Areal Flood Advisory 77, NWS Columbia, SC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE COLUMBIA SC
316 AM EDT MON SEP 17 2012

.SYNOPSIS...
ATMOSPHERIC MOISTURE WILL BE ON THE INCREASE THROUGH TUESDAY AHEAD
OF A COLD FRONT THAT WILL SLOWLY MOVE THROUGH LATE TUESDAY NIGHT
AND WEDNESDAY. DRIER AIR IS EXPECTED TO ENTER THE REGION
THURSDAY...PROVIDING FAIR WEATHER INTO SATURDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
CLOSED UPPER LOW OVER TX...WITH GENERALLY ZONAL UPPER FLOW ACROSS
THE EASTERN CONUS...AND NORTHERN STREAM TROUGH DIGGING INTO THE
NORTHERN PLAINS. SURFACE BOUNDARY CURRENTLY STRETCHED ACROSS THE
NORTHERN PART OF OUR FORECAST AREA (FA)...WITH HIGH PRESSURE
CURRENTLY CENTERED TO OUR NE OVER THE MID ATLANTIC COAST. RADAR
INDICATES A FEW SHOWERS IN PROGRESS NEAR THE FRONT OVER THE
NORTHERN FA. UPPER LOW WILL BEGIN TO OPEN AND SHIFT EAST TOWARDS
THE LOWER MISS VALLEY THROUGH TONIGHT...WITH UPPER FLOW BECOMING SW
OVER OUR REGION. LOW LEVEL FLOW OVER OUR REGION TO BECOME S TO SE
AND INCREASE TODAY AND TONIGHT AS HIGH TO OUR NE SLIPS EAST AND LOW
PRESSURE DEVELOPS TO OUR WEST. THIS MAY LEAD THE FRONT TO SHIFT
NORTH A LITTLE...AND WILL ALSO RESULT IN INCREASING ATMOSPHERIC
MOISTURE...THOUGH THE HIGHEST MOISTURE INCREASE WILL BE TO OUR WEST
TODAY. HIGH RESOLUTION NEAR TERM MODELS INDICATING FRONTAL
PRECIPITATION TO GRADUALLY DIMINISH AND SHIFT NORTH THROUGH THIS
MORNING...WHILE ADDITIONAL SCATTERED CONVECTION TO DEVELOP WITH
DIURNAL HEATING OVER THE FA LATER TODAY. WILL MENTION SLIGHT CHANCE
THUNDER TO ACCOMPANY THE CHANCE POPS THIS AFTERNOON...BUT SEVERE NOT
EXPECTED TODAY DUE TO WEAK INSTABILITY. GREATEST INCREASE IN
ATMOSPHERIC MOISTURE AND INSTABILITY TODAY APPEARS WILL BE MAINLY
JUST TO OUR WEST. SPC HAS SLIGHT RISK SEVERE MAINLY JUST TO OUR
WEST.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
UPPER ENERGY AND ACCOMPANYING SURFACE LOW WILL LIFT NE FROM THE
LOWER MISS VALLEY/GULF COAST REGION AHEAD OF AN APPROACHING UPPER
TROUGH AND COLD FRONT. SIGNIFICANT ATMOSPHERIC MOISTURE...WITH PW
VALUES 2 INCHES OR GREATER...WILL TRAVERSE OUR REGION TUESDAY.
CATEGORICAL POPS CONTINUE TO APPEAR IN ORDER. THE TRACK OF BEST
UPPER LIFT...SURFACE LOW...AND STRONGEST LOW LEVEL JET WILL STAY
JUST TO OUR NORTH...BUT MODERATE INSTABILITY AND CONSIDERABLE LOW
LEVEL JET TO PROVIDE A SEVERE THREAT WITH STRONG WIND GUSTS
POSSIBLE. SPC MAINTAINS REGION IN SLIGHT RISK TUE/TUE NT.

THE BEST MOISTURE WILL SHIFT TO OUR EAST TUESDAY NIGHT. COLD FRONT
APPEARS TO MOVE INTO THE REGION TUE NT...BUT THEN SLOWS DOWN AS
LATEST MODELS MAINTAINING UPPER TROUGHINESS JUST TO OUR WEST THROUGH
WEDNESDAY...WITH A SW UPPER FLOW PARALLEL TO THE FRONT AND
APPEARANCE OF SECONDARY LOW FORMATION ALONG THE FRONT NEAR OUR
REGION LATE TUE NT AND EARLY WEDNESDAY. THIS WILL NOT ALLOW THE
FRONT TO GET A PUSH THROUGH THE REGION...AND MAY PROVIDE FOR SOME
LINGERING CLOUDINESS AND MOISTURE WEDNESDAY.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
NO CHANGES WERE MADE TO THE ONGOING FORECAST FOR THIS TIME PERIOD.
PREVIOUS DISCUSSION FOLLOWS.

THE EXTENDED FORECAST FEATURES BELOW NORMAL TEMPERATURES FOR BOTH
MAXIMUM AND MINIMUM TEMPERATURES AND RELATIVELY DRY CONDITIONS.

