Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 271820
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
220 PM EDT FRI MAR 27 2015

.SYNOPSIS...
A STRONG COLD FRONT WILL PASS THROUGH THE AREA THIS AFTERNOON.
UNSEASONABLY COLD HIGH PRESSURE WILL PREVAIL THIS WEEKEND. FROST
AND AN INLAND FREEZE COULD OCCUR LATE SATURDAY NIGHT INTO EARLY
SUNDAY. ANOTHER COLD FRONT WILL MOVE THROUGH LATE MONDAY INTO
EARLY TUESDAY...EVENTUALLY LIFTING BACK NORTH AS A WARM
FRONT. WARMER HIGH PRESSURE WILL THEN BUILD OVER THE REGION
THROUGH THE MIDDLE OF NEXT WEEK. YET ANOTHER COLD FRONT COULD
APPROACH FRIDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
TODAY...A LINE OF CONVECTION AHEAD OF A STRONG COLD FRONT IS
CURRENTLY BISECTING THE FORECAST AREA AND STEADILY PROGRESSING
EAST. WEAK INSTABILITY HAS SUPPORTED ISOLATED THUNDER WITHIN THIS
LINE...HOWEVER THE MAJORITY OF THE ACTIVITY WILL BE LIGHT TO
MODERATE SHOWERS. RAIN COVERAGE WILL STEADILY END FROM SOUTHWEST
TO NORTHEAST AS THE FRONT SHIFTS TOWARD THE COAST DURING THE NEXT
SEVERAL HOURS AND CONVECTION LIFTS EAST/NORTHEAST ACCORDINGLY.

ALTHOUGH BREAKS IN THE THICK CLOUD COVER ARE POSSIBLE THROUGHOUT
THE DAY...WILL CONTINUE TO ADVERTISE MOSTLY CLOUDY CONDITIONS.
WARM ADVECTION IN ADVANCE OF THE FRONT...AS WELL AS PRE-FRONTAL
COMPRESSION EFFECTS...HAS ALLOWED TEMPERATURES TO RISE INTO THE
UPPER 60S TO LOW 70S. DEEP MIXING HAS DEVELOPED ACROSS PORTIONS OF
THE AREA THIS AFTERNOON AND SUPPORTED FREQUENT GUSTS.
WEST/NORTHWEST WINDS WILL PEAK IN THE 20 TO 25 MPH RANGE.

TONIGHT...ALTHOUGH THE DEEP MOISTURE WILL HAVE PULLED EAST WITH
THE COLD FRONT...THERE IS STILL SOME LINGERING LOW LEVEL MOISTURE
WITHIN THE FIRST COUPLE OF MILES OFF THE SURFACE. A POTENT SHORT
WAVE EMBEDDED WITHIN THE LONG WAVE TROUGH ALOFT WILL MOVE DIRECTLY
THROUGH THE FORECAST AREA AROUND 05-10Z. THIS WILL SUPPORT 20-30
POPS...MAINLY ACROSS THE CHARLESTON TRI-COUNTY REGION WHERE THE
BEST MOISTURE WILL REMAIN. ALTHOUGH COLDER AIR WILL ARRIVE BEHIND
THIS FEATURE...IT WILL BE SUFFICIENTLY DELAYED TO PREVENT ANYTHING
MORE THAN ISOLATED TO SCATTERED SHOWERS. HOWEVER...STEEP MID LEVEL
LAPSE RATES WITH 500 MB TEMPS AROUND -26 OR -27C COULD ALLOW FOR
SOME SOFT HAIL OR GRAUPEL TO OCCUR IN A FEW OF THE SHOWERS AS THE
SHORT WAVE SWINGS THROUGH THE AREA. 850 MB TEMPS FALL TO AN
ANOMALOUSLY COLD -1C SOUTH AND -4C NORTH AND WILL ALLOW FOR TEMPS
TO PLUNGE DOWN INTO THE UPPER 30S AND LOWER 40S BY DAYBREAK
SATURDAY. THIS WILL BE ABOUT 20-25F COLDER THAN THIS MORNING.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY/...
SATURDAY MORNING...A STRONG SHORTWAVE EMBEDDED WITHIN A LONGWAVE
TROUGH ALOFT 2 TO 3 STANDARD DEVIATIONS DEEPER THAN NORMAL FOR LATE
MARCH WILL PUSH THROUGH THE REGION. MODEL SOUNDINGS SUGGEST A WINDOW
OF OPPORTUNITY FOR CONVECTION SUPPORTED BY VERY COLD AIR
ALOFT/STEEPENING MID-LEVEL LAPSE RATES...BEFORE A SUBSIDENCE
INVERSION DEVELOPS SATURDAY AFTERNOON. HOWEVER...THE PROBABILITY FOR
ANY MEASURABLE PRECIPITATION IN THE FORM OF RAIN SHOWERS AND EVEN
ISOLATED GRAUPEL/LIGHTNING REMAINS TOO LOW...BELOW 15 PERCENT...TO
JUSTIFY A MENTION WITHIN PUBLIC FORECASTS. WILL CONTINUE TO ASSESS.

