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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 231744

Area Forecast Discussion
National Weather Service Charleston SC
144 PM EDT Thu May 23 2019

High pressure will dominate and will produce unseasonably hot
conditions from Friday into the later part of next week. 


As of 140 PM: I will update the forecast to adjust temperatures
and sky to align with the latest observations. In addition, I
will add a couple hours of SCHC PoPs for showers across northern
Charleston Co.

As of 1050 AM: Latest visible satellite and view out of the
window shows small-celled fair weather Cu across the forecast
area. I suspect that the Cu will likely start to mix out during
the early to mid afternoon. I will update the forecast to
increase sky cover through mid day, then decreasing clouds
during the afternoon. Temperatures and Wx appear on track.

Previous Discussion:
Initialized this update with a bit more cloud cover this 
morning per current satellite trends. Otherwise, the forecast 
was pretty much unchanged from the previous cycle.

Surface high pressure will extend across the region today. The 
mid and upper-level anticyclones centered over Georgia and South
Carolina will strengthen and retrograde slightly signaling the
beginning of a prolonged and potentially dangerous early season 
heat wave across the Southeast States. There is a window for 
areas south of I-16 to potentially see an isolated shower this 
morning through early afternoon in the vicinity of a narrow 925 
hPA theta-e axis aligned northwest-southeast. Other than the 
NSSL-WRF and ARW, the remaining HREF members are rain-free so 
will keep gridded pops below mentionable thresholds in this 
region for now. Forecast soundings show a capping inversion 
strengthening through the day with the cap reaching 2-3C by 
mid/late afternoon. Any risk for a shower should end by then, 
especially as the pure sea breeze circulation moves inland. 
Onshore winds will once again limit temperatures across the 
coastal counties, but H8 temperatures are progged to rise ~2C 
later in the day. This will result in another day with a large 
thermal gradient with highs ranging from the mid-90s across the 
far interior to the upper 70s/lower 80s at the beaches. Will 
likely see a few spots in the Statesboro- Reidsville-Metter 
corridor reach the upper 90s, similar to Wednesday.


Tonight: The surface high will shift to the south overnight
resulting in gradually shift to a more south to southwest flow
regime. The boundary layer will gradually decouple through the
night, first across the far interior, then working east to the
coast during the early morning hours Friday. Expect a least
modest radiational cooling to influence the low temperature
forecast, but it remains unclear how much stratus will develop
after midnight. Will trend to the cooler side of the guidance
envelop with lows ranging from the mid-upper 60s inland to the
mid 70s at the coast and Downtown Charleston. Dry conditions
will persist with shallow ground fog possible prior to daybreak

Friday through Sunday: Models continue to be in good agreement 
with strong upper level ridging persisting over the area through
the short term period. This will result in the beginning of a 
prolonged and dangerous heat wave. 

Friday: Large scale sinking motion and dry air in the mid and upper 
levels will result in nearly full sun with much of the energy going 
into the warming of the atmosphere. Downslope flow will keep the sea 
breeze pinned at or near the coast for much of the day so areas near 
the beaches will be rather hot too. Highs expected to be mainly in 
the mid to upper 90s away from the immediate coast, with heat 
indices topping out from 95 to 101. 

Saturday: Models in fairly good agreement that a weak backdoor cold 
front will push slowly south through at least a good portion of 
southeast South Carolina during the day before stalling out and then 
dissipating Saturday evening and overnight. Both the GFS and ECMWF 
show decent instability with CAPE values from 1000 to 2000 J/KG 
developing in the afternoon mainly along and north of the front. 
Despite strong ridging aloft, the 00Z GFS breaks out convection 
along and north of the front, mainly during the afternoon. This 
combined with a hint of this occurring with the NAM as well and the 
12Z GFS hinting at the same solution a bit further north, including 
a slight chance for thunderstorms in the forecast seems warranted. 
The backdoor front, onshore flow, and potential for at least a few 
thunderstorms will result in a slowing of the warmup for areas in 
South Carolina generally east of I-95. Highs should range from the 
mid to upper 80s at the beaches in southeast South Carolina to 
mainly the mid and upper 90s along and west of I-95. Heat indices 
could reach as high as 102 across interior southeast Georgia. 

