Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 280225 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 1025 PM EDT MON APR 27 2015 .SYNOPSIS... HIGH PRESSURE WILL REMAIN NORTH OF THE AREA THROUGH TUESDAY. A STORM SYSTEM LIFTING OUT OF THE GULF COAST REGION WILL MOVE INTO SOUTHERN GEORGIA TUESDAY NIGHT AND JUST OFFSHORE OF THE SOUTH CAROLINA COAST ON WEDNESDAY. AS THE LOW PULLS AWAY WEDNESDAY NIGHT AND THURSDAY...HIGH PRESSURE WILL BEGIN TO BUILD BACK IN FROM THE NORTHWEST...GRADUALLY SETTLING OVER THE AREA FOR THE WEEKEND INTO EARLY NEXT WEEK. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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HIGH PRESSURE TO THE NORTH WILL EXPAND INTO THE REGION OVERNIGHT...PUSHING A SECONDARY SURGE OF DRY/COOL AIR THROUGH THE AREA AS THE PRESSURE GRADIENT TIGHTENS BETWEEN THE HIGH TO THE NORTH AND LOW PRESSURE OVER THE NORTHERN GULF OF MEXICO. AS A RESULT...LIGHT/VARIABLE WINDS WILL BECOME NE 5-15 MPH. HIGH CLOUDS WILL MAINTAIN CLOUDY/MOSTLY CLOUDY SKIES OVERNIGHT...ALTHOUGH CLOUDS COULD THIN FROM TIME TO TIME. CLOUD COVER AND THE PROSPECTS FOR INCREASED BOUNDARY LAYER MIXING OVERNIGHT COULD TRANSLATE TO UNEVEN TEMPERATURE TRENDS ESPECIALLY LATER TONIGHT/TOWARD DAYBREAK. LOWS SHOULD RANGE FROM THE UPPER 40S/NEAR 50F INLAND/NORTH TO THE LOWER 60S ALONG THE IMMEDIATE COAST...WITH A WIDE SWATH OF 50S IN BETWEEN.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/... TUESDAY...UPPER LEVEL MOISTURE WILL CONTINUE TO INCREASE DURING THE DAY AS THE STRONG SUBTROPICAL JET FEEDS EASTERN PACIFIC MOISTURE ACROSS MEXICO AND INTO THE SOUTHEASTERN UNITED STATES...TYPICAL OF A STRENGTHENING EL NINO. MEANWHILE...A WEAK AREA OF HIGH PRESSURE AT THE SURFACE WILL HOLD ON JUST A LITTLE LONGER ACROSS SOUTH CAROLINA DURING THE MORNING...SUPPLYING SOME LOW LEVEL DRY AIR...BEFORE GRADUALLY LIFTING OUT TO THE NORTHEAST. THESE MEANS QUITE A BIT OF HIGH CLOUD COVER...BUT A STRUGGLE TO GET MUCH RAIN ACROSS SC. THE MOISTURE WILL BE DEEPER IN GEORGIA...ESPECIALLY TOWARDS THE ALTAMAHA...SO THE CHANCES OF RAIN WILL INCREASE SOME DURING THE DAY. HOWEVER...THE BEST LIFT WILL STILL LAG...SO I DO NOT EXPECT A LOT OF RAIN BEFORE EVENING. TEMPS WILL BE GENERALLY COOL DUE TO THE CLOUD COVER AND WEAK COOL ADVECTION IN LOW LEVELS...WITH TEMPS STRUGGLING TO GET ABOVE 70. TUESDAY NIGHT...GFS AND NAM DIVERGE QUITE A BIT DURING TUESDAY NIGHT AS THE NAM HAS THE MAIN LOW IN SOUTHWESTERN GEORGIA BY DAWN...WHILE THE GFS STILL HAS THE LOWEST PRESSURES IN THE NORTHEASTERN GULF OF MEXICO. THIS DIFFERENCE IS CRITICAL IN DETERMINING THE MODE OF RAIN/CONVECTION...AS WELL AS