175
FXUS62 KCHS 200607
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
107 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 /UNTIL 6 AM THIS MORNING/
AT 1 AM, KCLX DETECTED LITTLE TO NO RETURNS ACROSS SE GA/SC WITH
ISOLATED SHOWERS OVER THE ADJACENT WATERS. THE SHOWERS OVER THE
NEARSHORE WATERS SHOULD PUSH TO THE NORTH AND EAST OVER THE NEXT
HOUR OR TWO. THE FORECAST UPDATE WILL FEATURE A SHARP DECREASE
IN POPS, THEN DRY THROUGH THE REST OF THE NIGHT. TEMPERATURES
ARE ON PACE TO RANGE IN THE LOW 30S, WITH MID 30S ACROSS EXTREME
SE GA.
SHORT TERM /6 AM THIS MORNING THROUGH SATURDAY/
THURSDAY AND THURSDAY NIGHT: ALOFT, THE FORECAST AREA WILL SIT JUST
TO THE SOUTH WITHIN ZONAL FLOW AROUND A CLOSED LOW TRACKING EAST
FROM THE OHIO VALLEY INTO THE MID ATLANTIC AND NEW ENGLAND. AT THE
SURFACE, A DEEPENING LOW OFF THE EAST COAST WILL TRACK OFF TO THE
NORTHEAST AS COLD HIGH PRESSURE BUILDS INTO THE REGION. DURING THE
DAY, A WELL DEFINED SURFACE TROUGH WILL PUSH SOUTHEASTWARD OUT OF
THE WESTERN CAROLINAS AND SHOULD GET INTO THE FORECAST AREA DURING
THE AFTERNOON HOURS. MODEL GUIDANCE IS IN GOOD AGREEMENT THAT AN
AREA OF SHOWERS WILL TRY TO DEVELOP ALONG THE SURFACE TROUGH
UPSTREAM OF THE AREA AND THEN GRADUALLY DIMINISH AS IT APPROACHES
DURING THE AFTERNOON. THERE ARE SOME INDICATIONS FROM HI-RES MODEL
OUTPUT THAT A FEW SHOWERS COULD MAKE IT INTO PORTIONS OF THE
CHARLESTON TRI-COUNTY REGION, BUT CURRENT THINKING IS THAT DOWNSLOPE
FLOW SHOULD HELP TO NEGATE SHOWER DEVELOPMENT AS IT ARRIVES. SO
WHILE THERE COULD BE A FEW LIGHT RETURNS ON RADAR, NO MEASURABLE
PRECIPITATION IS EXPECTED AND WE ARE NOW CARRYING A DRY FORECAST.
EXPECT A VERY CHILLY DAY THANKS TO LOW-LEVEL COLD ADVECTION AND
LINGERING LOW-LEVEL CLOUD COVER FOR MUCH OF THE AREA THROUGH THE
AFTERNOON. FORECAST HIGHS RANGE FROM THE LOW TO MID 40S FOR THE TRI-
COUNTY REGION TO THE UPPER 40S AND LOW 50S ACROSS SOUTHEAST GA. SUCH
VALUES WILL BE ON THE ORDER OF 15-20 DEGREES BELOW NORMAL. TO MAKE
IT FEEL EVEN MORE CHILLY, NORTHWEST WINDS WILL FREQUENTLY GUST INTO
THE 20-25 MPH RANGE WHICH WILL YIELD WIND CHILL VALUES IN THE UPPER
30S TO LOW 40S FOR SOUTHEAST GA AND THE MID TO UPPER 30S FOR
SOUTHEAST SC. OVERNIGHT, SKIES WILL CLEAR OUT AND THE MAIN STORY
WILL BE A VERY COLD NIGHT. LOWS ARE FORECAST TO DIP INTO THE LOW TO
MID 20S. WITH NORTH WINDS STAYING UP IN THE 5-10 MPH RANGE THROUGH
THE OVERNIGHT, WIND CHILLS ARE FORECAST TO FALL INTO THE UPPER TEENS
FOR NEARLY THE ENTIRE AREA LATE THURSDAY NIGHT AND EARLY FRIDAY
MORNING. A COLD WEATHER ADVISORY WILL VERY LIKELY BE NEEDED FOR THE
ENTIRE FORECAST AREA.
