Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 202054
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
354 PM EST SAT DEC 20 2014

.SYNOPSIS...
UNSETTLED WEATHER IS EXPECTED THROUGH TUESDAY AS SEVERAL DISTURBANCES
MOVE OVER THE REGION. A STRONG COLD FRONT WILL APPROACH LATE TUESDAY
NIGHT...THEN SHIFT THROUGH THE AREA ON WEDNESDAY. DRY HIGH PRESSURE
WILL THEN SETTLE OVER THE SOUTHEAST THROUGH LATE WEEK...WITH A COLD
FRONT EXPECTED NEXT WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
THE PRIMARY UPPER VORT CAN BE SEEN ON WATER VAPOR IMAGERY SHIFTING
OFF THE SC COAST THIS AFTERNOON WITH SOME DRIER MID AND UPPER
LEVEL AIR MOVING INTO THE SOUTHEAST. THE RAIN TAPERED OFF OVER
MOST AREAS EARLIER THIS AFTERNOON WITH ONLY RESIDUAL PRECIP OVER
THE SC COASTAL WATERS. HOWEVER...MORE RECENTLY WE HAVE BEGUN TO
SEE SOME RADAR RETURNS ACROSS SOUTHERN GA MOVING NORTHEAST TOWARD
OUR FAR SW ZONES. THE 18Z NAM IS PICKING UP ON A WEAK 700 MB
DISTURBANCE LIFTING OUT OF THE CENTRAL GULF COAST REGION...PROGGED
TO LIFT THROUGH THE FORECAST AREA THROUGH THE OVERNIGHT HOURS. A
STRONGER MID-LEVEL SHORTWAVE WILL APPROACH FROM THE SW LATE
TONIGHT. ALTHOUGH FORCING AND MOISTURE WILL REMAIN LIMITED THROUGH
EARLY SUNDAY MORNING...WE MAINTAINED 20-40 POPS OVER MUCH OF THE
AREA.

THE HIGH PRESSURE WEDGE WILL BECOME FURTHER ENTRENCHED TONIGHT AS
THE SURFACE LOW STRENGTHENS OFF THE SC COAST. ABUNDANT BOUNDARY
LAYER MOISTURE WILL SUPPORT LOW OVERCAST SKIES...WITH THE
POTENTIAL FOR SOME BUILD-DOWN FOG. GEOSTROPHIC FLOW EXPECTED TO BE
GREATER THAN 15 KT OVERNIGHT WHICH MAY PRECLUDE A DENSE FOG EVENT.
ADDITIONALLY...NONE OF THE MODEL GUIDANCE IS CURRENTLY INDICATING
DENSE FOG POTENTIAL. WE MAINTAINED PATCHY FOG IN THE FORECAST
AFTER MIDNIGHT WITH CLOUDY SKIES. WEAK COLD ADVECTION WILL RESULT
IN LOW TEMPS IN THE LOWER TO MIDDLE 40S.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
SUNDAY...A WEAK SOUTHERN STREAM MID LEVEL PERTURBATION MOVING
THROUGH THE LOWER MS VALLEY FROM TONIGHT WILL CROSS THE AREA DURING
THE MORNING HOURS AND REINFORCES THE PRE-EXISTING STATIONARY FRONT
OVER THE ATLANTIC TO THE EAST/SE OF THE GULF STREAM. IT ALSO KEEPS
THE INLAND COOL HIGH PRESSURE WEDGE ANCHORED IN PLACE AS IT EXTENDS
SOUTH OF THE FORECAST AREA. THE IMPULSE ALOFT ALSO TRIGGERS WEAK
CYCLOGENESIS OFF THE SE COAST SOME 250 NM EAST OF SAVANNAH AS IT
MOVES LITTLE THROUGHOUT THE DAY. ISENTROPIC ASCENT WILL AGAIN BE THE
MAIN FACTOR FOR GENERATING RAINFALL...AS IT INCREASES IN MAGNITUDE
DURING THE AFTERNOON AND EVENING HOURS...AND POPS ACCORDINGLY WILL
BE ON THE INCREASE AS WELL DURING THE SECOND HALF OF THE DAY. WE/LL
START THE PERIOD WITH A LITTLE LIGHT RAIN...DRIZZLE AND/OR FOG
BEFORE POPS CLIMB INTO THE 30-50 PERCENTILE IN THE AFTERNOON.
GREATEST PROBABILITIES WILL BE OVER SE GA AND ESPECIALLY SOUTH OF
I-16 DURING THAT TIME. OVERCAST SKIES AND A CONTINUED FEED OF AIR
FROM THE NORTH/NE WITHIN THE LOW LEVEL WEDGE WILL KEEP TEMPS DOWN
MAINLY INTO THE 50S FOR MAX TEMPS.