OTHERWISE...THE MAIN STORY THIS WEEKEND WILL CENTER ON UNSEASONABLY
COOL TEMPERATURES...STRUGGLING TO REACH 60F MOST AREAS EACH
DAY...EXCEPT IN THE LOWER/MID 60S NEAR THE ALTAMAHA RIVER. THESE
HIGH TEMPERATURES WILL AVERAGE 10-14F BELOW NORMAL.

OF GREATEST IMPORT...COLD HIGH PRESSURE WILL SHIFT SOUTHWARD/TOWARD
THE REGION SATURDAY NIGHT/EARLY SUNDAY. THE POTENTIAL FOR BOUNDARY
LAYER DECOUPLING/EFFICIENT RADIATIONAL COOLING WILL INCREASE...AND
TEMPERATURES ARE EXPECTED TO FALL INTO THE 30S AWAY FROM THE
IMMEDIATE COAST. IN FACT...A COUPLE OF HOURS OF SUBFREEZING
TEMPERATURES COULD OCCUR WELL INLAND ESPECIALLY IF WINDS BECOME
CALM. PER COLLABORATION WITH WFOS CAE AND ILM OPTED NOT TO ISSUE
LATE 4TH PERIOD/EARLY 5TH PERIOD FREEZE WATCHES. HOWEVER...ADDED A
MENTION OF FROST AWAY FROM THE COAST...AND FREEZE WATCHES/WARNINGS
AND/OR FROST ADVISORIES COULD EVENTUALLY BE REQUIRED.

