Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 061543
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
1043 AM EST SAT FEB 6 2016

.SYNOPSIS...
HIGH PRESSURE TO THE NORTH WILL GIVE WAY TO A COASTAL TROUGH THAT
DEVELOPS OFFSHORE AND STRENGTHENS TODAY. LOW PRESSURE WILL TRACK
NORTH AND DEEPEN OFF THE SOUTHEAST COAST TONIGHT THROUGH
SUNDAY....THEN WILL MOVE AWAY TOWARD THE NORTHEAST SUNDAY NIGHT. A
COLD FRONT WILL CROSS THE AREA MONDAY. THEN...A DEEP TROUGH WILL
REMAIN IN PLACE THROUGH THE MIDDLE OF NEXT WEEK...BRINGING
UNSEASONABLY COOL CONDITIONS TO THE AREA.

&&

.NEAR TERM /THROUGH TONIGHT/...
FOR TODAY AND TONIGHT WE FIND AN IMPRESSIVE LOW LATITUDE AND
DEEPENING LOW AND ASSOCIATED SHORT WAVE THAT DROPS 500 MB HEIGHTS
DOWN TO SOME 2 STANDARD DEVIATIONS BELOW NORMAL BY 12Z SUNDAY.
THIS FEATURE THAT STARTS OUT POSITIVELY TILTED TODAY TAKES ON
NEUTRAL OR EVEN A SLIGHT NEGATIVE TILT BY DAYBREAK SUNDAY AS IT
SWEEPS IN FROM THE WEST. THIS INDUCES STRONG CYCLOGENESIS OVER THE
SW ATLANTIC...BUT THE EVOLUTION OF THIS SYSTEM HAS HUGE
IMPLICATIONS ON THE LOCAL FORECAST. WE CONTINUE TO IGNORE THE
WORST CASE SCENARIO FORECAST OF THE NAM WHICH HAS ACCUMULATING
SNOWS IN OUR CWFA WITH CONVECTIVE ELEMENTS AND NEGATIVE EPV...THE
RESULT OF THE LOW MUCH DEEPER AND CLOSER TO US THAN THE MODEL
CONSENSUS. THUS OUR FORECAST IS GEARED HEAVILY TOWARD THE HPC
FORECAST AND A BLEND OF OF THE GLOBAL MODELS...WHICH PROVIDES US
WITH A MORE CONSERVATIVE FORECAST IN REGARDS TO WINTER WEATHER FOR
LATE TONIGHT INTO SUNDAY.

A COASTAL TROUGH OFF THE SE COAST TODAY BECOMES MORE ORGANIZED
TODAY AS A 1031 MB HIGH TO OUR NORTH ELONGATES AND SPLIT INTO
SEVERAL DIFFERENT MESO CENTERS FROM THE SOUTHERN GREAT PLAINS TO
SE OF NEW ENGLAND. ISENTROPIC ASCENT WILL BEGIN LATE THIS MORNING
AND THIS AFTERNOON...RESULTING FIRST IN THE FORMATION AND/OR
ADVECTION OF MARITIME-INDUCED STRATOCUMULUS. THAT ALONG WITH SOME
HIGHER CIRRUS CLOUDS FROM THE WEST/SW WILL CAUSE AN INCREASE AND
THICKENING OF CLOUD COVER. EVENTUALLY JET DYNAMICS GET INVOLVED
AS THE LEFT EXIT REGION OF A 130-140 KT UPPER JET MOVES IN THIS
AFTERNOON...AND WILL CAUSE AT LEAST A SLIGHT CHANCE OR CHANCE OF
RAIN TO OCCUR. HIGHEST POPS WILL BE ACROSS THE COASTAL COUNTIES.

DEPENDING UPON HOW QUICKLY THE CLOUDINESS INCREASES WILL AID IN
HOW WARM WE/RE ABLE TO ACHIEVE TODAY. BUT LOWER 50S LOOKS TO BE
THE MAX AT BEST...WITH EVEN SOME UPPER 40S AT THE SHORELINE AND
PERHAPS EVEN CLOSER TO THE ALTAMAHA RIVER WHERE CLOUD COVER IS
EXPECTED TO THICKEN QUICKEST.

