587
FXUS62 KCHS 130742
AFDCHS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
342 AM EDT TUE AUG 13 2013
.SYNOPSIS...
A COLD FRONT WILL APPROACH LATE WEDNESDAY AND WEDNESDAY NIGHT. THE
FRONT WILL STALL OVER THE REGION THURSDAY AND REMAIN IN THE
VICINITY THROUGH THE WEEKEND...WHILE A WEDGE OF HIGH PRESSURE FORMS
IN ITS WAKE. THIS PATTERN WILL BRING UNSETTLED WEATHER FOR THE
SECOND HALF OF THE WEEK...WITH THE POTENTIAL FOR HEAVY RAIN AND
FLASH FLOODING. THE FRONT WILL EVENTUALLY LIFT TO THE NORTH EARLY
NEXT WEEK.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
EARLY THIS MORNING...ISOLATED TO SCATTERED SHOWERS/THUNDERSTORMS
ASSOCIATED WITH A PASSING 500 MB SHORTWAVE WILL CONTINUE TO STREAM
EASTWARD OVER THE NEXT COUPLE OF HOURS. COVERAGE WILL REMAIN
MAINLY ISOLATED AND I HAVE KEPT AT LEAST A SLIGHT CHANCE POP IN
THE FORECAST THROUGH THE REMAINDER OF THE MORNING.
TODAY...THE OVERALL SYNOPTIC PATTERN WILL BEGIN ITS TRANSITION TO
THE MORE ACTIVE PATTERN THAT HAS BEEN ADVERTISED FOR SEVERAL DAYS
NOW. THE WEST/EAST ORIENTED MID/UPPER LEVEL RIDGE WILL BE SUPPRESSED
SOUTH AND EVENTUALLY SPLIT AS THE PATTERN AMPLIFIES IN RESPONSE TO A
CLOSED LOW AND ASSOCIATED SHORTWAVE DIVING ACROSS THE GREAT LAKES
REGION. AT THE SURFACE...ATLANTIC HIGH PRESSURE WILL WEAKEN AND GIVE
WAY TO A COLD FRONT APPROACHING THE REGION FROM THE NORTHWEST. THIS
FRONT WILL REINFORCE THE INLAND TROUGH OF LOW PRESSURE AND LEAVE THE
AREA UNDER A LARGELY SOUTHWESTERLY FLOW IN THE LOW LEVELS. WINDS
WILL ONCE AGAIN TAKE ON A WESTERLY COMPONENT FOR MUCH OF THE
DAY...RESULTING IN YET ANOTHER WARM AND MUGGY DAY. HIGHS WILL REACH
THE MID 90S AND HEAT INDEX VALUES WILL AGAIN RANGE AROUND 105 FOR
SEVERAL HOURS THIS AFTERNOON. THE POP TREND FOR THE DAY WILL RAMP UP
IN THE LATE AFTERNOON AND INTO THE EVENING...CLOSELY ALIGNED WITH
THE APPROACH OF THE FRONT AND THE LOW LEVEL CONVERGENCE AND DEEPER
MOISTURE ASSOCIATED WITH IT. I HAVE STARTED THE DAY OFF WITH SLIGHT
CHANCES EVERYWHERE FOR THE MORNING TO ACCOUNT FOR ISOLATED
SHOWER/THUNDERSTORM DEVELOPMENT WITH THE ONSET OF SURFACE HEATING.
CHANCE POPS THEN INVADE FROM THE NORTH AND NORTHWEST AS THE FRONT
APPROACHES INTO THE EVENING. IN FACT...MOST OF THE MODEL SOLUTIONS
SHOW MOST OF THE ACTIVITY OCCURRING AFTER SUNSET ACROSS SOUTHEAST SC
AND ESPECIALLY THE TRI-COUNTY REGION. I HAVE STILL KEPT POPS IN THE
HIGH CHANCE RANGE...THOUGH I CAN CERTAINLY SEE A SCENARIO WHERE AN
AREA OF LIKELY POPS MAY BE REQUIRED WITH FUTURE FORECAST UPDATES.
