894
FXUS62 KCHS 201431
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
931 AM EST THU FEB 20 2025
SYNOPSIS
COLD HIGH PRESSURE WILL BUILD INTO THE REGION THROUGH THE START
OF THE WEEKEND. AN AREA OF LOW PRESSURE WILL DEVELOP OFF THE
COAST ON SUNDAY, FOLLOWED BY MORE HIGH PRESSURE INTO EARLY NEXT
WEEK.
NEAR TERM /THROUGH TONIGHT/
MINOR CHANGES WERE MADE TO BOTH THE HOURLY TEMPERATURES AND DEW
POINTS TO MATCH CURRENT OBSERVATIONS, OTHERWISE NO MAJOR CHANGES
WERE MADE WITH THE MID MORNING UPDATE.
MODELS INDICATE THE DOWNSLOPE FLOW WILL ERODE THE LOW CLOUDS FROM
WEST TO EAST, BEGINNING ALONG THE EAST FACING SLOPES OF THE
APPALACHIANS BETWEEN 13-14Z THIS MORNING. BASED ON HREF SIMULATED
CLOUD COVER, THE BACK EDGE OF THE STRATUS SHOULD REACH SE GA DURING
THE MID-DAY AND THE SC LOWCOUNTRY LATE THIS AFTERNOON. INCREASED
INSOLATION SHOULD ALLOW TEMPERATURES TO WARM TO AROUND 50 DEGREES
ACROSS SE GA, RESULTING IN MIXING TO 4 KFT. MIXING SHOULD PRODUCE A
PERIOD OF MOMENTUM TRANSFER BEGINNING AROUND 17Z, SUPPORTING GUSTS
BETWEEN 20 TO 25 MPH. CLOUD COVER WILL LIMIT HEATING ACROSS THE SC
LOWCOUNTRY, HIGH TEMPERATURES SHOULD BE LIMITED TO THE MID 40S.
RECENT RUNS OF THE HRRR SHOWS PATCHES OF DRIZZLE OR FLURRIES
DEVELOPING ACROSS THE UPSTATE OF SC THIS MORNING. THIS ACTIVITY
APPEARS LARGELY SUPPORTED BY THE JET CIRCULATION, ENTRANCE REGION,
OF A H25 160 KT JET. THE PRECIPITATION WILL BE TRANSPORTED BY NW
LLVL WINDS. GIVEN THE DOWNSLOPE FLOW, THE PRECIPITATION SHOULD
GRADUALLY DISSIPATE AS IT TRACKS SE. RECENT RUNS OF THE HRRR
SIMULATED REFLECTIVITY INDICATES THAT SUB 30 DBZ MAY CROSS THE
SC LOWCOUNTRY LATE THIS AFTERNOON. THE FORECAST WILL FEATURE
POPS LESS THAN 15 PERCENT, NO WEATHER WILL BE MENTIONED.
HOWEVER, A REPORT OF DRIZZLE OR SNOW GRAINS CANNOT BE RULED OUT
THIS AFTERNOON.
TONIGHT, AN ARCTIC AIR MASS WILL SPREAD ACROSS THE FORECAST
AREA TONIGHT. THE CENTER OF THE 1040 MB HIGH WILL REMAIN
CENTERED OVER THE GREAT PLAINS OVERNIGHT. GIVEN COLD LLVL
THICKNESSES AND CLEAR SKY, TEMPERATURES SHOULD STEADILY COOL
THROUGH THE NIGHT. USING A BLEND OF THE COOLER MOS MEMBERS, LOW
TEMPERATURES ARE FORECAST TO RANGE IN THE LOW TO MID 20S. IN
FACT, THE LOW TEMPERATURE AT KCHS MAY REACH THE RECORD LOW
TEMPERATURE OF 22 DEGREES SET IN 1958. NORTH WINDS ARE FORECAST
TO REMAIN BETWEEN 5 TO 10 MPH OVERNIGHT, RESULTING IN WIND CHILL
VALUES TO FALL TO 14 TO 17 DEGREES. A COLD WEATHER ADVISORY HAS
BEEN POSTED FOR THE ENTIRE FORECAST AREA TONIGHT, ENDING BY
MID-MORNING FRIDAY.
SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/
A FEW MORE HOURS OF BELOW 20 DEGREE WIND CHILLS FRIDAY MORNING AS
THE SUN RISES, GETTING ABOVE THE 20 DEGREE THRESHOLD BY THE MID-
MORNING HOURS. BE SURE TO BUNDLE UP BEFORE HEADING OUT THE DOOR,
ESPECIALLY THE KIDS WAITING FOR THE SCHOOL BUS. THROUGHOUT THE DAY A
SURFACE HIGH PRESSURE WILL BE MOVING EASTWARDS OUT OF THE CENTRAL
CONUS, KEEPING THE AREA DRY AND CHILLY WITH HIGHS IN THE MID 40S TO
LOWER 50S. ENSEMBLE SITUATIONAL AWARENESS TABLES (ESATS) SHOW
TEMPERATURES AT THE SURFACE REMAIN IN THE BOTTOM 2.5TH PERCENTILE OF
CLIMATOLOGY, SETTING UP CHS TO POSSIBLY BREAK A RECORD LOW MAX
TEMPERATURE (THE COLDEST HIGH TEMPERATURE FOR THE DATE) OF 47
DEGREES WITH A HIGH OF 46, THOUGH SAV AND CXM HAVE MUCH COLDER
RECORDS OUT OF REACH IN THE MID 30S.
AS THE HIGH PRESSURE CONTINUES TO SLIDE EASTWARDS, WE'LL HAVE ONE
MORE OVERNIGHT PERIOD INTO SATURDAY WITH LOWS IN THE MID 20S TO
LOWER 30S. WINDS LOOK TO REMAIN WEAK ENOUGH TO KEEP MINIMUM WIND
CHILLS IN THE MID 20S TO LOWER 30S, ABOVE CRITICAL THRESHOLDS.
QUIET CONDITIONS CONTINUE ON SATURDAY AS THE SURFACE HIGH
PRESSURE BEGINS TO WEAKEN, WITH SOUTHERLY WINDS EXPECTED BY THE
AFTERNOON HOURS AS AN 850MB HIGH DEVELOPS OFF TO OUR SOUTH. THIS
WILL SEND WARM AIR ADVECTION (WAA) BACK ACROSS THE REGION,
BRINGING THE REGION UP INTO THE LOWER TO MID 50S ACROSS SC AND
MID 50S TO LOWER 60S ACROSS GA.
THE WAA COMBINED WITH A SHORTWAVE MOVING THROUGH QUASI-ZONAL FLOW
WILL LEAD TO THE DEVELOPMENT OF A COASTAL LOW, WHICH IS EXPECTED TO
MOVE NORTHEAST UP THE COAST DURING THE EARLY MORNING HOURS ON
SUNDAY. DETERMINISTIC AND ENSEMBLE GUIDANCE CONTINUES TO DISAGREE ON
WHETHER OR NOT PRECIPITATION WILL REACH THE COASTAL AREAS, WITH THE
LATEST NBM KEEPING RAIN CHANCES OFF THE COAST. GIVEN THE 10-20%
CHANCES SEEN IN THE GEFS AND GEPS FOR SOME LIGHT RAIN FOR THE
CHARLESTON AREA, DID INTRODUCE 15-20% POPS FOR LIGHT RAIN DURING THE
LATE EVENING SATURDAY INTO SUNDAY MORNING, WHICH ALSO CARRIES
SUPPORT VIA ENSEMBLE CLUSTERING. SOME HINTS VIA THE LATEST 00.Z
GUIDANCE THAT CHANCES FOR RAIN MAY NEED TO BE INCREASED OVERNIGHT
INTO MONDAY, BUT FOR NOW THE NBM DOES HAVE SOME LOW (<20%) FOR LIGHT
SHOWERS ALONG THE COAST.
LONG TERM /MONDAY THROUGH WEDNESDAY/
HIGH PRESSURE BUILDS ALONG THE GULF COAST KEEPING THE AREA DRY AND
WARM THROUGH TUESDAY. WHILE THE TIMING IS NOT WELL AGREED UPON,
ENSEMBLE GUIDANCE DOES SHOW THAT A BROAD AREA OF LOW PRESSURE WILL
BE MOVING OVER THE GREAT LAKES REGION, WITH UPPER LEVEL TROUGHING
EXTENDING DOWN INTO THE FORECAST AREA OVERNIGHT INTO WEDNESDAY.
CHANCES FOR RAIN OUT OF THAT SYSTEM ARE NOT WELL AGREED UPON, WITH
THE NBM KEEPING THE REGION DRY WITH THAT PASSAGE.
DETERMINISTIC AND ENSEMBLES GUIDANCE SHOWS ANOTHER TROUGH DEVELOP
ACROSS THE ROCKIES ON WEDNESDAY, WITH A SURFACE LOW DEVELOPING ON
THE LEESIDE OF THE ROCKIES AS IT MOVES ONTO THE CENTRAL PLAINS. WILL
LIKELY SEE SOME RAIN OUT OF THE SYSTEM BY LATE THURSDAY INTO FRIDAY,
WITH TEMPERATURES LOOKING TO STAY SEASONABLE/WARM IN THE 60S AND
70S.