SUNDAY NIGHT INTO MONDAY...ANOTHER AND SLIGHTLY STRONGER SHORT WAVE
ARRIVES FROM THE GULF OF MEXICO AND FORCES THE ATLANTIC WAVE TO
DEEPEN A FEW MILLIBARS AS IT LIFTS NE ROUGHLY ALONG THE GULF STREAM
TO A POSITION OFF CAPE HATTERAS BY 00Z TUESDAY. THIS PATTERN
REINFORCES THE WEDGE INLAND...WHICH IN TURN ALLOWS FOR A FURTHER
INCREASE IN ISENTROPIC ASCENT. THAT ALONG WITH FORCING FROM THE
UPPER JET OVER THE SE CORNER OF THE COUNTRY AND CONVERGENCE
ASSOCIATED WITH THE NEARBY ATLANTIC LOW WILL CAUSE COVERAGE OF RAIN
TO EXPAND INTO AT LEAST THE 70-80 PERCENT RANGE SUNDAY
NIGHT...BEFORE COVERAGE DIMINISHES FROM WEST TO EST ON MONDAY AS THE
SURFACE LOW PULLS AWAY. PWATS OF 1.3 TO 1.4 INCHES AND THE FLOW
ALOFT OPENING MORE OUT OF THE GULF OF MEXICO SUPPORTS AT LEAST 1/4
TO 1/2 INCH AMOUNTS OF RAIN. IN ADDITION...STEEPER MID LEVEL LAPSE
RATES AND SHOWALTER INDICES DROPPING TO NEAR ZERO COULD SUPPORT A
LITTLE ELEVATED CONVECTION. LOW STRATUS DECKS WILL ALSO SUPPORT SOME
REDUCTION IN SURFACE VISIBILITIES SUNDAY NIGHT INTO MONDAY MORNING
AND WE HAVE ADDED PATCHY FOG TO THE FORECAST. THERE IS ONLY A SMALL
5-10 DEGREE TEMP SWING FROM DAYTIME HIGHS SUNDAY TO NIGHTTIME LOWS
SUNDAY NIGHT. THEN A FAIRLY LARGE NORTH/NW TO SOUTH/SE TEMP GRADIENT
MONDAY WITH HIGHS NEAR OR SLIGHTLY BELOW NORMAL.