REGARDING THE NATURE OF THE CONVECTION AND THE POTENTIAL FOR STRONG
TO SEVERE STORMS AND HEAVY RAIN...PWATS WITH THE DEEP MOISTURE BAND
AHEAD OF THE FRONT SURGE TO AROUND 2.2 INCHES. SEVERE WEATHER
PARAMETERS CERTAINLY AREN/T THAT IMPRESSIVE WITH MID LEVEL LAPSE
RATES AROUND 5 C/KM...FREEZING LEVELS OVER 16 KFT...AND WEAK FLOW
THROUGH THE DEPTH OF THE ATMOSPHERE. HOWEVER...WE CANNOT DISCOUNT A
FEW WET MICROBURSTS ESPECIALLY WHERE SIGNIFICANT MESOSCALE BOUNDARY
INTERACTIONS OCCUR. OF MORE CONCERN IS THE POTENTIAL FOR LOCALLY
HEAVY RAINFALL. THESE SHOWERS/STORMS WILL BE EFFICIENT RAINFALL
PRODUCERS AND THE FLOW REGIME IS SUCH THAT STORMS MAY MOVE OVER THE
SAME AREAS IN A WEST/EAST DIRECTION. STILL...THE OVERALL THREAT OF
SEVERE WEATHER AND HEAVY RAINFALL IS ISOLATED ENOUGH TO PRECLUDE A
MENTION IN THE HWO FOR NOW.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/...
TONIGHT...THE AFOREMENTIONED COLD FRONT WILL SAG INTO THE FORECAST
AREA ROUGHLY LINED UP RIGHT ACROSS THE NORTHERN HALF. THIS SHOULD
PROVIDE A FOCUS FOR ONGOING NOCTURNAL CONVECTION THAT WILL LIKELY
IMPACT SOUTHEAST SC THE MOST GIVEN THE ORIENTATION OF THE BOUNDARY
AND THE DEEP MOISTURE. THE HIGHEST POPS...HIGH CHANCE...ARE ACROSS
THE TRI-COUNTY REGION AND THE ADJACENT COAST. TEMPS WILL BE SIMILAR
TO PREVIOUS NIGHTS...FALLING INTO THE MID TO UPPER 70S IN MOST
AREAS.
THE SHORT TERM PERIOD WILL TURN OUT TO BE QUITE UNSETTLED AND WET AS
AN UNUSUAL SYNOPTIC PATTERN FOR MID AUGUST DEVELOPS ACROSS THE LOCAL
AREA.
WEDNESDAY...AN EXPANSIVE AND HIGHLY AMPLIFIED MID LATITUDE TROUGH
WILL BE FOUND COVERING MUCH OF THE EASTERN UNITED STATES...WHICH
PUSHES A COLD FRONT FROM THE NW AND NORTH INTO CLOSER PROXIMITY BY
LATE IN THE DAY AND DURING THE NIGHTTIME HOURS. IN ADVANCE OF THE
FRONT IS AN ALREADY EXISTING PRE-FRONTAL TROUGH...THAT WILL
EVENTUALLY BECOME ABSORBED WITHIN THE CYCLONIC CIRCULATION IN
ADVANCE OF THE APPROACHING COLD FRONT. CONDITIONS ARE VERY
FAVORABLE FOR INCREASING SHOWERS AND T-STORMS...WITH AMPLE LOW LEVEL
CONVERGENCE AND FORCING ALOFT WITH MID LEVEL IMPULSES AND THE NEARBY
RIGHT REAR QUAD OF THE UPPER LEVEL JET LATE IN THE DAY. WITH THE
MAIN EAST-NE TO WEST-SW MOISTURE AXIS AND BEST UPWARD OMEGA FIELDS
ALIGNED ACROSS THE FORECAST REGIME...THIS PROMISES TO BE A WET DAY.
PWATS ARE NEAR OR GREATER THAN 2 INCHES AND THE VERTICAL COLUMN IS
PRETTY WELL SATURATED TO ABOUT 300-400 MILLIBARS. WE ANTICIPATE THAT
AT LEAST MINOR FLOODING PROBLEMS WILL OCCUR...IF NOT LOCALIZED FLASH
FLOODING WITH BOUTS OF HEAVY RAINS IN MANY PARTS OF THE AREA. WE MAY
ALSO NEED TO CONSIDER A FLASH FLOOD WATCH AT SOME POINT.
