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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
000
FXUS62 KCHS 180839
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
439 AM EDT Thu Apr 18 2019

...A POWERFUL SPRING STORM TO BRING A NUMBER OF POTENTIAL 
HAZARDS TO THE REGION FRIDAY...

.SYNOPSIS...
High pressure will prevail today. A strong cold front will sweep
through the region Friday, then a trough of low pressure will
linger through Saturday. High pressure will build into the area
late this weekend and will prevail into the middle of next week. 

&&

.NEAR TERM /THROUGH TODAY/...
The synoptic pattern will begin to undergo a rapid transition
today as the mid/upper-level ridge across the Southeast U.S. 
breaks down and shifts offshore in response to a phasing of 
strong southern and northern stream energy over the Central 
Plains. The Central Plains phasing will yield a powerful Spring
storm which will impact much of the region on Friday. The region
will see one last quiet, warm day before active weather arrives
tomorrow. Although H8 temperatures are forecast to cool through
the day, increasing low-level thicknesses and a slightly warmer
start to the day should still allow afternoon highs to reach 
the lower-mid 80s away from the coast, warmest across interior 
Southeast Georgia where a pocket of 1000-850 hPa thicknesses are
forecast to rise to ~1395m by 18/21z. A steadily onshore flow 
will support cooler temperatures across the coastal counties 
with the beaches once again only topping out into the lower 70s.
Admittedly, forecasting the thermal gradient across the coastal
counties is proving to be a challenge with the sea breeze
circulation likely taking on a pure versus resultant 
configuration later in the day.

A few of the HREF members, including the H3R, ARW-E and NSSL- 
WRF, develop isolated showers off the lower South Carolina and
far northern Georgia coasts later in the afternoon within what 
appears to be a region of weak boundary layer moisture flux
convergence. Confidence is not overly high that anything 
measurable will develop and this behavior may very well be the 
result of a possible over development of stratocumulus within a 
region of deeper moisture lurking just offshore. Some single 
digit gridded pops were maintained in this region to reflect 
this, but no mentionable pops will be shown at this time. 

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...
Tonight: Warm air advection/isentropic assent will gradually
build through the night as low-level jetting intensifies ahead
of a powerful H5 low that will steadily close off across
Tennessee Valley. Guidance shows the strongest pre-frontal
moisture transport occurring across the western wall of the 
Gulf Stream and extending north into the northeast South 
Carolina coast through daybreak, only brushing portions of 
Charleston and Berkeley Counties. This suggests the bulk of any
convection that develops over the waters after midnight will be
directed more towards Myrtle Beach and the upper South Carolina
coast, which matches trends noted in a large majority of the
HREF members. Activity will be close enough to justify pops 
across the South Carolina coastal corridor prior to sunrise 
Friday. Pops will range from 40% across upper Charleston County 
to 30% across most of the Charleston Metro Area with 20% pops as
far west as Walterboro and parts of the Savannah Metro Area. A 
well defined squall line will likely be approaching the 
Reidsville-Millen corridor by 19/12z bringing a risk for severe
tstms with damaging winds, hail and tornadoes. Warm and breezy 
conditions will occur overnight with conditions likely becoming 
windy across the coastal counties by sunrise. Lows will range 
from the upper 60s inland to around 70 at the coast.

Friday: An active day of weather is becoming more likely during the 
early afternoon into early evening hours across the Southeast United 
States. The Storm Prediction Center has now included the entire 
Southeast South Carolina and Southeast Georgia area in a "Enhanced 
Risk" of severe weather.

A strong low pressure system will track over the Ohio River Valley 
with a southward extending cold front that sweeps from west to east 
across the Southeast United States. Aloft, a high amplitude trough 
of low pressure will help drive the cold front quickly towards the 
Southeast coast, most likely sometime between mid to late Friday 
afternoon. Timing of the front, along with showers/thunderstorms 
shifting onshore late morning will be a factor to how much 
instability develops across the area ahead of the front Friday 
afternoon. Latest guidance suggests much of the onshore activity 
during late morning hours potentially shifting north of the area by 
early afternoon, thus allowing SBCAPE to reach 1500-2000 J/kg as 
PWATs approach 1.75 inches and sfc dewpts remain in the mid/upper 
60s. This amount of instability/moisture combined with strong 
forcing and shear associated with the arrival of a 60 kt low-lvl 
jet, suggests widespread thunderstorms across the area, some being 
strong and/or severe Friday afternoon into early Friday evening, 
particularly along and east of the I-95 corridor where sfc temps 
peak near 80 degrees before fropa. 

Latest model soundings along with 850mb-500mb lvl crossover winds 
suggest mainly unidirectional wind fields during the most favorable 
time of convection, meaning the greatest threat with thunderstorms 
being damaging wind gusts within bowing segments. However, there is 
still some directional shear component as the cold front sweeps 
through the region, suggesting the potential for isolated tornadoes 
within a squall line of thunderstorms that quickly advances toward 
the coast by early Friday evening. Marginal mid-lvl lapse rates 
suggest hail to be small for this event. Once fropa occurs, the 
severe weather threat will come to an end, starting across inland 
areas mid/late afternoon, then near coastal areas late 
afternoon/early evening. 

