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000
FXUS62 KCHS 182000
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
400 PM EDT SAT MAY 18 2013

.SYNOPSIS...
AN UPPER LEVEL DISTURBANCE WILL GRADUALLY MOVE INTO THE AREA SUNDAY
INTO TUESDAY...THEN WILL STALL AND DISSIPATE AS ATLANTIC HIGH PRESSURE
BECOMES THE DOMINANT FEATURE THROUGH MUCH OF NEXT WEEK. A COLD FRONT
WILL APPROACH THE REGION BY NEXT WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
THIS AFTERNOON...ONGOING SHOWER/THUNDERSTORM DEVELOPMENT WILL
CONTINUE TO BE CONCENTRATED MAINLY ALONG AND WEST OF I-95 THROUGH
THE EARLY EVENING. SO FAR...THESE STORMS HAVE BEEN VERY PULSEY AND
UNABLE TO SUSTAIN GOOD CORES FOR VERY LONG. THE FREEZING LEVEL IS
AROUND 12.5 KFT AND THE -20C LEVEL IS AROUND 23.5 KFT. THE MAIN
THREATS WILL BE SMALL HAIL AND GUSTY WINDS...THOUGH ONE OR TWO
STORMS COULD BECOME STRONG ENOUGH TO WARRANT A SEVERE THUNDERSTORM
WARNING...ESPECIALLY WHERE OUTFLOW AND SEABREEZE BOUNDARY
INTERACTIONS OCCUR.

TONIGHT...WITH THE LOSS OF AFTERNOON HEATING ANY REMAINING
SHOWER/STORM ACTIVITY WILL DECREASE AND BECOME CONTAINED FURTHER TO
THE WEST. THROUGH THE OVERNIGHT...THE BAGGY UPPER TROUGH ALOFT WILL
CONTINUE TO MEANDER AROUND JUST TO THE NORTH AND NORTHWEST OF THE
FORECAST AREA. A PERUSAL OF MODEL SOUNDINGS SHOWS CONTINUED
MOISTENING AND THE PRESENCE OF ELEVATED INSTABILITY THROUGH
DAYBREAK. IN FACT...SHOWALTERS GO TO AROUND -1 OR -2. MANY OF THE
SUITE OF MODELS DEPICTS SHOWER/STORM COVERAGE INCREASING LATE
ACROSS MUCH OF THE AREA AND STREAMING IN FROM OFFSHORE. POPS SHOW A
DECREASING TREND THROUGH THE EVENING AND THEN RAMP BACK UP TO CHANCE
TOWARDS SUNRISE. PLENTIFUL CLOUD COVER AND WARM SOUTHERLY FLOW WILL
KEEP LOWS IN THE MID/UPPER 60S IN MOST AREAS.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
THE UPSTREAM SHORTWAVE TROUGH WILL SETTLE INTO THE REGION SUNDAY
THROUGH MONDAY...THEN WILL STALL AND SLOWLY UNRAVEL/WEAKEN OVER THE
REGION BY THE END OF THE FORECAST PERIOD. THE OVERALL REGIME WILL
FAVOR SCATTERED TO NUMEROUS SHOWERS/THUNDERSTORMS...SIGNIFICANT
CLOUD COVER AND TEMPERATURES NOT EXCEEDING THE LOWER TO MID 80S AT
MOST LOCALES. OF GREATEST INTEREST...LOCALLY HEAVY RAINFALL IS
POSSIBLE GIVEN PWATS INCREASING TO BETWEEN 1.5 AND 1.8 INCHES/
1 TO 3 STANDARD DEVIATIONS ABOVE NORMAL FOR MID-MAY AND MULTICELL
CONVECTION WITHIN A RELATIVELY WEAK STEERING FLOW REGIME/MODULATED
BY MESOSCALE BOUNDARIES.

GIVEN THE COMPLEX CONVECTIVE SCENARIO...FORECAST DETAILS WILL REMAIN
VERY DIFFICULT TO PIN DOWN BEYOND THE NEAR TERM FORECAST PERIOD. IN
GENERAL...PRECIPITATION COVERAGE SHOULD GENERALLY INCREASE PER
DIURNAL WARMING/DESTABILIZATION EACH AFTERNOON/EVENING. THE GREATEST
POTENTIAL COVERAGE WILL SUNDAY AND MONDAY PM...THEN DIURNAL
PRECIPITATION COVERAGE SHOULD BE RELATIVELY MORE SPARSE TUESDAY PM
DUE TO BUILDING HEIGHTS ALOFT AND AN ASSOCIATED WEAKENING OF THE
MID/UPPER VORTICITY MAXIMUM. HOWEVER...THE DEGREE OF DIURNAL
WARMING/DESTABILIZATION WILL DEPEND ON CLOUD COVER/ANTECEDENT
PRECIPITATION. INDEED...UPSTREAM/CLOSER TO THE UPPER LOW SATURDAY
AFTERNOON EARLIER PRECIPITATION AND LINGERING CLOUD COVER HELD
TEMPERATURES IN THE 70S AND HAD LIMITED COVERAGE/INTENSITY OF
PRECIPITATION...AND THIS SCENARIO CANNOT BE RULED OUT IN OUR AREA
SUNDAY/MONDAY.