ADDITIONALLY...WHERE BOUNDARY INTERACTION OCCURS THIS MAY SUPPORT
SOME STRONGER WET MICROBURST POTENTIAL. A GREATER AMOUNT OF
CLOUDINESS AND THE HIGHER RAIN PROBABILITIES SUGGEST THAT MAXIMUM
TEMPS WILL BE SEVERAL DEGREES LOWER THAN RECENTLY. HOWEVER...THERE
MAY BE JUST ENOUGH COMPRESSIONAL HEATING AHEAD OF THE FRONT AND SOME
DOWNSLOPE OFF THE MOUNTAINS TO SUPPORT HIGHS IN THE UPPER 80S TO
AROUND 90 BEFORE THE WIDESPREAD CONVECTION DEVELOPS. BUT THIS IS THE
LAST OF THE UNUSUALLY HOT WEATHER FOR AWHILE.
THURSDAY THROUGH FRIDAY...THE LARGE SCALE SHOWS A CONTINUATION OF
THE LONG WAVE TROUGH IN THE EAST...WITH A FEW MID LEVEL IMPULSES
FORECAST TO MOVE OVERHEAD AT TIMES. IN FACT THE ROUGH WILL START TO
SHARPEN FURTHER BY FRIDAY AS A STRONG SHORT WAVE DROPS SE THROUGH
THE CENTRAL PLAINS. THE BIG QUESTION THOUGH IS IN REGARDS TO HOW FAR
SOUTH IS THE COLD FRONT TRANSITIONING INTO A STATIONARY FRONT ABLE
TO PENETRATE. THE FRONT BECOMES ALIGNED WITH THE FLOW ALOFT AND
THERE DOESN/T APPEAR TO BE MUCH OF A PUSH TO THE NORTH OF THE FRONT
FROM HIGH PRESSURE STRETCHING FROM THE UPPER MIDWEST TO THE MID
ATLANTIC AND LONG ISLAND. BUT IF ENOUGH RAINFALL OCCURS THIS COULD
FORCE THE FRONT FURTHER SOUTHWARD. WE DO FEEL FAIRLY CONFIDENT THAT
COVERAGE OF CONVECTION WILL BE WELL ABOVE CLIMATOLOGY GIVEN
CONSIDERABLE FORCING BOTH IN THE LOWER LEVELS AND UPSTAIRS AND
ABUNDANT MOISTURE. HOWEVER...SINCE THERE IS SOME CONCERN ON WHERE
THE FRONT POSITIONS ITSELF WE HAVE CHOSEN NOT GO ANY HIGHER THAN
60-70 POPS THURSDAY AND 50-60 POPS FRIDAY. ADDITIONAL HEAVY RAINS
ARE EXPECTED WITH PWATS PERHAPS AS HIGH AS NEAR 2 STANDARD
DEVIATIONS ABOVE NORMAL. BULK SHEAR IS NOT BAD CONSIDERING ITS
AUGUST...SO THERE COULD BE SOME ORGANIZATION OF STORMS AND THIS MAY
LEAD TO SOME WET MICROBURSTS. BUT THIS RISK APPEARS SECONDARY TO THE
GREATER PROBABILITIES FOR AT LEAST LOCALIZED FLOODING RAINFALL.
WE/RE LOOKING FOR AT LEAST 1-3 INCH RAINFALL AREA-WIDE DURING THE
TWO DAY PERIOD...ALTHOUGH LOCALLY HIGHER AMOUNTS ARE CERTAINLY
POSSIBLE WHERE TRAINING OF CONVECTIVE CELLS TRANSPIRES. THERE IS
SOME POTENTIAL FOR A BUST IN REGARDS TO MAX TEMPS...BUT IT DOES LOOK
TO BE CONSIDERABLY LOWER IN MANY LOCATIONS THAN WE HAVE EXPERIENCED
IN SEVERAL WEEKS.
