FXUS62 KCHS 231748
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
1248 PM EST WED DEC 23 2009
.SYNOPSIS...
A HIGH PRES WEDGE WILL PERSIST INLAND THROUGH THU. A COMPLEX STORM
SYSTEM WILL IMPACT THE REGION THU NIGHT THROUGH FRI. HIGH PRES WILL
BUILD INTO THE REGION THIS WEEKEND AND REMAIN THE DOMINANT WEATHER
FEATURE THROUGH THE MIDDLE OF NEXT WEEK.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
HIGH CLOUDS CONTINUE TO STREAM INTO THE FORECAST AREA THIS
AFTERNOON...AHEAD OF A COMPLEX STORM SYSTEM DEVELOPING TO THE
WEST OVER THE PLAINS STATES. WEAK UPPER RIDGING PERSISTS OVER THE
SOUTHEAST STATES...WHILE A SURFACE HIGH PRESSURE WEDGE GRADUALLY
STRENGTHENS INLAND. THIS PATTERN WILL SUPPORT ANOTHER QUIET AND
DRY WEATHER SCENARIO FOR TODAY.
MODEL RELATIVE HUMIDITY FIELDS INDICATE CIRRUS INCREASING THROUGH
THE AFTERNOON...AND MAINTAIN A TREND OF SKY COVER BECOMING MORE
PARTLY CLOUDY DURING THE NEXT SEVERAL HOURS. THE INLAND WEDGE WILL
LIKELY CREATE A TYPICAL NORTHWEST TO SOUTHEAST HIGH TEMPERATURE
GRADIENT...WITH TEMPERATURES RANGING FROM THE UPPER 50S FAR
NORTHWEST PORTIONS OF BERKELEY AND DORCHESTER COUNTIES IN SOUTH
CAROLINA...TO PERHAPS THE MID 60S FAR SOUTHEAST GEORGIA COUNTIES.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
ANOTHER SIGNIFICANT SOUTHERN STREAM STORM SYSTEM WILL BE MOVING
OUT OF THE SOUTHERN ROCKIES THIS EVENING WITH MODELS INDICATING
UPPER RIDGE AMPLIFICATION FROM THE SW ATLC REGION INTO THE GREAT
LAKES BY THU AS THE COMPLEX STORM CONSOLIDATES OVER THE PLAINS
STATES. A WEAK SURFACE LOW OVER THE GULF STREAM IS FORECAST TO
WEAKEN TONIGHT BUT LOW LEVEL MOISTURE CONVERGENCE OFF THE COAST
COULD STILL INDUCE A FEW NEAR-COASTAL SHOWERS LATE. OVERALL WE
ARE LOOKING FOR DRY WEATHER TONIGHT WITH POCKETS OF LAYERED
CLOUDS AT TIMES. WE RAISED LOW TEMPERATURES A FEW DEGREES TO THE
S OF I-16 IN GEORGIA AS MODELS SHOW HIGHER SURFACE DEW POINTS AND
A POTENTIAL FOR SOME STRATOCUMULUS CLOUDS IN THAT REGION. INLAND
AREAS TO THE NW OF SAVANNAH AND INLAND FROM U.S. 17 IN SOUTH
CAROLINA WILL SEE SOME UPPER 30S LOWS TONIGHT.
ON THU...WE STILL SEE SOME MODEL TIMING DIFFERENCES BUT MOST OF OUR
REGION SHOULD SQUEEZE OUT ONE LAST DRY DAY. THE GFS REMAINS THE
FAST AND STRONG OUTLIER FOR THE ONSET OF ISENTROPIC LIFT TOWARD
LATE AFTERNOON AND WE RESPECT THAT SOLUTION ENOUGH TO KEEP 20-30
POPS AFTER MID AFTERNOON. THE SURFACE WEDGE WILL REALLY BE DIGGING
IN AND NE BREEZES WILL BE ON THE INCREASE. THE LOW LEVEL PRES
GRADIENT SHOULD STRENGTHEN IN RESPONSE TO TIGHTENING THERMAL AND
MOISTURE GRADIENTS AND THE ONSET OF BACKING UPPER FLOW JUST TO OUR
W. LAYERED CLOUDS SHOULD PREVAIL AS THE WEDGE PATTERN HOLDS AND
WARM AIR ADVECTION ABOVE THE BOUNDARY LAYER INCREASES. HIGHS
EXPECTED TO RANGE FROM THE MID 50S NW OF CHARLESTON TO THE MID 60S
TO THE S OF I-16 IN GEORGIA.
&&
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
THURSDAY NIGHT...THE PRIMARY SURFACE LOW WILL LIFT NORTH THROUGH THE
MS RIVER VALLEY...AND A WARM FRONT WILL APPROACH THE REGION FROM THE
S/SW. INITIALLY...A PLUME OF RICH MOISTURE FEATURING PWATS 1.5 TO
1/75 INCHES WILL OVERRUN THE SURFACE WEDGE OF COOL AIR...AND DEEP
LAYER ASCENT WILL ENSURE THAT RAIN OVERSPREADS THE REGION FROM W/SW
TO E/SE JUSTIFYING CATEGORICAL POPS.