Strong wind fields associated with this system will also support 
gusty southerly winds across the Southeast United States outside of 
shower/thunderstorm activity. A Wind Advisory will be possible over 
the area, especially near the coast in Southeast South Carolina. 
Strong cold air advection behind the front will likely support gusty 
winds at least into the evening. A few showers could persist behind 
the departing front Friday night, but the bulk of activity should be 
offshore within a few hours of sunset. Lows will be much cooler than 
the previous night as high pressure begins to push into the area 
late. In general, lows should dip into the lower 50s away from the 
coast. 

Saturday and Sunday: A large mid/upper lvl low will be slow to shift 
off the Eastern Seaboard this weekend, suggesting the potential for 
a few showers across the area as some moisture wraps around its 
southern periphery. However, the bulk of deep moisture and precip 
activity should be offshore Saturday afternoon/evening, followed by 
a period of drier weather through Sunday as sfc high pressure builds 
across the region. Temps will be noticeably cooler over the weekend, 
but will modify quickly early next week. In general, high temps 
should only reach the mid/upper 60s across most areas Saturday 
afternoon. Saturday night lows should dip into the mid/upper 40s 
inland to low/mid 50s near the coast. On Sunday, temps will rebound 
into the mid 70s under sunny skies. 

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
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 /07Z THURSDAY THROUGH MONDAY/...
VFR. 

Extended Aviation Outlook: Flight restrictions are then likely 
at both terminals Friday with showers/thunderstorms shifting 
over the terminals, potentially from activity shifting onshore 
early at the CHS terminal, followed by strong 
showers/thunderstorms with a passing cold front Friday afternoon
into the evening at both CHS and SAV terminals. Gusty south 
winds are also likely Friday and Friday night outside of showers
and thunderstorms. Flight restrictions could persist into early
Saturday before improving to VFR by Saturday afternoon. VFR 
conditions should then prevail under high pressure Sunday and 
Monday. 


&&

.MARINE...
Today: South to southeast winds will prevail today with with 
speeds generally less than 15 kt. Locally enhanced winds around
15 kt are possible in Charleston Harbor with the afternoon sea 
breeze. Seas will average 2-3 ft nearshore and 3-4 ft offshore.

Tonight: Conditions will deteriorate over the coastal waters
tonight as low-level jetting takes hold ahead of a strong storm
system to the west. South winds are expected to increase to
20-25 kt with gusts to 30 kt across nearshore waters with 25-30
kt with gusts to 35 kt over the Georgia offshore waters. Small
Craft Advisories remain in force for all nearshore legs,
including Charleston Harbor. A Gale Warning has been posted for
the Georgia offshore legs where frequent gusts of 35 kt are
expected after 08z. Seas will build overnight, reaching 4-7 ft
nearshore and 6-8 ft offshore by daybreak Friday.

Friday through Monday: Conditions will continue to deteriorate 
Friday morning into the afternoon as a strong pressure gradient and 
associated 60 kt low-lvl jet spreads across the waters in advance of 
a cold front shifting offshore Friday afternoon into early Friday 
evening. At least Small Craft Advisory level conditions will occur 
for all coastal waters Friday, followed by a period of gale force 
wind gusts late Friday morning into Friday evening. A Gale Warning 
is now in effect for offshore Georgia waters until 04Z Saturday. All 
remaining waters, including the CHS harbor continue to be under a 
Gale Watch Friday into early Friday night. Seas should also build up 
to 5-8 ft across GA nearshore waters and 9-12 ft across offshore 
Georgia waters and beyond 10 nm from the coast in nearshore SC 
waters. 

Thunderstorms could be strong and/or severe over coastal waters 
Friday afternoon into early Friday night along/near the passing cold 
front. Damaging winds and isolated waterspouts will be the primary 
threat. Conditions should then improve Saturday, but Small Craft 
Advisories will likely persist behind the departing front as cooler 
high pressure builds over the waters. By daybreak Sunday, high 
pressure should become centered along the Southeast coast, favoring 
winds and seas that remain below Small Craft Advisory level 
conditions through Monday.

Surf Zone Hazards: Strong winds could produce high surf and an 
elevated risk for rip currents Friday. A High Surf Advisory may 
be needed for breakers of 5 ft or greater. 

&&

.TIDES/COASTAL FLOODING...
Elevated tides are expected for the evening high tide cycle.
Levels in the Charleston Harbor could approach or slightly
exceed 7.0 ft MLLW. A Coastal Flood Advisory could be needed.
Levels are not expected to reach advisory levels at Fort
Pulaski.

Confidence is decreasing that shallow coastal flooding will
occur with the evening high tide Friday as winds are expected to
shift more westerly by then. However, there is still a low-end
risk, especially with winds are slow to shift than expected.

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

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Gale Watch from Friday morning through Friday evening for 
     AMZ330-350-352-354.
     Small Craft Advisory from midnight tonight to 6 AM EDT Friday 
     for AMZ330-350-352-354.
     Small Craft Advisory from 8 PM this evening to 4 AM EDT Friday 
     for AMZ374.
     Gale Warning from 4 AM Friday to midnight EDT Friday night for 
     AMZ374.

&&

$$