Sunday: Northwest flow in the lower levels returns, but it may not 
be quite strong enough to keep sea breeze pinned at the coast all 
day. With downslope flow, full sun, a dry ground, and 850 mb 
temperatures likely to be from 19 to 20 degrees Celsius, expect high 
temperatures to be from 85 to 90 at the coast and generally from 97 
to 102 inland. Heat indices away from the coast are expected to be 
from 102 to 106 degrees. As a result, heat advisories may be needed 
for some areas.


There does not appear to be any relief in the dangerous and 
prolonged heat wave through at least Thursday. Continued sinking 
motion, a dry atmosphere above the boundary layer which will result 
in nearly full sun, a dry ground, and 850 mb temperatures generally 
within a degree or two of 20 Celsius all point to high temperatures 
from 97 to 103 degrees away from the coast. Heat indices at or above 
105 which would prompt a heat advisory may occur on any of the days. 
The need for a heat advisory seems mostly likely to be needed on 
Monday and/or Tuesday. Air quality issues may develop at any point 
during this period due to the excessive heat and generally stagnant 
air pattern. Even though winds will be on the lighter side, hot 
conditions and dry fuels could aid in the development of wild fires.



Extended Aviation Outlook: VFR.


Through Tonight: East to southeast winds will veer to the south
and southwest tonight as high pressure sinks to the south. A
period of elevated winds will occur near the coast and
Charleston Harbor once again as a pure sea breeze circulation
moves inland. Winds should peak 15 kt with gusts to 20 kt there
with 10-15 kt elsewhere. Winds will diminish closer to 10 kt
overnight. 2-4 ft seas this morning will subside to 1-3 ft
through the period.

Friday through Tuesday: Other than east to southeast winds which 
develop mainly in the South Carolina waters on Saturday, south to 
southwest winds will prevail through the period. Wind speed will be 
below 15 knots except possibly a bit higher near the coast and in 
various harbors associated with the afternoon sea breeze. Seas will 
mainly be from 1 to 3 feet.


A very significant, hazardous and prolonged heat wave will challenge 
or break many of these records.


Station     Record High Min   Year
-------     ---------------   ---- 
KCXM            77            1998 


Station     Record High       Year(s)
-------     -----------       -------
KCHS            97            1953
KSAV            98            2011, 1953


Station     Record High       Year(s)
-------     -----------       -------
KCHS            97            2000, 1953
KSAV           100            1953

Station     Record High Min   Year(s)
-------     ---------------   -------
KCHS            75            1953
KSAV            76            1878
KCXM            79            1998


Station     Record High       Year(s)
-------     -----------       -------
KCHS            98            1953
KSAV           100            1953

Station     Record High Min   Year(s)
-------     ---------------   -------
KCHS            75            1998
KCXM            79            1998


Station     Record High       Year(s)
-------     -----------       -------
KCHS            98            1989
KSAV            98            1989, 1962
KCXM            95            1962, 1926

Station     Record High Min   Year(s)
-------     ---------------   -------
KCHS            76            1991
KSAV            77            1878
KCXM            78            1991


Station     Record High       Year(s)
-------     -----------       -------
KCHS            97            1967, 1964
KSAV            96            1964, 1898
KCXM            93            2000

Station     Record High Min   Year(s)
-------     ---------------   -------
KCHS            76            2000
KSAV            76            1885
KCXM            80            2000


Station     Record High       Year(s)
-------     -----------       -------
KCHS            95            1945
KSAV            98            1945, 1898

Station     Record High Min   Year(s)
-------     ---------------   -------
KCHS            76            2018
KSAV            75            1885
KCXM            77            1998


Station     Record High       Year(s)
-------     -----------       -------
KCHS            95            2004
KSAV           101            1898

Station     Record High Min   Year(s)
-------     ---------------   -------
KCHS            75            1982
KSAV            75            1924
KCXM            78            1998