THE THREAT FOR ANY SEVERE WEATHER. RIGHT NOW...I AM BLENDING ROUGHLY 2/3 GFS AND 1/3 NAM...THINKING THE GFS MAY HAVE A BIT BETTER HANDLE ON THE LEADING AIR MASS...WHICH SHOULD FORCE THE CONVECTION TO REMAIN OVER THE GULF TUESDAY NIGHT...ALONG WITH THE LOWEST PRESSURE. THEREFORE...I HAVE GONE WITH A LESS AGGRESSIVE RAIN SOLUTION TUESDAY NIGHT AND KEPT THE POPS BELOW MET GUIDANCE. WEDNESDAY...I EXPECT MOST IF NOT ALL THE AREA WILL SEE SHOWERS OR THUNDERSTORMS AT SOME POINT WEDNESDAY...BUT DUE TO DIFFERENCES IN THE MODELS I AM NOT THRILLED WITH TRYING TO PIN DOWN DETAILS 48 HOURS IN ADVANCE OF THE THREAT. IF STRONG CONVECTION DOES DEVELOP...I THINK IT SHOULD MAINLY BE CONFINED TO INTERSTATE 16 AND SOUTHWARD IN GEORGIA...AND POSSIBLY ALONG THE IMMEDIATE SOUTHERN SC COAST IF THE WARM SECTOR MANAGES TO SNEAK ONSHORE. THIS WILL BE ANOTHER CASE WHERE THE LIMITING FACTOR WILL BE THE LACK OF INSTABILITY. IN FACT...IF THE STORM GOES AS FAR SOUTH AS THE LATEST GFS INDICATES...THERE MAY BE VERY LITTLE OF THE WARM SECTOR MAKING IT INTO THE CWA. THE POSITIONING OF THE LOW TRACK AND ASSOCIATED WARM SECTOR WILL ALSO PLAY A ROLE IN MAXIMUM TEMPERATURES. THERE COULD BE A WIDE RANGE OF MAX TEMPS ON WEDNESDAY AS INTERIOR SOUTH CAROLINA LOOKS BARELY CERTAIN TO STAY IN THE COOL SECTOR...WHERE TEMPS WILL HAVE TO FIGHT RAIN...CLOUDS...AND COLD ADVECTION. EVEN MY CURRENT 68-70 FORECAST FOR THE NORTHWESTERN TIER MAY BE OPTIMISTIC. HOWEVER...I DO HAVE THE WARM SECTOR SNEAKING INTO FAR SOUTHEASTERN GEORGIA...WHERE 80 OR HIGHER IS POSSIBLE IF BREAKS IN THE CLOUDS DEVELOP. WEDNESDAY NIGHT...BY THIS TIME...THE SURFACE LOW SHOULD BE GRADUALLY MOVING NORTHEASTWARD AWAY FROM THE AREA. NORTHERLY AND NORTHWESTERLY WINDS WILL KICK IN BEHIND THE STORM...BUT LOW LEVEL MOISTURE WILL REMAIN STUBBORN...SO THERE SHOULD STILL BE QUITE A BIT OF CLOUD COVER OVERNIGHT...ALONG WITH A FEW SHOWERS. TEMPS SHOULD GENERALLY BOTTOM OUT IN THE 50S. THURSDAY...WHILE THE SURFACE LOW WILL CONTINUE TO PULL AWAY...A STRONG UPPER LOW WILL DIVE INTO THE MEAN TROF IN THE EAST. THE CYCLONIC FLOW ALOFT WILL KEEP SOME CLOUDS AROUND...AND A FEW SHOWERS MAY DEVELOP OVER THE SOUTH CAROLINA COUNTIES AS A RESULT OF SOME STEEPENING MID LEVEL LAPSE RATES. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... MEDIUM RANGE GUIDANCE IS IN GOOD AGREEMENT WITH THE OVERALL SYNOPTIC PATTERN IN THE LONG TERM PERIOD. IN THE MID AND UPPER LEVELS...A LONG WAVE TROUGH WILL LINGER OVER THE EASTERN U.S. FRIDAY AND SATURDAY BEFORE RIDGING GRADUALLY BUILDS FROM THE WEST INTO EARLY NEXT WEEK. MEANWHILE AT THE SURFACE...THE REGION WILL BE POSITIONED BETWEEN A