MONDAY NIGHT AND TUESDAY...YET ANOTHER QUICK MOVING SOUTHERN STREAM
SHORT WAVE ARRIVES LATE MONDAY NIGHT AND TUESDAY FROM THE GULF OF
MEXICO. FURTHER UPSTREAM A MORE PRONOUNCED SHORT WAVE STARTS TO DIG
THROUGH THE CENTRAL STATES...WHICH IN TURN AMPLIFIES THE LARGE SCALE
PATTERN ACROSS THE CONUS. THIS BACKS OUR FLOW ALOFT FURTHER TO THE
SW AND OPENS THE DOOR FOR A FEED OF EVEN DEEPER AND TROPICAL-LIKE
MOISTURE TO MOVE IN. PWATS CLIMB TO NEAR 2 STANDARD DEVIATIONS ABOVE
NORMAL...AROUND 1.6 TO 1.7 INCHES SO YET ANOTHER ROUND OF RAINFALL
WILL OCCUR. POPS WILL BE IN THE CHANCE TO LIKELY RANGE MONDAY NIGHT
THEN SHOULD PEAK IN THE CATEGORICAL RANGE TUESDAY. ADDITIONAL QPF
WILL BE AT LEAST 1/2 TO 3/4 INCHES...WITH POTENTIAL HIGHER AMOUNTS
IN THIS PATTERN. THE SHORT WAVE IN THE PLAINS WILL ALSO TRIGGER
CYCLOGENESIS NEAR THE ARKLATEX REGION BY TUESDAY...WHICH IN TURN
ATTEMPTS TO LIFT OUR COASTAL WARM FRONT NORTHWARD. NOT SURE HOW THIS
PROGRESSION WILL ACTUAL PAN OUT GIVEN THAT THE INLAND WEDGE WILL BE
RELUCTANT TO GIVE UP ITS HOLD. THIS WILL ALLOW FOR WARM ADVECTION TO
BOOST TEMPS AT LEAST INTO THE LOWER OR MIDDLE 60S...PERHAPS EVEN
HIGHER NEAR THE ALTAMAHA. THIS IN TURN CREATES SOME SURFACE BASED
INSTABILITY ON THE ORDER OF 500-800 J/KG IN SYNC WITH STEEP MID
LEVEL LAPSE RATES OVER OUR COASTAL ZONES AND SOUTH OF I-16. IT/S IN
THESE AREAS WHERE WE HAVE ISOLATED T-STORMS MENTIONED IN THE LATEST
FORECAST.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
TUESDAY NIGHT TEMPS WILL REMAIN RATHER MILD AHEAD OF A COLD FRONT TO
THE WEST. WE COULD SEE A FEW THUNDERSTORMS BEFORE THE FRONT SHIFTS
OFFSHORE WEDNESDAY AFTERNOON. GUSTY WINDS SHOULD ALSO ACCOMPANY THE
FRONT AS STRONG COLD AIR ADVECTION OCCURS BEHIND IT. A MUCH
QUIETER/DRY PERIOD OF WEATHER IS THEN ANTICIPATED AS HIGH PRESSURE
BUILDS OVER THE REGION BEHIND THE FRONT STARTING WEDNESDAY NIGHT.
TEMPS WILL BE SLOW TO WARM THROUGH THURSDAY...UNTIL HIGH PRESSURE
BECOMES MORE CENTERED OVER THE WESTERN ATLANTIC. OVERALL HIGH TEMPS
SHOULD RANGE IN THE MID/UPPER 50S THURSDAY...BEFORE SOUTHERLY FLOW
RETURNS TO THE AREA ALONG THE WESTERN EDGE OF HIGH PRESSURE ON
FRIDAY. TEMPS SHOULD PEAK INTO THE LOW/MID 60S BY LATE WEEK. ANOTHER
COLD FRONT COULD APPROACH NEXT WEEKEND...BRINGING THE NEXT CHANCE OF
PRECIP TO THE AREA.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
CEILINGS WILL CONTINUE TO BOUNCE ALL OVER THE PLACE THIS AFTERNOON
AS POCKETS OF DRY AIR INTERACT WITH AN OTHERWISE DEEP MOIST LAYER
IN PLACE. SPORADIC IFR CONDITIONS THROUGH MID AFTERNOON WILL
RETURN TO VFR FOR MUCH OF THE EVENING AS DRIER MID AND HIGH LEVEL
AIR SHIFTS IN. OVERNIGHT...THE MODELS ARE SPLIT REGARDING THE
POTENTIAL FOR IFR CEILINGS AND BUILD-DOWN FOG. MODEL SOUNDINGS AND
SEASONAL CLIMATOLOGY BOTH SUPPORT PREVAILING IFR CEILINGS AND
3-4SM VSBYS FROM LATE THIS EVENING THROUGH DAYBREAK SUNDAY. AT
LEAST MVFR CEILINGS ARE LIKELY TO PERSIST THROUGH MUCH OF THE DAY
SUNDAY. RAIN WILL OVERSPREAD THE AREA FROM THE SOUTH DURING THE
DAY SUNDAY BUT THE BEST CHANCE WILL BE AFTER 18Z SO WE DID NOT
INCORPORATE INTO THE CURRENT TAF SET.

EXTENDED AVIATION OUTLOOK...PERIODS OF MVFR OR LOWER CONDITIONS
ARE LIKELY WITH RAIN/SHOWERS AND LOW STRATUS THROUGH EARLY TUESDAY
WHILE A WEDGE OF HIGH PRESSURE PERSISTS INLAND AND WAVES OF LOW
PRESSURE LIFT NORTHEAST ACROSS THE NEARBY ATLANTIC. ADDITIONAL
FLIGHT RESTRICTIONS APPEAR LIKELY TUESDAY AFTERNOON THROUGH
WEDNESDAY AFTERNOON AS A STRONG COLD FRONT APPROACHES FROM THE
WEST WITH SHOWERS AND POSSIBLY THUNDERSTORMS. WINDY CONDITIONS
SHOULD ALSO OCCUR NEAR THE TIME OF FRONTAL PASSAGE ON WEDNESDAY.

&&

.MARINE...
THE TIGHT NORTHEAST GRADIENT WILL PERSIST THROUGH TONIGHT AS HIGH
PRESSURE CONTINUES TO WEDGE DOWN THE EASTERN SEABOARD AND LOW
PRESSURE SPINS OFF THE SC COAST. A STEADY 15-20 KT WIND EXPECTED
OVER MOST OF THE MARINE AREA OVERNIGHT WITH SEAS BUILDING TO 5 FT
OVER OUTER PORTIONS BY LATE TONIGHT.