OTHERWISE...CANNOT RULE OUT SHOWERS/THUNDERSTORMS AT ANY TIME
THROUGH THE PERIOD. FOR EXAMPLE...18/12Z GUIDANCE GENERALLY
INDICATES AN INCREASE IN DEEP LAYER MOISTURE AND AN ASSOCIATED
INCREASE IN SHOWER COVERAGE OVERNIGHT SUNDAY NIGHT/EARLY
MONDAY...ESPECIALLY ACROSS NORTHERN/EASTERN COUNTIES. WHILE
CONFIDENCE IN THIS SCENARIO IS NOT NECESSARILY HIGH...MODEL TRENDS
JUSTIFY AN INCREASE IN POPS OVERNIGHT SUNDAY NIGHT INTO EARLY
MONDAY. FURTHER...OTHERWISE...HIGHEST POPS ARE CAPPED IN THE HIGH
CHANCE/LOW LIKELY CATEGORY INLAND DURING THE SUNDAY/MONDAY PM
PERIOD...BUT SIGNIFICANT REFINEMENTS WILL BE REQUIRED WITHIN
SUBSEQUENT FORECAST PACKAGES.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
A RATHER WARM PATTERN PERSISTS IN THE LONG TERM AS THE AREA SITS ON
THE WESTERN SIDE OF AN UPPER RIDGE WITH ATLANTIC HIGH PRESSURE
OFFSHORE. RAIN CHANCES WILL INCREASE A BIT LATE IN THE WEEK AS AN
UPPER TROUGH APPROACHES FROM THE WEST AND A COLD FRONT DROPS INTO
THE AREA FRIDAY NIGHT OR SATURDAY.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
AT KSAV...THE MOST IMMEDIATE CONCERN IS THE POTENTIAL FOR
THUNDERSTORM DEVELOPMENT IN THE VICINITY OVER THE NEXT FEW HOURS.
MOST HI-RES MODEL RUNS SHOW STORMS LINING UP ROUGHLY NORTH/SOUTH
WITH THE EASTERN EXTENT BEING RIGHT NEAR THE AIRFIELD. CONFIDENCE
IS LOW IN DIRECT IMPACTS SO I HAVE ADDED A VCTS/CB FOR THE 19-21Z
TIME FRAME. WILL AMEND BASED ON RADAR TRENDS. THEN
OVERNIGHT...MODEL GUIDANCE SHOWS THE POTENTIAL FOR DEVELOPING MVFR
CEILINGS LATE WHICH THEN CONTINUES THROUGH MUCH OF THE MORNING.
STILL NOT CONFIDENT ENOUGH TO ADVERTISE PREVAILING MVFR CONDITIONS
SO I HAVE JUST ADDED A MENTION OF SCATTERED MVFR LEVEL CLOUDS.

AT KCHS...AFTERNOON THUNDERSTORMS ARE EXPECTED TO REMAIN WEST OF
THE AREA. THE MAIN THING TO KEEP AN EYE ON IS THE OVERNIGHT PERIOD
WHEN MUCH OF THE MODEL GUIDANCE SHOWS ISOLATED/SCATTERED SHOWERS
DEVELOPING ALONG THE SC COASTLINE WITHIN SOME WEAK CONVERGENCE. I
HAVE INTRODUCED VCSH STARTING AT 09Z AND WITH ANY SHOWERS THAT DO
DEVELOP MVFR CEILINGS ARE POSSIBLE.

EXTENDED AVIATION OUTLOOK...PRIMARILY VFR. DIRECT/TRANSIENT IMPACTS
INCLUDING FLIGHT RESTRICTIONS FROM ISOLATED TO SCATTERED
SHOWERS/THUNDERSTORMS WILL LIKELY OCCUR THROUGH EARLY NEXT WEEK.

&&

.MARINE...
TONIGHT...NO CHANGE TO THE SETUP WITH ATLANTIC HIGH PRESSURE DRIVING
PERSISTENT S TO SE FLOW. WIND SPEEDS WILL REMAIN AT OR BELOW 15 KTS
WITH SEAS 2-4 FT.

PERSISTENT ATLANTIC HIGH PRESSURE WILL PRODUCE SOUTH TO SOUTHEAST
WINDS RANGING FROM 10 TO 15 KNOTS...WITH LOCALLY/BRIEFLY STRONGER
ONSHORE WINDS AS HIGH AS 20 KT DUE TO THE DIURNAL SEA BREEZE
CIRCULATION EACH DAY. SEAS WILL AVERAGE 2 TO 4 FEET MUCH OF THE
TIME...ALTHOUGH SEAS AS HIGH AS 5-6 FT COULD BEGIN TO IMPINGE UPON
WATERS BEYOND 20 NM BY MID TO LATE WEEK.

&&

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

&&

$$

NEAR TERM...BSH
SHORT TERM...SPR
LONG TERM...JRL
AVIATION...BSH
MARINE...BSH/SPR