&&
.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
THERE REMAINS A HIGHLY AMPLIFIED BLOCKING PATTERN ACROSS THE CONUS
AND WESTERN ATLANTIC THROUGH THE WEEKEND...BEFORE SLOWLY STARTING TO
SHOW SOME SIGNS OF CHANGING EARLY NEXT WEEK. A LONG WAVE TROUGH FROM
THE GREAT LAKES TO THE LOWER MISSISSIPPI VALLEY IS SANDWICHED
BETWEEN THE SUB-TROPICAL RIDGE OVER THE SW ATLANTIC AND ANOTHER
RIDGE IN THE DESERT SW. EVENTUALLY THE TROUGH BEGINS TO LIFT OUT ON
MONDAY...ALLOWING THE OCEANIC RIDGE TO POKE WESTWARD INTO THE LOCAL
AREA. AT THE SURFACE THE STATIONARY FRONT WILL REMAIN OVER OR NEARBY
INTO AT LEAST SATURDAY...MAYBE EVEN INTO SUNDAY...BEFORE IT LIFTS
NORTH. UNTIL THAT HAPPENS...MOISTURE REMAINS PLENTIFUL WITH PWATS
FAR ABOVE TYPICAL AUGUST VALUES. THUS MORE WET WEATHER IS IN THE
OFFING THROUGH THE WEEKEND...BEFORE SOME DRYING OUT TAKES PLACES ON
MONDAY. WITH THE MAIN PLUME OF SUB-TROPICAL AND TROPICAL MOISTURE TO
EXTEND ACROSS SOUTHERN SOUTH CAROLINA AND SE GEORGIA...EXCESSIVE
RAINFALL TOTALS ARE QUITE POSSIBLE. THE WPC 7 DAY QPF THROUGH 00Z
NEXT TUESDAY SHOWS A GOOD 4-6 INCHES ACROSS THE LOCAL ZONES.
MEANWHILE IN REGARDS TO DAYTIME HIGHS...THEY WILL GENERALLY BE BELOW
SEASONAL NORMALS WITH EXTENSIVE CLOUDINESS AND ELEVATED RAIN
CHANCES.
WORTH NOTING IS THAT WE CONTINUE TO KEEP A CLOSE WEATHER WATCH ON
THE GULF OF MEXICO...WHERE SOMETHING TROPICAL COULD FORM LATE THIS
WEEK. FOR FURTHER INFORMATION PLEASE REFER TO THE TROPICAL WEATHER
OUTLOOK ISSUED BY NHC.
&&
.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
THE MOST IMMEDIATE CONCERN AT KCHS AND KSAV IS THE POTENTIAL FOR
ADDITIONAL IMPACTS FROM ONGOING SHOWERS/THUNDERSTORMS. CURRENT
ACTIVITY IS QUITE ISOLATED AND WILL NOT MENTION ANYTHING IN THE
TAF FOR THE PRE-DAWN HOURS. THE FOCUS THEN TURNS TO TUESDAY. A
COLD FRONT WILL BEGIN TO APPROACH THE REGION FROM THE NORTHWEST
LATE TUESDAY AFTERNOON AND INTO THE EVENING...BRINGING WITH IT
INCREASED LOW LEVEL CONVERGENCE AND DEEP MOISTURE. THUNDERSTORM
ACTIVITY WILL LIKELY BE AT ITS BEST COVERAGE LATE IN THE PERIOD AS
THE FRONT DRAWS CLOSER. I HAVE HANDLED CONVECTION WITH A VCTS
STARTING AT 22Z AT BOTH SITES THAT CONTINUES THROUGH THE 01-02Z
TIME PERIOD. THEREAFTER...KCHS WILL LIKELY SEE THE BEST CHANCE OF
PRECIP THROUGH THE REMAINDER OF THE FORECAST PERIOD AND A SHRA
MENTION HAS BEEN ADDED THERE THROUGH 06Z. I HAVE KEPT THINGS VFR
IN THE VCTS GROUP FOR NOW...ALLOWING FUTURE FORECAST UPDATES TO
HONE IN ON BETTER TIMING AND FLIGHT RESTRICTIONS AS CONVECTIVE
TRENDS BECOME CLEARER.
EXTENDED AVIATION OUTLOOK...THERE IS A MODERATE TO HIGH RISK OF AT
LEAST PERIODIC FLIGHT RESTRICTIONS THROUGH SATURDAY...AS A COLD
FRONT TRANSITIONS INTO A STATIONARY FRONT AS IT STALLS OVER OR NEAR
THE AREA. OCCASIONAL SHRA AND TSRA ALONG WITH POSSIBLE HEAVY RAINS
AND MAYBE EVEN GUSTY WINDS WILL BE COMMON.