STRONG LOW OFF THE MID-ATLANTIC/NORTHEAST COAST AND STRONG HIGH PRESSURE CENTERED OVER THE MISSISSIPPI VALLEY FRIDAY AND SATURDAY. THE HIGH WILL THEN BUILD OVER THE REGION THROUGH THE WEEKEND...BEFORE SHIFTING INTO THE ATLANTIC ON MONDAY. OVERALL...THIS PATTERN WILL FAVOR RAIN-FREE CONDITIONS WITH STRONG SUBSIDENCE AND LIMITED MOISTURE...ALTHOUGH SOME ISOLATED CONVECTION IS POSSIBLE MONDAY ONCE AN ONSHORE FLOW DEVELOPS AND THE SEA BREEZE BECOMES MORE PRONOUNCED. TEMPERATURES WILL MODERATE ON FRIDAY...THEN RETURN TO NEAR NORMAL OVER THE WEEKEND AND INTO EARLY NEXT WEEK. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... VFR. THE PROBABILITY FOR MVFR CEILINGS WILL BEGIN TO INCREASE AT KSAV TOWARD THE END OF THE 00Z TAF PERIOD. EXTENDED AVIATION OUTLOOK...MVFR OR LOWER CONDITIONS ARE PROBABLE LATE TUESDAY NIGHT AS A WARM FRONT LIFTS TOWARD THE AREA AND WEDNESDAY INTO WEDNESDAY NIGHT AS LOW PRESSURE PASSES NEARBY AND SHOWERS/THUNDERSTORMS BECOME LIKELY. VFR CONDITIONS ARE EXPECTED TO PREVAIL THURSDAY THROUGH SATURDAY. && .MARINE...
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OVERNIGHT...A WEAK PRESSURE PATTERN AND LIGHT/VARIABLE WINDS WILL GIVE WAY TO A SURGE OF NORTHEAST WINDS AS HIGH AS 15-20 KT AS EXPANDING HIGH PRESSURE PUSHES A SECONDARY COLD FRONT THROUGH THE REGION. SEAS OF 1-3 FT WILL BUILD SOMEWHAT TO 2-4 FT LATER TONIGHT. DESPITE THE TIGHTENING PRESSURE GRADIENT/ASSOCIATED SURGE OF STRONGER NE WINDS...WINDS AND SEAS SHOULD REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS OVERNIGHT. TUESDAY THROUGH SATURDAY...WEAK HIGH PRESSURE WILL HOLD ON TUESDAY...BUT THEN RETREAT AS THE AREA OF LOW PRESSURE STARTS MOVING TOWARDS THE WATERS FROM THE SOUTHWEST. WINDS WILL INCREASE SOME TUESDAY NIGHT...BUT CURRENT THINKING IS THAT THEY WILL REMAIN BELOW 22 KNOTS SUSTAINED...SO SCAS ARE NOT EXPECTED. CONDITIONS SHOULD THEN DETERIORATE AS THE LOW PRESSURE SYSTEM PASSES OVER THE COASTAL WATERS WEDNESDAY AND WEDNESDAY NIGHT. A PERIOD OF SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED FOR ALL WATERS LATE WEDNESDAY OR WEDNESDAY NIGHT BEHIND THE DEPARTING LOW PRESSURE SYSTEM. WINDS/SEAS SHOULD THEN SLOWLY IMPROVE ON THURSDAY AS THE LOW PRESSURE SYSTEM SHIFTS NORTH/NORTHEAST AND AWAY FROM THE AREA...BEFORE IMPROVING FURTHER AS WEAK HIGH PRESSURE SETTLES OVER THE WATERS ON FRIDAY AND SATURDAY.
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&& .CHS WATCHES/WARNINGS/ADVISORIES... GA...NONE. SC...NONE. MARINE...NONE. && $$ SYNOPSIS... NEAR TERM...SPR SHORT TERM... LONG TERM... AVIATION...SPR MARINE...SPR

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