafts given the strengthening low-level
wind fields, but not anticipating anything severe at this 
point. This activity should move north of the Santee River by 
mid-morning. Will have to watch Downtown Charleston for 
possible urban flooding as this activity could impact the 
peninsula during the 837 AM high tide with tide levels expected 
to peak 6.7-6.9 ft MLLW.

Today: A very busy day is ahead. Water vapor imagery shows a
a potent upper low digging into the lower Mississippi Valley
early this morning. Strong forcing ahead of the low is
supporting a squall line across eastern Alabama which has shown
signs of strengthening over the past hour. The squall line will
continue to march east this morning and is on target to push 
into far interior Southeast Georgia by late morning-early 
afternoon, reaching the Savannah River and the Savannah Metro 
Area by early-mid afternoon and southern South Carolina and the 
Charleston Metro mid-late afternoon. Mesoscale convective 
processes will ultimately determine how fast the line propagates
to the east which is hard to pin down this far out. Utilized a 
blend of the various HREF members to depict the timing and
position of the squall line in the gridded forecasts with
gridded pops maxed out at 100% all areas at various times 
through the day.

The risk for severe tstms remains elevated today given the
favorable juxtaposition of deep moisture, instability, strong
wind fields and forcing for assent across the region per model 
time heights. The squall line is expected to cross the forecast 
area during the typically favorable diurnal maximum with 
modified soundings showing SBCAPE values reaching 2000-3000 J/kg
(temps near 80 with dewpoints around 70) within a highly 
sheared environment. What is most troubling is there are signals
that the squall line could become regionally enhanced and 
potentially quite dangerous as it crosses the Savannah River 
into the South Carolina Midlands and Lowcountry later this 
afternoon. This is in response to deep layered omega rapidly 
intensifying in response to the approaching upper low becoming 
negatively tilted and the associated right entrance region of 
the upper jet becoming oriented from south-north atop a belt of 
strengthening wind fields. This should provide a favorable 
environment for a potentially widespread damaging wind threat 
across mainly southern South Carolina where surface based 
instability will be the highest. While the primary threat looks 
to be damaging winds, model hodographs support a risk for a few 
tornadoes with EHI's peaking 3-5 and 0-1km helicity values 
ranging from 300-400 m2/s2 with SigTor values rising above 4. 
The tornado risk will be highest where any supercells develop 
ahead of the squall line or within the squall line itself. 

It does not appear at this time that early morning coastal 
convection will temper the severe weather threat later today and
any possible stabilizing influences from a shallow marine-layer
should be easily overwhelmed by the intense quasi-geostrophic 
forcing noted in the various model guidance. 

Breezy to very windy conditions will occur today outside any 
convection that forms. The highest non-tstm winds look to occur
east of a Walterboro-Savannah line where the strongest low-
level wind fields will be found. A Wind Advisory has been 
issued for this region for winds 20-30 mph with gusts to 40-45 
mph. For Charleston and Tidal Berkeley, including much of the
Charleston Metro Area, mixed layer winds at KCHS rise to about 
35 kt by 18z with momentum transfer algorithms supporting wind 
gusts approaching 50 kt for about a 3-4 hour period early-mid 
afternoon. Will cap winds at 25-35 mph with gusts to 50 mph for 
now and keep conditions just below High Wind Warning thresholds.
However, this will need to be watched carefully, especially if 
a period of enhanced insolation occurs.


Tonight: Expect rapidly improving conditions as a strong cold
front pushes offshore. Lows will drop into the upper 40s inland
to the mid-upper 50s at the coast. 

Saturday: The mid-levels will consist of a unseasonable strong 
trough over the eastern half of the country in the morning with low 
pressure near the OH Valley. In fact, NAEFS shows 500 mb heights 
near the core of the trough about 4 standard deviations below normal 
for this time of year. As time progresses, the trough will shift to 
the east, with its southern extent beginning to move offshore by 
daybreak Sunday. Meanwhile, the low continues to meander near the OH 
Valley. At the surface, in the morning a strong cold front will be 
moving off the East Coast while low pressure is near the OH Valley. 
Weak troughing will extend from the low into the TN Valley. As time 
progresses, the cold front will continue to shift further offshore 
while the low meanders near the OH Valley. The trough will shift to 
the northeast. Drier air will build into our area with PWATs falling 
as low as 0.5". Models hint that during the day there could be some 
remnant showers far offshore (with the departing front) and towards 
the mountains (with the trough). But it appears we should stay 
within the dry air and see only clouds. The dry forecast continues 
into the night as high pressure in the Gulf builds towards us. The 
pressure gradient will be elevated, leading to gusty southwest winds 
in the afternoon, especially at the beaches. Below normal 850 mb 
temperatures and low heights will lead to below normal temperatures. 
Highs across the area may not reach the 70 degree mark. This will 
carry over into the night with lows falling into the 40s, warmer at 
the beaches.