WITH THE APPROACH OF THE MID/UPPER LOW TONIGHT AND IT/S TREND
TOWARD A NEGATIVE TILT WILL COMBINE WITH A STRONG 35-45 KT LOW
LEVEL JET TO PRODUCE POTENT FORCING FOR ASCENT. UPWARD VERTICAL
VELOCITY WILL STRENGTHEN AND DEEP LAYERED MOISTURE ARRIVES IN
CONCERT WITH DEEPENING LOW PRESSURE THAT FORMS OFF THE FLORIDA
COAST AND LIFTS NORTH/NE SEVERAL HUNDRED MILES OFF THE LOCAL
COAST. IN RESPONSE THERE IS A RAPID ONSET OF RAIN THAT OCCURS WITH
PLENTY OF LOW LEVEL FORCING IN TANDEM WITH UPPER DIFLUENCE. AN
ORGANIZED RAINFALL EVENT WILL TRANSPIRE WITH WIDESPREAD RAINS
EXPECTED NEAR AND EAST OF I-95...WITH GRADUATED COVERAGE FURTHER
WEST WHERE A SHARP GRADIENT WILL BE FOUND. QPF LOOKS TO RANGE FROM
ABOUT 1/10 INCH FAR INTERIOR SE GA TO AS MUCH AS 2/3 OR 3/4 INCHES
IN THE CHARLESTON TRI-COUNTY DISTRICT. NOT ENOUGH FOR A FLASH
FLOOD WATCH...BUT ENOUGH TO CAUSE AT LEAST SOME NUISANCE FLOODING
GIVEN THE SATURATED GROUNDS FROM THE CONSIDERABLE RAINS EARLIER IN
THE WEEK. THERE COULD BE A CONCERN FOR FLOODING IN DOWNTOWN
CHARLESTON WITH RAINS ONGOING WITH THE HIGH TIDE EARLY SUNDAY
MORNING.

WHILE NOT IN THE FORECAST THERE CERTAINLY COULD BE SOME ELEVATED
CONVECTION GIVEN THE POTENCY OF THIS SYSTEM.

WHILE THE PRECIP WILL BE ALL RAIN THE FIRST PART OF THE
NIGHT...COLD ADVECTION TO THE NW OF THE SURFACE LOW WILL PUSH
TOWARD THE COAST AFTER MIDNIGHT WHICH WILL CAUSE A CONCERN IN
REGARDS TO WINTER-TYPE PRECIP. THERE WILL BE PLENTY OF ICE IN THE
SNOW GROWTH ZONE ALOFT WITH STRONG UPWARD MOTION. AND THE FREEZING
LEVEL IS DOWN TO AROUND 850 MB...SUPPORTING A MIX OR POSSIBLE
CHANGEOVER TO SNOW AFTER 3 OR 4 AM. GIVEN THE COMPLEXITY OF THE
FORECAST SITUATION AND THE UNCERTAINTY OF THE EXACT TRACK AND
STRENGTH OF THE SURFACE ATLANTIC LOW...WE HAVEN/T STRAYED TOO FAR
FROM THE PREVIOUS FORECAST. UTILIZING SURFACE TEMPS THAT ARE LESS
THAN 35F IS WHERE WE HAVE SHOWN THE PROBABILITY OF SNOW THAT
EITHER MIXES WITH OR CHANGES OVER FROM THE ONGOING RAINS. THIS IS
ROUGHLY WEST OF A LINE FROM JAMESTOWN TO GOOSE CREEK...WALTERBORO
AND HARDEEVILLE IN SC. AND WEST OF A LINE FROM APPROXIMATELY
RINCON TO HINESVILLE AND LUDOWICI. THE BEST CHANCE OF ANY SMALL
ACCUMULATING SNOWS WOULD BE IN THE SC COUNTIES WHERE THE
CHANGEOVER OR MIXTURE OCCURS. BUT EVEN HERE THOSE CHANCES ARE LOW
AT BEST THROUGH 12Z SUNDAY. STILL...IF THE WORST CASE SCENARIO
SUCH AS THAT DEPICTED ON THE NAM HAS ANY VALIDITY...A WINTER
WEATHER ADVISORY OR MAYBE A WINTER STORM WATCH/WARNING MAY YET BE
REQUIRED.