MONDAY...THE NEXT COLD FRONT WILL APPROACH THE REGION. AHEAD OF THIS
FEATURE...TEMPERATURES SHOULD RECOVER INTO THE UPPER 60S/LOWER 70S.
ALSO...AT LEAST SLIGHT CHANCE/CHANCE POPS AS AN AXIS OF FORCING FOR
ASCENT/AN ASSOCIATED BAND OF ENHANCED MOISTURE ADVANCES INTO THE
REGION FROM THE NORTHWEST. ISOLATED THUNDERSTORMS CANNOT BE RULED
OUT...BUT INSTABILITY APPEARS MARGINAL THUS OPTED NOT TO ADD A
MENTION OF THUNDER WITHIN THE FRIDAY MORNING FORECAST.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
THE LATE MONDAY/EARLY TUESDAY COLD FRONT WILL WEAKEN...STALL THEN
WILL EITHER DISSIPATE OR PUSH NORTH AS A WARM FRONT. THUS...A
WARMING TREND IS EXPECTED WITH TEMPERATURES NEAR NORMAL TUESDAY
THEN ABOVE NORMAL...AT LEAST IN THE MID/UPPER 70S WEDNESDAY
THROUGH LATE WEEK. MORE SUNSHINE THAN EXPECTED WEDNESDAY WILL
SUPPORT HIGHS AROUND 80F. THE NEXT OPPORTUNITY FOR SIGNIFICANT
PRECIPITATION WILL OCCUR AFTER WEDNESDAY AS A SOUTHERN SHORTWAVE
TROUGH PUSHES THROUGH THE REGION. TIMING REMAINS UNCERTAIN...POPS
COULD INCREASE AS EARLY AS LATER WEDNESDAY OR WEDNESDAY NIGHT AND
COULD LINGER INTO FRIDAY AS ANOTHER WEAK COLD FRONT ADVANCES INTO
THE REGION...BUT HIGHEST POPS...CAPPED AT CHANCE FOR NOW...ARE IN
ORDER THURSDAY INTO THURSDAY NIGHT. ALSO...MODELS DEPICT
SUFFICIENT INSTABILITY FOR JUSTIFY A MENTION OF THUNDER THURSDAY
AND FRIDAY.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
CEILINGS AT BOTH TERMINALS WILL LIKELY WAVER BETWEEN MARGINAL
MVFR AND VFR THROUGH THE AFTERNOON AHEAD OF A STRONG COLD FRONT
PROGRESSING THROUGH THE FORECAST AREA. A BAND OF SHOWERS AND
ISOLATED THUNDERSTORMS ARE STEADILY SHIFTING TOWARD BOTH
TERMINALS BY 2 PM EDT. HAVE INCLUDED A TEMPO GROUP TO ADDRESS THE
POTENTIAL FOR SHOWERS TO AFFECT KCHS AND KSAV THROUGH ABOUT
20Z...AND WILL MONITOR TRENDS IN CASE THUNDER NEEDS TO BE ADDED TO
THE TAF. CONDITIONS WILL IMPROVE BACK TO PREVAILING VFR THIS
EVENING AND TONIGHT BEHIND THE COLD FRONT...AS DRIER AIR STEADILY
ADVECTS INTO THE REGION.

SOUTHWEST/WEST WINDS WILL BE GUSTY THROUGH THE AFTERNOON AND
PERHAPS INTO THE EVENING HOURS...BEFORE DIMINISHING OVERNIGHT.
COMPLETE DECOUPLING IS NOT EXPECTED DUE TO A TIGHTENED PRESSURE
GRADIENT BEHIND THE DEPARTING COLD FRONT.

EXTENDED AVIATION OUTLOOK...FLIGHT RESTRICTIONS POSSIBLE IN SHOWERS
MONDAY. OTHERWISE...MAINLY VFR.

&&

.MARINE...
TODAY...A STRONG COLD FRONT WILL REACH THE COASTAL WATERS IN THE
NEXT SEVERAL HOURS...PASSING TO THE EAST/SOUTHEAST BY LATE
AFTERNOON. DESPITE SOME MARINE LAYERING EFFECTS WITHIN THE WARM
ADVECTION...A FAIRLY DECENT PRESSURE GRADIENT WILL ASSIST IN
INCREASING SOUTHWEST WINDS INTO THE 15 TO 20 KT RANGE. THERE WILL
EVEN BE FREQUENT GUSTS TO NEAR 25 KT ON THE OUTER GEORGIA WATERS
AND INFREQUENT GUSTS TO 25 KT ON THE CHARLESTON COUNTY ATLANTIC
WATERS. THIS WILL BE SUFFICIENT TO ALLOW SEAS TO BUILD TO 6 FT ON
THESE AFOREMENTIONED MARINE ZONES...SUPPORTING MARGINAL SMALL
CRAFT ADVISORIES. THE OTHER ZONES WILL STAY JUST BELOW ANY
ADVISORY CONDITIONS THROUGH THE DAY. MEANWHILE...PATCHY SEA FOG
WILL PERSIST INTO THE AFTERNOON...UNTIL WINDS VEER TO WEST AND
SCOUR OUT THE FOG. MARINERS ARE ALSO ADVISED THAT A FEW
THUNDERSTORMS COULD MOVE THROUGH THIS AFTERNOON.