AVIATION /15Z THURSDAY THROUGH MONDAY/
PRIOR TO THE 12Z TAFS, IR SATELLITE AND GROUND BASED
OBSERVATIONS INDICATED WIDESPREAD IFR, WITH POCKETS OF LIFR,
CONDITIONS ACROSS THE COASTAL PLAIN OF GA/SC. THE TAFS WILL BE
INITIALIZED WITH IFR CEILINGS AND STEADY NORTH-NORTHWEST WINDS.
CONDITIONS SHOULD CHANGE LITTLE THROUGH EARLY THIS MORNING.
MODELS INDICATE THE DOWNSLOPE FLOW WILL ERODE THE LOW CLOUDS
FROM WEST TO EAST, BEGINNING ALONG THE EAST FACING SLOPES OF THE
APPALACHIANS BY 13Z. GIVEN SOME DAYTIME HEATING, CLOUD BASES AT
KCHS/KJZI ARE FORECAST TO LIFT TO MVFR BETWEEN 17-18Z. BASED ON
HREF SIMULATED CLOUD COVER, THE BACK EDGE OF THE RESTRICTIVE
CEILINGS SHOULD REACH KSAV BY 18Z AND KCHS/KJZI LATE THIS
AFTERNOON. INCREASED INSOLATION SHOULD ALLOW TEMPERATURES TO
WARM TO AROUND 50 DEGREES AT KSAV, RESULTING IN MIXING TO 4 KFT.
MIXING SHOULD PRODUCE A PERIOD OF MOMENTUM TRANSFER BEGINNING
AROUND 18Z. KSAV IS FORECAST TO SEE NW WINDS INCREASE TO 15KTS
WITH GUSTS BETWEEN 20-25 KTS AT 18Z UNTIL 23Z. BY EARLY THIS
EVENING, ALL TERMINALS SHOULD BECOME CLEAR TO MOSTLY CLEAR WITH
NW WINDS AROUND 10 KTS.
EXTENDED AVIATION OUTLOOK: VFR CONDITIONS ARE EXPECTED TO
PREVAIL.
MARINE
TODAY THROUGH TONIGHT, THE COMBINATION OF A STEEPENING PRESSURE
GRADIENT AND COLD AIR ADVECTION SHOULD SUPPORT GUSTY NW WINDS. GUSTS
ARE FORECAST TO RANGE BETWEEN 25 TO 30 KTS ACROSS THE CHARLESTON
COUNTY NEARSHORE AND OUTER GA WATERS. SEAS WILL RANGE BETWEEN 3 TO 6
FT. SMALL CRAFT ADVISORIES WILL REMAIN IN EFFECT TO HIGHLIGHT THE
WIND AND WAVE HEIGHTS.
FRIDAY THROUGH TUESDAY: HIGH PRESSURE WILL RETURN TO THE REGION
FRIDAY THROUGH SATURDAY. WINDS AND SEAS SHOULD REMAIN BELOW SCA
CRITERIA. ANOTHER COASTAL LOW MAY DEVELOP OVER THE WATERS ON SUNDAY,
MOVING AWAY FROM THE REGION ON MONDAY. THERE MAY BE AN INCREASE IN
WINDS AND SEAS, BUT CONDITIONS SHOULD REMAIN BELOW ADVISORY
CRITERIA. HIGH PRESSURE WILL EXPAND ACROSS THE MARINE ZONES ON
TUESDAY. SOUTHWEST WINDS ARE FORECAST TO RANGE BETWEEN 10-15 KTS WITH
SEAS BETWEEN 2-3 FT.
CLIMATE
RECORD LOW MAXIMUM TEMPERATURES:
FEBRUARY 21:
KCHS: 47/2020
RECORD LOW TEMPERATURES:
FEBRUARY 21:
KCHS: 22/1958
CHS WATCHES/WARNINGS/ADVISORIES
GA...COLD WEATHER ADVISORY FROM 10 PM THIS EVENING TO 10 AM EST
FRIDAY FOR GAZ087-088-099>101-114>119-137>141.
SC...COLD WEATHER ADVISORY FROM 10 PM THIS EVENING TO 10 AM EST
FRIDAY FOR SCZ040-042>045-047>052.
MARINE...SMALL CRAFT ADVISORY UNTIL 10 AM EST FRIDAY FOR AMZ350.
SMALL CRAFT ADVISORY UNTIL 5 PM EST FRIDAY FOR AMZ374.
NEAR TERM...CPM/NED
SHORT TERM...
LONG TERM...
AVIATION...NED
MARINE...NED
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