Sunday: The mid-levels will consist of a strong trough along the 
East Coast in the morning. The trough will weaken and lift to the 
northeast with time, allowing heights to rise over our area.
At the surface, broad high pressure centered over FL will spread 
across the Southeast into the night. Strong subsidence will bring 
dry conditions and sunny skies. Warmer 850 mb temperatures and 
rising heights will lead to seasonal temperatures.

Monday: The mid-levels will consist of a trough over the 
Northeastern U.S. in the morning. As this trough continues to lift 
to the northeast with time, ridging will build over the Southeast. 
At the surface, high pressure will prevail over the Southeast. The 
dry weather and sunny skies continues. Temperatures should rise a 
few degrees above normal, but remain cooler at the beaches due to 
the sea breeze.


High pressure over the Southeast Monday night will remain nearly 
stationary into Tuesday, bringing dry conditions and above normal 
temperatures. The high starts to shift offshore Wednesday as a front 
approaches from the north. Models have this front weakening as it 
gets closer to us, meaning the rain threat should be low. Thursday 
the models diverge on the upper level pattern and the overall rain 
threat, so we went with a blend for the forecast.


KCHS: Areas of convection near KJAX is expected to lift north
and expand through daybreak. Activity is targeted to impact the
terminal roughly 12-14z. Expect dry conditions for several hours
thereafter with very gusty winds reaching as high as 40-45 kt at
times. Squall line is expected to push through the terminal
roughly 20-23z brining a risk for heavy rainfall, erratic wind
shifts and IFR vsbys. Once the line passes, expect steadily
improving conditions with VFR prevailing sunset through 20/06z.

KSAV: Convection near KJAX is expected to pass east of the
terminal through daybreak. Will carry VCTS 11-14z to cover. Dry
conditions will prevail through the morning with a squall line
expected to cross the terminal roughly 18-21z. Line will bring
heavy rainfall, erratic winds and IFR vsbys. Otherwise, gusty
winds will continue to impact the region for much of the day.
Gusts could reach as high as 30-35 kt. VFR will prevail from
late afternoon through 20/06z.

Extended Aviation Outlook: Gusty southwest winds Saturday


Today and Tonight: A Gale Warning is in effect for all waters
through this evening. Dangerous conditions will develop across 
the waters later this morning with gale force winds expected, 
especially in frequent gusts. Winds may actually reach sustained
gales for the Charleston County and Georgia offshore waters 
where gusts could approach 45 kt. There is a concern that a few 
storm force wind gusts could occur in the Charleston Harbor 
where diurnal heating could locally enhance the winds there. Do
not think any storm force gusts will be frequent enough for a 
Storm Warning in the Charleston Harbor at this point. A squall 
line will begin to move into the waters by late afternoon 
brining a risk for enhanced winds and waterspouts. Seas will 
peak 7-11 ft nearshore waters and 11-13 ft over the Georgia 
offshore waters. Winds should diminish below gale force early 
tonight with the Gale Warning likely being replaced with a Small
Craft Advisory at that time.

Saturday through Tuesday: A strong cold front will be moving 
off the East Coast Saturday morning while low pressure is near 
the OH Valley. The pressure gradient across the coastal waters 
will be elevated, leading to gusty southwest winds in the 
morning. Saturday afternoon the gradient will lower as the front
moves further offshore, allowing winds to ease. But it's a safe
bet that Small Craft Advisories will be up for all of the 
coastal waters and maybe even Charleston Harbor Saturday morning
due to winds and seas. These advisories should drop off by 
Saturday night. Broad high pressure will spread across the 
Southeast Sunday night, then prevail towards the middle of next 
week, bringing tranquil conditions.

Surf Zone Hazards: Increasing winds and seas will support a high
risk for rip currents at all beaches today. In addition,
breakers in the surf zone are expected to reach 6-8 ft for the
Charleston County beaches and 4-6 for all remaining beaches. A
High Surf Advisory has been issued to address this.


GA...High Rip Current Risk through this evening for GAZ117-119-139-
     High Surf Advisory from 9 AM this morning to 8 PM EDT this 
     evening for GAZ117-119-139-141.
     Wind Advisory from 9 AM this morning to 5 PM EDT this 
     afternoon for GAZ119.
SC...High Rip Current Risk through this evening for SCZ048>051.
     High Surf Advisory from 9 AM this morning to 8 PM EDT this 
     evening for SCZ048-049-051.
     Wind Advisory from 9 AM this morning to 5 PM EDT this 
     afternoon for SCZ043>045-048-049-051.
     Wind Advisory from 9 AM this morning to 8 PM EDT this evening 
     for SCZ050-052.
     High Surf Advisory from 9 AM this morning to 11 PM EDT this 
     evening for SCZ050.
MARINE...Gale Warning until 11 PM EDT this evening for AMZ350-352-374.
     Gale Warning until 5 PM EDT this afternoon for AMZ354.
     Gale Warning until 8 PM EDT this evening for AMZ330.