THE COLDER AIR WILL COMBINE WITH STRENGTHENING NW AND NORTH WINDS
AROUND THE SURFACE CYCLONE TO DROP TEMPS TO THE LOWER AND MIDDLE
30S INLAND AND UPPER 30S ON THE SHORE...WHILE ASSOCIATED WIND
CHILLS ARE DOWN IN THE 20S.

LAKE WINDS...THE TIGHT PRESSURE GRADIENT AROUND THE ATLANTIC
CYCLONE...PLUS COLD ADVECTION ATOP THE WARMER WATERS OF LAKE
MOULTRIE HAS CAUSED US TO ISSUE A LAKE WIND ADVISORY FROM LATE
TONIGHT THROUGH SUNDAY.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
SUNDAY AND SUNDAY NIGHT...THE FOCUS FOR THE SHORT TERM PERIOD FALLS
SQUARELY ON THE SUNDAY TIME FRAME AND THE POTENTIAL FOR SNOW ACROSS
THE FORECAST AREA. THE OVERALL SCENARIO IS QUITE COMPLEX AND THE
DETAILS OF THE FORECAST WILL HINGE ALMOST ENTIRELY ON THE PROXIMITY
TO THE COAST OF THE DEVELOPING SURFACE LOW. FIRST...LETS JUST GET IT
OUT OF THE WAY AND SAY THAT THE 00Z AND 06Z NAM SOLUTIONS ARE...AT
THIS TIME...CONSIDERED TO BE DISTINCT OUTLIERS. THERE IS ALMOST NO
SUPPORT FOR THESE SOLUTIONS FROM OTHER GLOBAL AND HI-RES MODELS. THE
NAM IS QUITE INTERESTING THOUGH AS THE DYNAMICS AND THERMODYNAMICS
IT PORTRAYS ARE OFF THE CHARTS IMPRESSIVE AND COULD RESULT IN A
SIGNIFICANT SNOW EVENT FOR THE MIDLANDS/LOWCOUNTRY/PEE DEE REGIONS.
THE MODEL CONSENSUS USED FOR THE FORECAST HOWEVER FEATURES A LOW
TRACK THAT IS ABOUT 150 MILES FURTHER TO THE EAST AND KEEPS THE BEST
DYNAMICS JUST OFF THE COAST. THE MORNING IS EXPECTED TO START OFF
WITH LIKELY TO CATEGORICAL POPS ALONG AND EAST OF ROUGHLY THE I-95
CORRIDOR. THIS SHIELD OF PRECIPITATION IS THEN EXPECTED TO STEADILY
PUSH TO THE EAST SUCH THAT BY THE AFTERNOON ONLY EASTERN CHARLESTON
COUNTY WILL STILL BE IMPACTED. EVEN WITH THE FAVORED FURTHER EAST
TRACK...A CHANGEOVER OR TRANSITION TO WET SNOW IS EXPECTED ACROSS
MUCH OF THE FORECAST AREA. GIVEN THE ISOTHERMAL NATURE OF THE
THERMODYNAMIC PROFILE...A WET SNOW WOULD BE EXPECTED BUT CURRENT
THINKING STILL FAVORS LITTLE TO NO ACCUMULATION. SOME LIGHT
ACCUMULATIONS COULD BE POSSIBLE ON ELEVATED SURFACES...BUT
CONFIDENCE REMAINS TOO LOW FOR ANY KIND OF WATCH/WARNING/ADVISORY AT
THIS POINT. THOUGH THE NAM SOLUTION IS ON AN ISLAND AND CONSIDERED
AN OUTLIER...THERE IS A LOW END CHANCE THAT A SIMILAR SCENARIO COULD
OCCUR. WE WILL NEED TO CAREFULLY WATCH SUBSEQUENT MODEL RUNS OVER
THE NEXT 24 HOURS TO SEE HOW THE CONSENSUS SHIFTS IN REGARDS TO THE
TRACK OF THE YET TO DEVELOP SURFACE LOW. REGARDLESS...THE OVERNIGHT
IS EXPECTED TO BE DRY AS THE LOW DEEPENS AND PULLS AWAY TO THE
NORTHEAST. LOWS ARE EXPECTED TO DIP INTO THE MID 30S IN MOST AREAS.