FRIDAY THROUGH SATURDAY: HIGH PRESSURE WILL PREVAIL ACROSS THE
REGION FOR FRIDAY AND SATURDAY. FRIDAY WILL BRING THE COLDEST
TEMPERATURES OF THE PERIOD WITH THE MORNING STARTING IN THE LOW TO
MID 20S AND HIGHS ONLY REACHING THE UPPER 40S IN MOST PLACES, WHICH
WOULD BE ON THE ORDER OF 15-18 DEGREES BELOW NORMAL. KCHS WILL
LIKELY COME CLOSE TO BOTH A RECORD LOW TEMPERATURE AND A RECORD LOW
MAXIMUM TEMPERATURE FOR 2/21 (SEE CLIMATE SECTION BELOW). FRIDAY
NIGHT WILL BE ANOTHER VERY COLD NIGHT, WITH LOWS IN THE MID TO UPPER
20S AWAY FROM THE IMMEDIATE COAST. SATURDAY IS EXPECTED TO BRING A
SOLID REBOUND IN TEMPERATURES WITH HIGHS IN THE MID TO UPPER 50S
ACROSS THE AREA.
LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/
AN AREA OF LOW PRESSURE IS EXPECTED TO DEVELOP OFF THE COAST ON
SUNDAY AND MOVE AWAY FROM THE AREA INTO MONDAY. WE ARE CARRYING A
DRY FORECAST, THOUGH THERE IS SOME SUPPORT IN DETERMINISTIC MODEL
OUTPUT FOR RAINFALL LATER SUNDAY INTO MONDAY MORNING. THEREAFTER,
HIGH PRESSURE SHOULD RETURN TO THE REGION AND THE FORECAST APPEARS
QUIET. TEMPERATURES SHOULD TREND WARMER THROUGH THE PERIOD, PERHAPS
GETTING BACK INTO THE 70S BY TUESDAY.
AVIATION /06Z THURSDAY THROUGH MONDAY/
PRIOR TO THE 6Z TAFS, IR SATELLITE AND GROUND BASED OBSERVATIONS
INDICATED WIDESPREAD IFR CONDITIONS ACROSS THE COASTAL PLAIN OF
GA/SC. THE TAFS WILL BE INITIALIZED WITH IFR CEILINGS AND STEADY
NORTH WINDS. CONDITIONS SHOULD CHANGE LITTLE THROUGH DAYBREAK
THURSDAY. 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. GIVEN SOME DAYTIME HEATING,
CLOUD BASES AT KCHS/KJZI IS 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 17Z 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 BEGIN
AROUND 17Z. KSAV IS FORECAST TO SEE NW WINDS INCREASE TO
15KTS WITH GUSTS BETWEEN 20-25 KTS AT 17Z 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
DECENT NORTH GRADIENT WILL CONTINUE TONIGHT, THOUGH THE GRADIENT
WILL RELAX A BIT LATER TONIGHT. 25 KT GUSTS AND 6 FT SEAS WILL
CONTINUE IN THE CHARLESTON NEARSHORE AND GA OFFSHORE WATERS.
OCCASIONAL GUSTS TO 25 KT REMAIN POSSIBLE IN THE REMAINDER OF
THE NEARSHORE WATERS BUT THE COVERAGE AND DURATION WILL BE
LIMITED.
THURSDAY THROUGH MONDAY: HIGH PRESSURE WILL BUILD IN FROM THE
NORTHWEST AND THE COMBINATION OF COLD ADVECTION AND A TIGHTENING
PRESSURE GRADIENT WILL PRODUCE STRONG NORTHWESTERLY TO NORTHERLY
FLOW ACROSS THE LOCAL WATERS. WINDS WILL PEAK THURSDAY NIGHT INTO
FRIDAY MORNING, WITH SUSTAINED WINDS OF 20-25 KNOTS AND GUSTS UP TO
30 KNOTS. SMALL CRAFT ADVISORIES ARE ALREADY IN EFFECT FOR THE
CHARLESTON COUNTY WATERS AND THE OUTER GA WATERS AND HAVE BEEN
EXTENDED THROUGH FRIDAY MORNING. WE WILL LIKELY ALSO NEED ADVISORIES
FOR THE SOUTHERN SC WATERS AND THE NEARSHORE GA WATERS, BUT THERE IS
ENOUGH OF A LAG TO HOLD OFF ON ISSUING FOR NOW. CONDITIONS WILL THEN
STEADILY IMPROVE FRIDAY AND INTO THE WEEKEND. OVERALL CONDITIONS
SHOULD REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS INTO EARLY NEXT
WEEK.
CLIMATE
RECORD LOW MAXIMUM TEMPERATURES:
FEBRUARY 21:
KCHS: 47/2020
RECORD LOW TEMPERATURES:
FEBRUARY 21:
KCHS: 22/1958
CHS WATCHES/WARNINGS/ADVISORIES
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 10 AM EST FRIDAY FOR AMZ350.
SMALL CRAFT ADVISORY UNTIL 1 PM EST FRIDAY FOR AMZ374.
NEAR TERM...NED
SHORT TERM...BSH
LONG TERM...BSH
AVIATION...NED
MARINE...BSH/CPM/JRL
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