Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 261656 AFDCHS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHARLESTON SC 1256 PM EDT THU MAR 26 2015 .SYNOPSIS... ATLANTIC HIGH PRESSURE WILL WEAKEN AHEAD OF A STRONG COLD FRONT APPROACHING TONIGHT AND SWEEPING THROUGH DURING FRIDAY. COLDER HIGH PRESSURE WILL THEN BUILD FOR THE WEEKEND...SUPPORTING A POSSIBLE FREEZE FOR SOME INLAND AREAS EARLY SUNDAY. HIGH PRESSURE THEN SHIFTS SOUTH OF THE REGION MONDAY...BEFORE ANOTHER COLD FRONT MOVES THROUGH LATE MONDAY OR EARLY TUESDAY. HIGH PRESSURE WILL THEN PREVAIL INTO THE MIDDLE OF NEXT WEEK. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... NO MAJOR CHANGES WERE MADE WITH THE EARLY AFTERNOON UPDATE. LOCALLY DENSE SEA FOG WILL AFFECT THE COASTAL COMMUNITIES FROM EDISTO BEACH NORTHEAST ALONG THE CHARLESTON COUNTY COAST FOR A FEW HOURS THIS AFTERNOON. IT IS UNCLEAR HOW FAR INLAND THE FOG BANK WILL PENETRATE AS THE SEA BREEZE DEVELOPS...BUT TRENDS ARE BEING WATCHED. THE RISK FOR SHOWER ACTIVITY LOOKS MINIMAL WITH MODIFIED OBSERVED AND FORECAST SOUNDINGS SHOWING LITTLE IN THE WAY OF INSTABILITY. THIS IS SUPPORTED BY THE LATEST RAP AND H3R RUNS. THE BEST RAIN CHANCES WILL HOLD OFF UNTIL AFTER 6 PM AS A COLD FRONT AND ASSOCIATED SHORTWAVE ENERGY DRAW CLOSER. COULD SEE AN ISOLATED SHOWER DEVELOP ALONG THE RESULTANT SEA BREEZE ITSELF WHERE LOW- LEVEL MOISTURE FLUX CONVERGENCE WILL BE LOCALLY MAXIMIZED... BUT THE OVERALL RISK FOR THIS IS TOO SMALL TO JUSTIFY A MENTIONABLE POP ATTM. WILL CONTINUE TO MONITOR CONVECTIVE TRENDS THROUGH THE DAY. && .SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/... TONIGHT...RAIN CHANCES INCREASE AS THE FRONT APPROACHES FROM THE WEST AND LARGE SCALE LIFT/DEEP MOISTURE MOVE INTO THE AREA. INSTABILITY APPEARS WEAK BUT CANNOT RULE OUT A FEW THUNDERSTORMS GIVEN THE INCREASING LOW-LEVEL JET/DEEP LAYER SHEAR. GIVEN THE WARM AND HUMID AIR MASS ADVECTING INTO THE AREA THERE MIGHT STILL BE SOME SEA FOG NEAR THE COAST...ESPECIALLY UNTIL WINDS BECOME MORE SOUTHWEST AFTER MIDNIGHT. TEMPS WILL LIKELY NOT DROP BELOW 60 DEGREES. FRIDAY...A DEEPENING AND AMPLIFYING LONG WAVE TROUGH WILL PREVAIL OVER THE EASTERN STATES...WITH SOME PHASING OF A SHORT WAVE OVER THE LOWER MS VALLEY WITH ANOTHER AND STRONGER SHORT WAVE THAT DIVES FROM THE MID MS VALLEY INTO THE TN VALLEY IN THE AFTERNOON. THIS TRANSITION WILL CAUSE HEIGHTS ALOFT TO STEADILY FALL AND WILL SHOVE A STRONG COLD FRONT INTO THE NW TIER DURING THE MID-LATE MORNING...TOWARD