CHRISTMAS DAY WILL START WITH RAIN NORTH OF THE SAVANNAH RIVER...AND
CATEGORICAL POPS ARE JUSTIFIED THERE DURING THE MORNING. ALONG/SOUTH
OF THE SAVANNAH RIVER...THE PROGRESS OF A DEEP LAYER DRY SLOT
REMAINS UNCERTAIN...THUS TAPERED FRIDAY MORNING POPS TO LIKELY/CHANCE
ACROSS SE GA. THE SCENARIO REMAINS UNCERTAIN FRIDAY AFTERNOON INTO
FRIDAY NIGHT. BY 12Z FRIDAY...GUIDANCE SUGGESTS THAT A SECONDARY
SURFACE LOW PRESSURE WILL DEVELOP AT THE TRIPLE POINT ACROSS
INTERIOR SECTIONS OF GA OR SC...AND THIS FEATURE IS THEN FORECAST TO
PROGRESS TOWARD THE NE. THE QUESTION IS WHETHER THE PLUME OF DEEP
MOISTURE CONTINUES TO SWING TOWARD THE E/NE...DESPITE THE PRESENCE
OF THE SECONDARY SURFACE LOW...ESSENTIALLY ENDING THE PRECIPITATION
EVENT AS INDICATED BY MOST GUIDANCE...OR WHETHER THE SECONDARY LOW
CAN DELAY THE PROGRESS OF THE DRY SLOT...PROLONGING SIGNIFICANT
PRECIPITATION AS SUGGESTED BY THE 00Z NAM. PREFER THE MORE
PROGRESSIVE SCENARIO...WITH MOST PRECIPITATION ENDING FRIDAY
AFTERNOON...BUT CONFIDENCE IS INSUFFICIENT TO JUSTIFY MAJOR FORECAST
CHANGES. THUS...MAINTAINED AFTERNOON POPS GRADUATED FROM LIKELY N/NE
TO SLIGHT CHANCE FAR INLAND...THE GRADUALLY ENDED PRECIPITATION
ACROSS THE REGION FRIDAY NIGHT. SIGNIFICANT CHANGES MAY BE REQUIRED
IN LATER FORECAST PACKAGES.
HIGH PWATS/DEEP LAYER ASCENT SUGGEST THE POTENTIAL FOR ANOTHER
SIGNIFICANT RAIN EVENT THURSDAY NIGHT INTO FRIDAY...AND LOCALIZED
FLOODING COULD OCCUR.
ALSO OF NOTE...00Z GUIDANCE SUGGESTS THAT AS THE WEDGE RETREATS AND
THE WARM SECTOR SURGES NORTH AHEAD OF THE SECONDARY SURFACE
LOW...TEMPERATURES IN THE 65-70F RANGE AND DEWPOINTS AROUND 60F
COULD SUPPORT A NARROW AXIS OF SURFACE BASED INSTABILITY ESPECIALLY
ACROSS S/E COUNTIES OF THE FORECAST AREA. IN A WORST CASE
SCENARIO...STRONG WIND FIELDS AND LOW LEVEL CONVERGENCE NEAR THE
SECONDARY SURFACE LOW/TRAILING COLD FRONT COULD SUPPORT A FEW
THUNDERSTORMS WITH A LOW CHANCE FOR DAMAGING WIND GUSTS.
HOWEVER...THERE REMAINS MUCH UNCERTAINTY REGARDING ONGOING
PRECIPITATION COVERAGE...THE PROGRESS OF THE WARM SECTOR/MAGNITUDE
OF INSTABILITY AND OTHER CRITICAL FORECAST DETAILS TO JUSTIFY A
CONFIDENT MENTION OF THUNDERSTORMS WITHIN THIS FORECAST.
FRIDAY NIGHT...A COLD FRONT WILL CROSS THE REGION...THEN HIGH
PRESSURE SHOULD PROVIDE A RAIN-FREE AND RELATIVELY TRANQUIL REGIME
THIS WEEKEND THROUGH MIDWEEK. AFTER SATURDAY...WHEN HIGH
TEMPERATURES WILL BE AROUND NORMAL...EXPECT BELOW NORMAL
TEMPERATURES SUNDAY THROUGH MIDWEEK.
LATE THIS WEEKEND INTO EARLY NEXT WEEK...THE SUBTROPICAL JET WILL
REMAIN LADEN WITH MOISTURE AND LOCATED NOT TOO FAR S OF THE REGION.
IF ANY SIGNIFICANT WAVES DEVELOP WITHIN THIS JET...CLOUDS AND
PERHAPS EVEN SOME PRECIPITATION COULD SPREAD BACK INTO THE FORECAST
AREA ESPECIALLY S/E. HOWEVER...THIS SCENARIO REMAINS UNLIKELY AND IS
NOT REFLECTED WITHIN THE LATEST FORECAST.