TONIGHT...STEADY ISALLOBARIC PRESSURE RISES WILL DEVELOP BEHIND
THE DEPARTING COLD FRONT...KEEPING NW WINDS ELEVATED. AS A RESULT
HAVE RAISED ANOTHER SMALL CRAFT ADVISORY FOR THE NEARSHORE GEORGIA
WATERS WHERE FREQUENT GUSTS TO 25 KT ARE ANTICIPATED WITHIN THE
COLD ADVECTION. THIS WILL GO ALONG WITH AN ALREADY PRE-EXISTING
ADVISORY ON THE OUTER GEORGIA WATERS FOR SEAS OF 6 FT AND GUSTS
OF 25 OR 30 KT. CONDITIONS WILL BE CLOSE TO ADVISORY THRESHOLDS
ON THE SOUTH CAROLINA ATLANTIC WATERS...AND MAY STILL NEED TO
RAISE ADDITIONAL ADVISORIES AT A LATER TIME.

SATURDAY THROUGH WEDNESDAY...ELEVATED OFFSHORE WINDS AND ASSOCIATED
SMALL CRAFT ADVISORY CONDITIONS SHOULD RELAX AS HIGH PRESSURE SHIFTS
TOWARD THE WATERS SATURDAY AFTERNOON THROUGH SATURDAY EVENING.
HOWEVER...LATE SATURDAY NIGHT INTO SUNDAY GUIDANCE DEPICTS A SURGE
OF NORTHEAST WINDS WHICH COULD PRODUCE A FEW HOURS OF LOCALIZED
SMALL CRAFT ADVISORY CONDITIONS. THEN...AS HIGH PRESSURE SHIFTS
SOUTH OF THE WATERS SUNDAY NIGHT INTO MONDAY...ONSHORE WINDS 10 KT
OR LESS SHOULD VEER TOWARD THE SOUTHWEST AND INCREASE IN
SPEED...ESPECIALLY MONDAY AHEAD OF AN APPROACHING COLD FRONT.

AFTER A WEAKENING COLD FRONT PASSES THEN STALLS MONDAY NIGHT OR
EARLY TUESDAY...WINDS AND SEAS WILL REMAIN WELL BELOW SCA LEVELS
THROUGH MID WEEK.

SEAS WILL CHANGE DURING VIRTUALLY EACH PERIOD OF THIS FORECAST. SEAS
WILL BEGIN SATURDAY AT 2-4 FT WITHIN NEARSHORE WATERS AND 4-6 FT
BEYOND 20 NM WILL SUBSIDE TO 1-3 FT BY SATURDAY EVENING...THEN WILL
BUILD TO 2-4 FT AND PERHAPS AS HIGH AS 5 FT BEYOND 20 NM LATE
SATURDAY NIGHT THROUGH SUNDAY MORNING. SUNDAY NIGHT...SEAS SHOULD
SUBSIDE TO 1-2 FT...THEN INCREASING WINDS WILL SUPPORT SEAS BUILDING
TO AT LEAST 3-5 FT MONDAY. SEAS SHOULD THEN SUBSIDE TO 1-2 FT AGAIN
BY MID WEEK.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 1 PM EDT SATURDAY FOR AMZ374.
     SMALL CRAFT ADVISORY UNTIL 11 PM EDT THIS EVENING FOR AMZ350.
     SMALL CRAFT ADVISORY FROM 11 PM THIS EVENING TO 11 AM EDT
     SATURDAY FOR AMZ354.

&&

$$
SYNOPSIS...
NEAR TERM...WMS/
SHORT TERM...SPR
LONG TERM...SPR
AVIATION...WMS/
MARINE...


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