OUR COASTAL CORRIDOR BY EARLY AFTERNOON...AND INTO THE ATLANTIC THEREAFTER. LOW LEVEL CONVERGENCE WILL COMBINE WITH LARGER SCALE FORCING FOR ASCENT DUE TO STRONG VERTICAL WIND FIELDS THROUGHOUT THE TROPOSPHERE...MOST NOTABLY A 30-40 KT LOW LEVEL JET AND A 130-150 UPPER JET. THESE DYNAMICS WILL HAVE CONSIDERABLE MOISTURE TO WORK WITH...AS PWATS ARE AS MUCH AS 1-2 STANDARD DEVIATIONS ABOVE NORMAL FOR LATE MARCH. THIS WILL RESULT IN AT LEAST 60-70 POPS MAINLY IN THE FORM OF SHOWERS. HOWEVER...GIVEN 50- 60 KT OF BULK SHEAR AND SBCAPES THAT COULD BE AS GREAT AS 300-500 J/KG THERE CERTAINLY COULD BE A FEW RUMBLES OF THUNDER. RAIN CHANCES WILL PEAK IN THE MORNING...THEN TAIL OFF FROM WEST TO EAST IN THE AFTERNOON. WARM ADVECTION IN ADVANCE OF THE COLD FRONT WILL ASSIST IN NEGATING THE LACK OF INSOLATION...BUT FRIDAY WILL CERTAINLY BE NOTICEABLY COOLER THAN IT WILL BE TODAY. IN FACT AS COLD ADVECTION ARRIVES IN THE AFTERNOON THERE COULD ACTUALLY BE SOME SLOW FALL TO SOME OF OUR TEMPS THE REST OF THE DAY. FRIDAY NIGHT...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 DECK. A POTENT SHORT WAVE EMBEDDED WITHIN THE LONG WAVE TROUGH ALOFT WILL MOVE DIRECTLY THROUGH THE CWFA AROUND 05-08Z. THIS WILL SUPPORT 20-30 POPS AND MAINLY ACROSS THE CHARLESTON TRI-COUNTY REGION WHERE THE BEST MOISTURE WILL REMAIN. ALTHOUGH COLDER AIR WILL BE FILTERING IN...AS PER SOUNDING PROFILES IT LOOKS LIKE ONE OF THOSE EVENTS WHERE THE COLD AIR IS TRYING TO CATCH UP TO THE DEPARTING MOISTURE AND IT WILL ARRIVE TOO LATE TO PRODUCE ANYTHING OTHER THAN LIQUID. BUT DUE TO THE STEEP MID LEVEL LAPSE RATES WHERE 500 MB TEMPS ARE NEAR MINUS 27C...THERE APPEARS TO BE A RISK FOR SOME GRAUPEL OR SOFT HAIL. 850 MB TEMPS WILL PLUNGE TO -3C SOUTH AND -6C NORTH...SUPPORTIVE OF LOWEST TEMPS IN THE MID 30S TO LOWER 40S. BUT THERE IS TOO MUCH WIND AND THE GROUNDS ARE TOO WET FOR ANY FROST. SATURDAY...THE LARGE SCALE EAST COAST TROUGH REMAINS IN PLACE...WHILE SIMULTANEOUSLY SURFACE HIGH PRESSURE GRADUALLY BUILDS FROM THE WEST/NW. ANOTHER SHORT WAVE EMBEDDED WITHIN THE DEEP NW FLOW WILL DROP THROUGH THE UPPER MIDWEST TO NEAR THE SOUTHERN APPALACHIANS LATE. THIS WILL RESULT IN AN INCREASE IN MID/HIGH LEVEL CLOUDS OVER THE SW PART OF THE CWFA. ALTHOUGH THERE IS DECENT INSOLATION...WITH 850 MB TEMPS CLOSE TO 2 STANDARD DEVIATIONS BELOW NORMAL WE/LL BE HARD PRESSED TO REACH AN MORE THAN 55-60F. SATURDAY NIGHT...THE AFOREMENTIONED SHORT WAVE WILL PULL THROUGH OVERNIGHT AND USHER IN A 1033 ARCTIC