MONDAY THROUGH TUESDAY...THE DEEP AND HIGH AMPLITUDE LONGWAVE TROUGH
WILL CARVE FURTHER SOUTHWARD INTO TUESDAY...PUSHING A CLIPPER TYPE
LOW JUST NORTH OF THE REGION. A COLD FRONT ASSOCIATED WITH THIS
SYSTEM WILL CROSS THE AREA LATE MONDAY AND THE ECMWF/GFS CONTINUE TO
SHOW ISOLATED SHOWERS POPPING UP AS THE TROUGH PASSES THROUGH ALOFT
COINCIDENT WITH THE SURFACE SYSTEM. MONDAY WILL SEE INCREASING CLOUD
COVER AND BREEZY CONDITIONS...WITH HIGHS IN THE MID/UPPER 50S.
BEHIND THE FRONT...MUCH COOLER AIR WILL BE USHERED IN FOR TUESDAY AS
HIGHS WILL STRUGGLE TO REACH THE UPPER 40S.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
THE FORECAST AREA WILL REMAIN SOLIDLY UNDER THE INFLUENCE OF THE
COLD TROUGH ALOFT THROUGH MID TO LATE WEEK. HIGH PRESSURE WILL
GRADUALLY BUILD IN FROM THE WEST AND THE OVERALL FORECAST CONTINUES
TO LOOK COOL AND DRY. IN FACT...TEMPERATURES WILL BE WELL BELOW
NORMAL WEDNESDAY AND THURSDAY BEFORE MODIFYING TO NEAR NORMAL BY THE
END OF THE WEEK AND THE START OF THE WEEKEND.

&&

.AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
THROUGH 12Z SUNDAY...BKN-OVC VFR CEILINGS WILL PREVAIL TODAY.
LIGHT RAINS WILL OCCUR IN THE AFTERNOON AND CONTINUE INTO THIS
EVENING IN RESPONSE TO A COASTAL TROUGH OFFSHORE AND ENERGY ALOFT
AS CEILINGS LOWER TOWARD LOW-END VFR OR POSSIBLY MVFR THIS
EVENING. AS LOW PRESSURE FORMS NORTH OF THE BAHAMAS TONIGHT THE
ATMOSPHERE BECOMES FURTHER ENERGIZED...LEADING TO MODERATE OR
PERHAPS LOCALLY HEAVY RAINS TONIGHT. THIS IN TURNS SOLID MVFR OR
POSSIBLE IFR WEATHER AT BOTH SITES. OUTSIDE CHANCE OF SNOW MIXING
IN WITH THE RAIN CLOSER TO DAYBREAK SUNDAY...ESPECIALLY AT KCHS.

NORTH/NE WINDS WILL AVERAGE AROUND 10 KT TODAY...BACKING SOME
20-40 DEGREES TONIGHT AS SPEEDS INCREASE AS THE SURFACE LOW
STRENGTHENS IN THE ATLANTIC. GUSTS WILL PEAK AROUND 20 KT AT KSAV
AND CLOSE TO 30 KT AT KCHS LATE.

EXTENDED AVIATION OUTLOOK...THERE WILL BE A CHANCE FOR RESTRICTED
CEILINGS SATURDAY NIGHT INTO SUNDAY MORNING. OTHERWISE VFR
CONDITIONS WILL PREVAIL.

&&

.MARINE...
TODAY...A PINCHING OF THE GRADIENT WILL DEVELOP BETWEEN A
DEVELOPING AND ORGANIZING COASTAL TROUGH OUT NEAR THE GULF STREAM
AND HIGH PRESSURE TO THE NORTH. THIS CAUSES THE NE WINDS OF AT
LEAST 15-20 KT THIS MORNING ON THE SC WATERS...AND 20-25 KT AND
GUSTY ON THE GA WATERS. WINDS WILL INCREASE A BIT FURTHER ALL
WATERS THIS AFTERNOON. SMALL CRAFT ADVISORIES ARE IN EFFECT. SEAS
WILL PEAK AT 4-6 FT WITHIN 20 NM AND 7-9 FT ON THE OUTER GA
WATERS...WITH 1-2 FT WAVES IN CHARLESTON HARBOR.

