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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 241446

Area Forecast Discussion
National Weather Service Charleston SC
1046 AM EDT Fri May 24 2019

Unseasonably hot high pressure will dominate into late next


As of 1045 AM: Visible satellite indicated a well developed 
land breeze over the coastal waters, with no cloud cover over 
land. I will update the forecast to reduce sky cover over the 
CWA through the day. In addition, based on latest MOS, I will 
update to lower high temperatures a few degrees. However, 
dewpoints will remain in the mid to upper 60s along the coast, 
with low to mid 60s inland, resulting in a small change in peak 
heat index.

Previous Discussion:
The RAP and H3R have trended about an hour slower with the 
progression of the pinned sea breeze this afternoon. They both 
have also increased highs by another degree with a sliver of 
highs 99-100 from roughly Metter and Reidsville east to near 
Springfield. Did not make any changes to the high temperature 
forecast with the sunrise update, but if a slight increase in 
highs may need to be introduced later this morning if short term
models continue to trend in this direction.

Today will be the first in a prolonged series of unseasonably 
hot days for Southeast South Carolina and Southeast Georgia. 
Deep layered high pressure will remain anchored across the 
Southeast U.S. today with the local forecast area positioned 
along the eastern flanks of the mid-upper level anticyclone 
centered over west-central Georgia. The combination of a 
downslope northwest flow aloft and rising H8 temperature to near
19C will promote an unseasonably hot day with little cloud 
cover. Noted the latest NAEFS 1000 hPa temperatures are forecast
to be ~2-2.5 standard deviations above average for late May.

The synoptic pattern favors a pinned sea breeze scenario with 
the circulation likely not making any inland progress until late
afternoon. This typically allows hot temperatures to reach all 
the way to the Intracoastal Waterway. The various statistical 
guidance packages typically under forecast high temperatures in 
downslope flow regimes, especially in drought conditions, and 
this was taken into account when constructing today's high 
temperature forecast. Opted to nudge highs closer to the RAP/H3R
consensus which is about 2-3 degrees warmer yesterday's 
afternoon forecast cycle. This places highs ranging from the 
upper 90s inland to as far east as the Highway 17 corridor in 
Southeast South Carolina and I-95 in Southeast Georgia to the 
upper 80s/near 90 at the beaches. Even Downtown Charleston could
reach 95-97 degrees with west winds before cooling off the 
passage of the sea breeze. There are signals that a few spots 
in interior Southeast Georgia could reach 100 degrees, especially 
in the Metter-Claxton-Reidsville corridor. Dewpoints are 
expected to mix out into the lower-mid 60s away from the coast, 
but will pool back into the lower 70s across parts of the 
coastal counties in the vicinity of the late afternoon sea 
breeze. The resulting heat indices should peak roughly in the 
100-103 range for most locations which is below the usual pre-
July 1 Heat Advisory criteria of 105.

Since this is the first day of what will likely become a 
prolonged heat event, an impact-based Heat Advisory will not be 
issued for today per coordination with the neighboring NWS 
Weather Forecast Offices (WFOs). A Special Weather Statement 
will be issued to address today's heat instead. The need for 
impact-based Heat Advisories will be coordinated over the coming
days on a case-by-case basis.


Tonight: Modest radiational cooling is expected as the boundary
layer is poised to decouple quickly after sunset. A weak
backdoor cold front will approach the Santee River by daybreak
Saturday. Dry weather will continue under mainly clear skies.
Lows will range from the upper 60s/lower 70s inland to the 
upper 70s at the beaches and Downtown Charleston.

Saturday through Monday: The mid-levels will consist of strong 
high pressure hovering over the Southeast Saturday and Sunday, 
with the high drifting southwards into the northern Gulf of 
Mexico on Monday. Heights over our area will slowly drop through
the short term. However, this won't be very noticeable because 
the high is unseasonably strong. NAEFS shows 500 mb heights 
peaking at about 3 standard deviations above normal. It also 
shows above normal temperatures at almost all levels of the 
atmosphere. At the surface, a weakening front will pass just 
north and then offshore of our area on Saturday. Otherwise, 
areas of high pressure will stretch from the western Atlantic 
into the Southeast states. The only risk of "convection" could 
be across the Charleston Tri- County area Saturday afternoon. 
But this will be a big hurdle to jump over given all of the dry 
air in place across the area as shown by model soundings. Hence,
we have slight chance POPs in these locations and it's possible
this may be overdone. Otherwise, the main moisture will be fair
weather clouds each afternoon. Temperatures are forecasted to 
approach or exceed record values throughout the short term. For 
the high temperatures, we went with a blend of models, thickness
values, and accounted for the sea breeze (if any). Saturday 
will be the "coolest" day with temperatures ranging from the 
mid/upper 90s for most locations and maybe a few century marks 
near the Altamaha River. Sunday, expect widespread upper 90s to 
lower 100s. Monday may be the hottest day of the bunch because 
the sea breeze may remain pinned to the coast. If so, most 
locations away from the immediate coast could reach at least 100
degrees for highs. Lows each night will cool down into the 70s.
Heat indices are forecasted to be above 100 degrees each day. 
Heat Advisories could be needed, especially Sunday and Monday. 
Users who are curious about the records can look at the short 
Climate section below.


The mid-level ridge will remain in place while surface high pressure 
persists. Temperatures may trend downward a few degrees each day. 
However, they are still forecasted to remain well above normal 
throughout the long term. Additionally, no rainfall is in the 
forecast. Heat advisories, poor air quality, and dry fuels that 
could increase the risk of wildfires are all concerns. But the 
details will need to be better defined as this time period gets 



Extended Aviation Outlook: VFR.


Today: West winds will back during the day as sea breeze
circulation develops along the beaches. Winds will increase in
the afternoon as an intense thermal gradient develops between
the ocean and inland locations. Winds are expected to peak 10-15
kt for most locations with 15 kt or 15-20 kt for the Charleston
County waters. Seas will average 1-3 ft.

Tonight: southwest winds will slowly veer to the west after
midnight as a backdoor cold front approaches from the north.
winds will remain 15 kt or less with seas 1-3 ft.

Saturday through Tuesday: Atlantic high pressure will prevail. 
The sea breeze should develop along the coast each afternoon, 
bringing periods of gusty winds. However, no advisories are 
expected. Winds are then forecasted to ease somewhat each 
evening. Waves will generally peak in the 3-4 ft range.


A significant, prolonged heat wave will challenge or break many 
of these records:

KCHS: 99 set May 21, 1938.
KSAV: 101 set May 30, 1898 and May 31, 1945.
KCXM: 99 set May 21, 1938 and May 26, 1953.


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