HIGH FROM THE OHIO VALLEY. THE ASSOCIATED COLD ADVECTION WILL RESULT IN UNSEASONABLY COLD AIR PENETRATING INTO THE BULK OF THE FORECAST DISTRICT. THERE LOOKS TO BE TOO MUCH MIXING TO PREVENT A HARD FREEZE...BUT A LIGHT FREEZE IS CERTAINLY POSSIBLE ACROSS AT LEAST THE NW TIER. IF WINDS DO DROP OFF ENOUGH THEN TEMPS COULD BE EVEN COLDER THAN 30-32F IN THESE SECTIONS AND FROST WOULD ALSO BE A CONCERN IN MANY OTHER COMMUNITIES SOUTH AND EAST. SUNDAY...THE EAST COAST TROUGH LIFTS OUT AND GIVES WAY TO SHORT WAVE RIDGING AND ITS ASSOCIATED RISING HEIGHTS...AS THE CORE OF THE ARCTIC HIGH SHIFTS DIRECTLY OVERHEAD. DESPITE PLENTIFUL SUNSHINE AND THE AIR MASS MODIFYING...TEMPS WILL ONLY BE ABLE TO MUSTER THE UPPER 50S AND LOWER 60S. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... THE MID LEVEL PATTERN WILL FLATTEN THROUGH THE EARLY AND MIDDLE PARTS OF NEXT WEEK...ALTHOUGH A SUBTLE IMPULSE WILL MOVE THROUGH LATE MONDAY/EARLY TUESDAY. THIS WILL FORCE A COLD FRONT THROUGH THE FORECAST ZONES...ACCOMPANYING ONLY BY MODEST MOISTURE AND JUST LIMITED CHANCES FOR SHOWERS. THE FRONT THEN WASHES OUT AND GIVES WAY TO HIGH PRESSURE THROUGH MID WEEK. WARMER CONDITIONS WILL RETURN AS EVEN THE AIR MASS BEHIND THE COLD FRONT IS OF PACIFIC ORIGIN SO TEMPS SHOULD GET BACK NEAR OR EVEN ABOVE SEASONAL NORMS. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... VFR THROUGH THE AFTERNOON INTO THIS EVENING...WITH INCREASING CHANCES FOR MVFR CIGS AND RAIN SHOWERS AFTER 06Z. THE BULK OF THE HEAVIEST RAINS WILL LIKELY HOLD OFF UNTIL CLOSER TO SUNRISE FRIDAY...ALTHOUGH SOME GUIDANCE SUGGESTS EVEN LATER THAN THAT. WILL INCLUDE PREVAILING 5SM -SHRA FROM 10Z ON AT BOTH TERMINALS. WILL ALSO INCLUDE A TEMPO FOR BRIEF SHOWERS AT KSAV FROM ROUGHLY 23-01Z AS HIGH RESOLUTION DATA SUPPORT SHOWERS IN THAT AREA ALONG THE SEA BREEZE. EXTENDED AVIATION OUTLOOK...FLIGHT RESTRICTIONS LIKELY FRIDAY IN ASSOCIATION WITH SCATTERED TO NUMEROUS SHRA AND POSSIBLE TSRA AS A COLD FRONT MOVES THROUGH. VFR WEATHER WILL THEN PREVAIL LATE FRIDAY INTO MONDAY...WITH POSSIBLE SUB-VFR WEATHER WITH ANOTHER COLD FRONT LATE MONDAY. && .MARINE... EXPANDED THE MARINE DENSE FOG ADV NORTHEAST TO THE CHARLESTON COUNTY WATERS AND THE GEORGIA OFFSHORE WATER PER SATELLITE TRENDS AND WEBCAM OBSERVATIONS. TODAY...WEBCAMS OFF HILTON HEAD AND EDISTO BEACH SUGGEST THE SEA FOG HAS LIFTED SOME OVER THE PAST 1-2 HOURS. HOWEVER...WINDS ARE FORECAST TO RETURN TO A MOVE SOUTHERLY DIRECTION AS THE MORNING LAND BREEZE DISSIPATES