&&
.MARINE...
TODAY AND TONIGHT...A COLD FRONT WILL MOVE INTO THE LEE OF THE
SOUTHERN APPALACHIANS DURING THE PERIOD LEADING TO A TIGHTENING OF
THE PRESSURE GRADIENT AND INCREASED SOUTHWEST FLOW OVER THE LOCAL
WATERS. WINDS WILL PICK UP THIS AFTERNOON AND LIKELY REACH THEIR
HIGHEST POINT IN THE LATE EVENING HOURS...PERHAPS EXTENDING INTO THE
EARLY WEDNESDAY MORNING TIME PERIOD. IN FACT...WINDS MAY BRIEFLY
TOUGH SMALL CRAFT ADVISORY CRITERIA AT TIMES. A SOLID 15-20 KTS IS
EXPECTED ACROSS ALL ZONES...INCLUDING THE LAND/SEA INTERFACE AND THE
CHARLESTON HARBOR. WILL KEEP CONDITIONS BELOW ADVISORY THRESHOLDS
FOR NOW...THOUGH IT WILL BE CLOSE. SEAS WILL GENERALLY RANGE 2-3 FT
WITHIN 20 NM...AND UP TO 4 FT OVER THE CHARLESTON COUNTY
WATERS...AND 3-4 FT BEYOND 20 NM.
WEDNESDAY AND WEDNESDAY NIGHT...AN UPSTREAM COLD FRONT WILL DROP SE
AND WILL BEGIN TO MERGE WITH AN ALREADY EXISTING TROUGH ACROSS THE
CAROLINAS. THE FRONT WILL STILL REMAIN OUTSIDE THE LOCAL WATERS
DURING THE DAYLIGHT HOURS...BUT WILL TRY AND SLIP INTO THE MARINE
AREA WEDNESDAY NIGHT. GIVEN A VERY RELAXED PRESSURE GRADIENT THERE
IS NO RISK FOR ANYTHING HIGHER THAN ABOUT 10-15 KT WINDS AND SEAS OF
2-4 FT /HIGHEST ACROSS THE OUTERMOST GEORGIA WATERS/.
THURSDAY THROUGH SUNDAY...THE COLD FRONT WILL STALL OUT OVER OR NEAR
THE MARINE COMMUNITY...BUT THE EXACT FORECAST IS HIGHLY DEPENDENT
UPON WHERE THIS STATIONARY FRONT ACTUALLY IS FOUND INTO THE WEEKEND.
THERE DOESN/T APPEAR TO BE MUCH PUSH SOUTH FROM HIGH PRESSURE TO THE
NORTH...SO WE/RE NOT QUITE SURE ON HOW FAR SOUTH THE FRONT WILL MAKE
IT. UNLESS THERE IS STRONG PINCHING NORTH OF THE FRONT AND/OR SOME
SORT OF TROPICAL SYSTEM IN THE NORTHERN GULF THAT CREATES A TIGHTER
GRADIENT...THE PROBABILITIES FOR ANY SMALL CRAFT CONDITIONS IS
EXTREMELY LOW.
MARINERS ADVISED THAT SHOWER AND T-STORM ACTIVITY WILL BE AT LEAST
SCATTERED OR NUMEROUS THROUGH SUNDAY...SOME OF WHICH WILL PRODUCE
HAZARDS SUCH AS GUSTY WINDS...FREQUENT LIGHTNING AND HEAVY RAINFALL.
&&
.TIDES/COASTAL FLOODING...
THE PERIGEAN SPRING TIDES WILL BE UPCOMING EARLY NEXT WEEK AND THIS
COULD LEAD TO POSSIBLE MINOR COASTAL FLOOD CONCERNS...ESPECIALLY IF
WINDS ARE ONSHORE. PROBABILITIES/CONFIDENCE HOWEVER ARE NOT YET
SUFFICIENT ENOUGH TO INCLUDE IN THE HAZARDOUS WEATHER OUTLOOK.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...NONE.
&&
$$
SYNOPSIS...
NEAR TERM...BSH
SHORT TERM...33
LONG TERM...33
AVIATION...BSH
MARINE...BSH/33
TIDES/COASTAL FLOODING...33