TONIGHT...HAZARDOUS MARINE CONDITIONS WILL OCCUR AS CYCLOGENESIS
OCCURS OFF THE SE COAST. THIS HAS PROMPTED A GALE WATCH FOR BOTH
AMZ350 AND AMZ374...AND LIKELY THIS WILL BE UPGRADED TO A GALE
WARNING LATER TODAY AS WINDS RISE TO AS HIGH AS 35 OR 40 KT. THE
GALES CERTAINLY COULD SPREAD INTO THE OTHER WATERS...INCLUDING
CHARLESTON HARBOR. BUT FOR NOW WE HAVE HIGH-END SMALL CRAFT
ADVISORIES ON THESE WATERS. WAVE WATCH GUIDANCE LOOKS TO BE A
LITTLE UNDERDONE ON WAVE HEIGHTS AND HAVE BEEN ADJUSTED
ACCORDINGLY.

SUNDAY THROUGH THURSDAY...CONDITIONS ACROSS THE LOCAL WATERS WILL
START OFF VERY POOR SUNDAY MORNING AS AN AREA OF LOW PRESSURE
PASSES WILL OFFSHORE AND DEEPENS. FREQUENT GALE FORCE GUSTS ARE
EXPECTED TO BE ONGOING ACROSS THE CHARLESTON COUNTY WATERS AND THE
OUTER GEORGIA WATERS...AND THE GALE WATCH REMAINS IN EFFECT.
ELSEWHERE...HIGH END SMALL CRAFT ADVISORY CONDITIONS WILL BE IN
PLACE THROUGH THE DAY ON SUNDAY. IF THE OFFSHORE LOW ENDS UP
TRACKING CLOSER TO THE COAST...GALE WATCHES/WARNINGS MAY BE NEED
TO BE EXPANDED. CONDITIONS WILL BRIEFLY DIMINISH SUNDAY NIGHT
BEFORE RAMPING BACK UP MONDAY AND MONDAY NIGHT AS A REINFORCING
COLD FRONT CROSSES THE WATERS. ADVISORIES WILL BE NEEDED DURING
THIS TIME...AND THERE COULD EVEN BE ANOTHER ROUND OF GALES FOR THE
OUTER GEORGIA WATERS. ADVISORIES WILL LIKELY LINGER THROUGH
TUESDAY AND WEDNESDAY AS A TIGHT PRESSURE GRADIENT CONTINUES IN
THE WAKE OF THE DEPARTED COLD FRONT.

&&

.TIDES/COASTAL FLOODING...
ASTRONOMICAL HIGH TIDES WILL SOON BE UPON US AND DEPARTURES ARE
ALREADY RUNNING AS MUCH AS 1 FOOT ABOVE PREDICTED ALONG THE ENTIRE
STRETCH OF THE COAST. THIS SUGGESTS AT LEAST A LOW END CHANCE OF
SHALLOW COASTAL FLOODING WITH THE MORNING HIGH TIDES ALONG PARTS
OF THE SC COAST EARLY THE UPCOMING WEEK.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...LAKE WIND ADVISORY FROM 4 AM TO 6 PM EST SUNDAY FOR SCZ045.
MARINE...SMALL CRAFT ADVISORY UNTIL 4 AM EST MONDAY FOR AMZ352.
     GALE WATCH FROM LATE TONIGHT THROUGH SUNDAY AFTERNOON FOR
     AMZ350.
     SMALL CRAFT ADVISORY UNTIL 1 AM EST SUNDAY FOR AMZ350.
     SMALL CRAFT ADVISORY UNTIL 1 AM EST MONDAY FOR AMZ354.
     GALE WATCH FROM 10 PM EST THIS EVENING THROUGH SUNDAY
     AFTERNOON FOR AMZ374.
     SMALL CRAFT ADVISORY UNTIL 10 PM EST THIS EVENING FOR AMZ374.
     SMALL CRAFT ADVISORY FROM 7 PM THIS EVENING TO 6 PM EST SUNDAY
     FOR AMZ330.

&&

$$
SYNOPSIS...
NEAR TERM...
SHORT TERM...
LONG TERM...BSH
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...


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