AND A SEA BREEZE INITIATES. THIS WILL RETURN THE LOW- LEVEL FLOW TO A MORE FAVORABLE GEOMETRY FOR SEA FOG DEVELOPMENT. WILL MAINTAIN THE MARINE DENSE FOG ADVISORY FOR THE NEARSHORE WATERS FROM EDISTO BEACH SOUTH. THE FOG COULD EXPAND INTO THE CHARLESTON COUNTY WATERS LATER THIS AFTERNOON...SO THE ADVISORY MAY NEED TO BE EXPANDED WITH LATER FORECAST CYCLES. TONIGHT...VEERING WINDS AHEAD OF AN APPROACHING COLD FRONT LATE TONIGHT. WARM AND MOIST AIR MASS WILL REMAIN IN PLACE CREATING A GOOD ENVIRONMENT FOR SEA FOG. WINDS/SEAS WILL APPROACH SMALL CRAFT ADVISORY LEVELS LATE TONIGHT BEYOND 20 NM. FRIDAY...A FAIRLY TIGHT AND CYCLONIC SW PRESSURE PATTERN WILL PREVAIL IN ADVANCE OF THE COLD FRONT THAT DOESN/T MOVE THROUGH AFTER 1 OR 2 PM. WARM ADVECTION WILL RESTRICT THE AMOUNT OF MIXING...BUT GIVEN GEOSTROPHIC WINDS OF 30 KT OR SO THIS WILL SUPPORT MARGINAL SMALL CRAFT ADVISORIES ACROSS THE AMZ374 WATERS FOR GUSTS UP NEAR 25 KT AND SEAS AS HIGH AS 6 FT. WE STILL MIGHT NEED TO EXPAND THE ADVISORY TO ENCOMPASS AMZ350...BUT FOR NOW THOSE WATERS LOOK TO REMAIN JUST BELOW ANY ADVISORY CONDITIONS. EVEN THOUGH WIND FIELDS WILL BE SOMEWHAT ELEVATED...SEA FOG WILL CONTINUE UNTIL THE FRONT SCOURS THINGS OUT AND VISIBILITIES IMPROVE. MARINERS SHOULD ALSO REMAIN ALERT FOR POSSIBLE T-STORMS...MOST ESPECIALLY ACROSS THE GULF STREAM WATERS. FRIDAY NIGHT AND SATURDAY...A MIXTURE OF COLD ADVECTION AND GOOD ISALLOBARIC PRESSURE RISES WILL LIKELY SUPPORT ADDITIONAL SMALL CRAFT ADVISORIES AS NW WINDS APPROACH 20-25 KT IN WAKE OF THE COLD FRONT AND STRENGTHENING ATLANTIC LOW PRESSURE SOUTH/SE OF NEW ENGLAND. SATURDAY NIGHT AND SUNDAY...STRONG CONTINENTAL HIGH PRESSURE WILL BUILD FROM THE NW AND WILL BE ACCOMPANIED BY A SURGE OF NORTH/NE WINDS AROUND DAYBREAK SUNDAY...BUT LIKELY BENEATH ANY ADVISORY CRITERIA. THE HIGH WILL THEN SETTLE ATOP THE WATERS BY LATE IN THE DAY AND WINDS AND SEAS WILL COME DOWN. MONDAY AND TUESDAY...HIGH PRESSURE EARLY NEXT WEEK WILL GIVE WAY TO A COLD FRONT THAT MOVES THROUGH BY EARLY TUESDAY...BEFORE BECOMING DIFFUSE. MARINE CONDITIONS WILL BE AT OR LESS THAN 15 OR 20 KT FOR WINDS AND LESS THAN 3-5 FOOT SEAS. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...NONE. SC...NONE. MARINE...DENSE FOG ADVISORY UNTIL 11 PM EDT THIS EVENING FOR AMZ352-354. DENSE FOG ADVISORY UNTIL 5 PM EDT THIS AFTERNOON FOR AMZ350- 374. SMALL CRAFT ADVISORY FROM 6 AM FRIDAY TO 1 PM EDT SATURDAY FOR AMZ374. && $$ ST

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