MEDIUM RANGE MODELS ARE IN REASONABLE AGREEMENT WITH THE UPPER LEVEL
PATTERN THROUGH THE PERIOD.  AN EXPANSIVE UPPER LEVEL TROUGH WILL BE
SITUATED ACROSS MUCH OF THE COUNTRY EAST OF THE ROCKY MOUNTAINS ON
THURSDAY WITH A FRONTAL BOUNDARY LOCATED JUST OFF THE EAST THE
COAST.  ANOTHER SURGE OF UPPER ENERGY WILL DROP INTO THE BACKSIDE OF
THE UPPER TROUGH THURSDAY NIGHT AND FRIDAY...REINFORCING AND
AMPLIFYING THE UPPER TROUGH AS IT PERSISTS INTO THE WEEKEND.
SURFACE HIGH PRESSURE WILL BE CENTERED OVER THE NORTHEASTERN STATES
THUR/FRI AND RIDGE DOWN THE EAST COAST INTO THE FORECAST AREA
LIMITING PRECIPITATION CHANCES.  PWAT VALUES THURSDAY THROUGH
SATURDAY WILL REMAIN LOW...LESS THAN OR AROUND AN INCH...FURTHER
LIMITING PRECIPITATION CHANCES.

THE UPPER TROUGH AXIS SHIFTS CLOSER TO THE FORECAST AREA OVER THE
WEEKEND AS ATMOSPHERIC MOISTURE INCREASES WITH PWAT VALUES RETURNING
TO AROUND 1.5 INCHES.  ATMOSPHERE BECOMES A BIT MORE UNSTABLE WITH
HEATING OF THE MODIFIED AIR MASS AND INCREASED MOISTURE AND WILL
INCLUDE SLIGHT CHANCE POPS ON SUNDAY AS A FRONTAL BOUNDARY
APPROACHES THE REGION.  IT IS WORTH NOTING THAT THERE IS A
DIFFERENCE BETWEEN THE ECMWF/GFS WHERE THE ECMWF BRINGS A CLOSED LOW
ACROSS THE WESTERN OHIO VALLEY BY SUNDAY.  MAX TEMPERATURES ARE
EXPECTED IN THE LOWER 80S THUR/FRI AND POSSIBLY REACH THE MID 80S
NEXT WEEKEND WHILE OVERNIGHT LOWS SHOULD BE IN THE LOWER 60S WITH
SOME POSSIBLE UPPER 50S THUR/FRI MORNING.

&&

.AVIATION /07Z MONDAY THROUGH FRIDAY/...
WEAK ONSHORE FLOW CONTINUES BUT WEAK FRONTAL BOUNDARY REMAINS
WELL NORTH OF TAF SITES. ONLY IFR CONDITIONS ARE NEAR THE CLT
AREA TO THE NORTH OF FRONT WHERE THERE IS WEAK ISENTROPIC LIFT.
ACROSS REGION FROM GEORGIA INTO SOUTH CAROLINA JUST MID LEVEL
CEILINGS AROUND 10 KFT. THERE ARE ALSO SOME LIGHT SHOWERS NORTH
OF TAF SITES BUT SHOULD NOT MOVE INTO AREA LATER CLOSE TO DAYBREAK.
DUE TO UNCERTAINTY AS TO TIMING AND COVERAGE...DECIDED NOT TO
MENTION. ALSO...WILL KEEP CONDITIONS ABOVE IFR. MODELS BRING IFR
CONDITIONS IN BY 12Z. WEAK FRONTAL BOUNDARY IS EXPECTED TO DRIFT
FARTHER NORTHWARD WITH LITTLE AFFECT ON TAF SITES. SREF IFR
PROBABILITIES GIVE LESS THAN A 20 PERCENT CHANCE OF IFR CEILINGS
THIS MORNING. IN ADDITION SATELLITE IMAGERY SHOWS A GOOD SWATHE OF
MID LEVEL CLOUDS CONTINUING TO MOVE EASTWARD ACROSS THE REGION. WILL
START WITH VFR CONDITIONS GRADUALLY LOWERING TO MVFR WITH CEILINGS
1-2 KFT AND VSBYS 3-4 MILES IN FOG FOR A BRIEF PERIOD 12Z-15Z THIS
MORNING. AFTER 15Z...MAINLY VFR CONDITIONS. THERE WILL PROBABLY BE
SOME SHOWERS AND MAY A THUNDERSTORM OR TWO LATER THIS AFTERNOON ON
INTO THE EARLY EVENING HOURS. SPC JUST HAS AREA IN SEE TEXT. DECIDED
NOT MENTION IN TAF SITES DUE TO UNCERTAINTY AS TO TIMING AND
COVERAGE. LATER THIS EVENING AS MOISTURE INCREASES TO THE FORECAST
AREA...DECIDED TO GO MVFR CEILINGS AROUND 2 KFT. SURFACE VSBYS
EXPECTED TO REMAIN 7 MILES AS SURFACE WINDS SHOULD BE 5-6 KNOTS.
HOWEVER...THERE COULD BE BRIEF IFR-MVFR CONDITIONS IN SHOWERS OR
POSSIBLY A THUNDERSTORM. AGAIN...DID NOT MENTION DUE TO UNCERTAINTY.

EXTENDED AVIATION OUTLOOK...WIDESPREAD MVFR OR IFR CONDITIONS MAY
OCCUR MONDAY NIGHT INTO WEDNESDAY ASSOCIATED WITH A SLOW MOVING
FRONTAL SYSTEM.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
SC...NONE.
GA...NONE.
&&

DCM/TTH