&&
.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
HIGH CLOUDS WILL CONTINUE TO DOMINATE SKY COVER THROUGH THE
REMAINDER OF THE AFTERNOON. MAIN CONCERN FOR THE 18Z TAF PERIOD
WILL BE THE DEVELOPMENT OF MVFR OR LOWER VISIBILITIES/CEILINGS AT
THE TERMINALS OVERNIGHT.
LATEST MODEL SOUNDINGS INDICATE A DECENT LOW LEVEL JET THROUGH
THE NIGHTTIME HOURS...SUGGESTING THAT FOG IS LESS LIKELY. MORE
QUESTIONS REMAIN REGARDING CEILINGS...AS NAM MODEL SOUNDINGS SHOW
LOW LEVEL MOISTURE BECOMING TRAPPED UNDER THE INVERSION BEFORE
SUNRISE THURSDAY. CONFIDENCE IN MVFR CEILINGS IS HIGHER FURTHER
SOUTH AT KSAV...WHERE SREF HAS NEARLY 50 PERCENT PROBABILITIES THAT
1 KFT OR LESS CLOUDS WILL DEVELOP. HAVE THUS INCLUDED MVFR
CEILINGS AT KSAV BEGINNING AT 09Z AND PERSISTING THROUGH THE TAF
PERIOD...ESPECIALLY CONSIDERING A WARM FRONT WILL BE TAKING SHAPE
TO THE SOUTH OF THE TERMINAL AND INCREASE MOISTURE CONVERGENCE
ACCORDINGLY. HAVE NOT INCLUDED MVFR CEILINGS AT KCHS WITH THE 18Z
ISSUANCE.
LESS CONFIDENCE IN IFR CONDITIONS AT THIS TIME...AND PREFER TO
KEEP MENTION OUT OF THE TAFS SO FAR IN ADVANCE. HAVE JUST HINTED
AT THE POTENTIAL BY INCLUDING SCATTERED IFR CLOUDS AT BOTH KCHS
AND KSAV.
EXTENDED AVIATION...
PERIODS OF IFR LIKELY IN RAIN THURSDAY NIGHT THROUGH FRIDAY. MAINLY
VFR SATURDAY THROUGH EARLY NEXT WEEK.
&&
.MARINE...
AS AN INLAND HIGH PRESSURE WEDGE STRENGTHENS TODAY...EXPECT
NORTHEAST FLOW OVER THE COASTAL WATERS TO STEADILY RISE TOWARD 15
KT BY THE AFTERNOON HOURS. LATEST TRENDS INDICATE WIND SPEEDS
ACROSS THE NORTHERN SOUTH CAROLINA AND OFFSHORE GEORGIA WATERS
HAVE ALREADY RISEN INTO THE 10 TO 15 KT RANGE LATE THIS
MORNING...AND HAVE BUMPED UP FORECAST WINDS ACCORDINGLY.
WINDS AND SEAS WILL FURTHER INCREASE TONIGHT...AND ESPECIALLY
PICK UP ON THU...AS SURFACE HIGH PRES BUILDS INTO THE MID ATLC
REGION. THE PINCHING WILL BECOME TIGHTER WITH TIME AS THE STORM
SYSTEM APPROACHES FROM THE W ON THU. IT CERTAINLY APPEARS THAT
SMALL CRAFT ADVISORY CONDITIONS WILL DEVELOP DURING CHRISTMAS EVE
AS NE WINDS SURGE AT OR ABOVE 20 KT AND SEAS BUILD TO 5-8 FT BY
LATE DAY. TIMING UNCERTAINTY OF WHEN GRADIENT PINCHING REALLY
PINCHES STRONG SUGGESTING HOLDING OFF ON ISSUING ADVISORIES JUST
YET...BUT LOOK FOR THEM TO BE HOISTED LATER TODAY OR TONIGHT.
SMALL CRAFT ADVISORY WINDS/SEAS APPEAR LIKELY THURSDAY NIGHT AND
FRIDAY...AND A PERIOD OF GALES CANNOT BE RULED OUT THURSDAY NIGHT. A
COLD FRONT WILL CROSS THE WATERS FRIDAY NIGHT...AND THE INITIAL
SURGE OF COLD ADVECTION SHOULD MAINTAIN SMALL CRAFT ADVISORY WINDS
AND/OR SEAS. THEN...CONDITIONS SHOULD IMPROVE SATURDAY THROUGH
SUNDAY. EARLY NEXT WEEK...ANOTHER SURGE OF W/NW WINDS COULD AGAIN
SUPPORT SMALL CRAFT ADVISORY CONDITIONS ESPECIALLY BEYOND 20 NM.
&&
.TIDES/COASTAL FLOODING...
STRONG ONSHORE WINDS COULD PRODUCE HIGH SURF...ELEVATED TIDES...
SHALLOW COASTAL FLOODING AND BEACH EROSION THURSDAY NIGHT INTO
FRIDAY.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...NONE.
&&
$$