SUNDAY...A PINCHED PRESSURE GRADIENT BETWEEN DEVELOPING LOW PRESSURE
OFF THE GA COAST NEAR THE GULF STREAM AND A STRONG INLAND WEDGE OF
HIGH PRESSURE WILL PERSIST. THIS WILL RESULT IN NORTH/NE WINDS OF AT
LEAST 15-20 KT...WITH GUSTS AT TIMES TO NEAR 25 KT. IF THE GUSTS
BECOME FREQUENT ENOUGH WE/D NEED A SMALL CRAFT ADVISORY...WHICH
WE/LL DEFER TO FUTURE SHIFTS TO SEE IF WE INDEED SHOULD HOIST THE
ADVISORY HEADLINE. THE FETCH IS FAVORABLE ENOUGH FOR SEAS TO BE AS
HIGH AS 3-5 FT...GREATEST OVER AMZ374 AND THE EASTERNMOST PARTS OF
AMZ350.

SUNDAY NIGHT AND MONDAY...THE ATLANTIC WAVE DEEPENS SOME AS IT LIFTS
NE ALONG THE GULF STREAM...WHILE THE INLAND WEDGE HOLDS FIRM. THE
GRADIENT THOUGH DOES SLACKEN SOME OF THE LOW SHIFTS NORTH OF THE
LOCAL WATERS...AND WINDS AND SEAS WILL BE LESS AND BELOW ANY
ADVISORY CRITERIA.

MONDAY NIGHT AND TUESDAY...THE INLAND WEDGE IS STILL IN PLACE DURING
MUCH OF THE PERIOD...BUT IT DOES ATTEMPT TO ERODE BY TUESDAY NIGHT
AS THE COASTAL WARM FRONT STARTS TO LIFT NORTH/NW. UNTIL THIS
TRANSITION THOUGH OUR WINDS WILL STRUGGLE TO VEER AROUND...AS SPEEDS
HOLD NEAR OR BELOW 15 KT AND SEAS ARE LIMITED TO 3 OR 4 FT.

TUESDAY NIGHT AND WEDNESDAY...FINALLY THE COASTAL WARM FRONT WILL
SHIFT NORTH OF THE LOCAL WATERS TUESDAY NIGHT AS A STRONG COLD FRONT
MOVES THROUGH WEDNESDAY. WARM ADVECTION AND THE RESULTING
MARINE-LAYERING EFFECTS WILL LIMIT THE STRENGTH OF WINDS AHEAD OF
THE FRONT...BUT SMALL CRAFT ADVISORIES ARE CERTAINLY POSSIBLE.
BEHIND THE FRONT THE COMBINATION OF COLD ADVECTION AND STRONG
ISALLOBARIC PRESSURE RISES MAY SUPPORT GALES...OR AT THE LEAST SMALL
CRAFT ADVISORIES. MARINERS ARE ALSO ADVISED THAT SOME STRONGER
T-STORMS ARE POSSIBLE IN ADVANCE OF THE COLD FRONT...CAPABLE OF
PRODUCING GUSTY WINDS AND LIGHTNING STRIKES.

THURSDAY...CONDITIONS WILL IMPROVE OVER ALL WATERS AS HIGH PRESSURE
BECOMES CENTERED JUST SOUTH OF THE AREA. WINDS/SEAS SHOULD
EVENTUALLY LOWER BELOW SMALL CRAFT ADVISORY LEVELS FOR ALL MARINE
ZONES AS COLD ADVECTION WANES AND THE GRADIENT RELAXES.

SEA FOG...LOW STRATUS DECKS COULD EQUATE TO REDUCED VISIBILITIES
INTO TUESDAY. ACTUAL SEA FOG MAY HAVE A WINDOW TO DEVELOP AS
SLIGHTLY WARMER AIR ARRIVES ATOP MID/UPPER 50 SHELF WATER TEMPS
WITHIN A SOUTH AND SOUTHWEST FLOW LATE TUESDAY INTO TUESDAY NIGHT
PRIOR TO WINDS INCREASING ENOUGH TO SCOUR IT OUT WEDNESDAY. NIGHT.

&&

.TIDES/COASTAL FLOODING...
ALTHOUGH THERE ARE NO IMMEDIATE CONCERNS IN REGARDS TO TIDES...GIVEN
THE PERIGEAN SPRING TIDES EARLY NEXT WEEK...THERE IS AN INCREASING
RISK FOR SHALLOW COASTAL FLOODING MONDAY AND TUESDAY AROUND THE TIME
OF THE MORNING HIGH TIDE. THIS SITUATION MIGHT BE FURTHER EXACERBATED
DUE TO STEADY RAINS PRIOR TO AND DURING THE HIGH TIDAL CYCLE.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...NONE.

&&

$$

NEAR TERM...JRL
SHORT TERM...33
LONG TERM...DPB
AVIATION...33/JRL
MARINE...33/JRL
TIDES/COASTAL FLOODING...33





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