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000
FXUS62 KFFC 181147
AFDFFC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
747 AM EDT SAT MAY 18 2013



.PREV DISCUSSION... /ISSUED 438 AM EDT SAT MAY 18 2013/

SHORT TERM /TODAY THROUGH SUNDAY/...
COMPLEX AND DIFFICULT FORECAST THE NEXT 24 TO 36 HOURS.  SLOW MOVING
AND WEAK UPPER LEVEL LOW NOTED IN WATER VAPOR OVER CENTRAL TN
RESPONSIBLE FOR SHOWERS AND A FEW STORMS MOVING INTO WESTERN GA NOW
THROUGH SAT AM. MODELS CONTINUE TO KEEP THE REGION IN A GENERAL
NORTHWEST FLOW PATTERN THROUGH SUNDAY WITH WEAK DISTURBANCES MOVING
SOUTHEAST ON THE SOUTHWEST SIDE OF THE WEAK UPPER LOW.  ALL OF THE
MODELS AGREE ON CHANCE TO LIKELY POPS FOR A PORTION OF THE AREA BUT
DISAGREE ON EXACT LOCATION AND TIMING...MOST LIKELY DUE TO
DIFFICULTIES RESOLVING THESE FEATURES.  FOR TODAY...BIG QUESTION
WITH THIS ONGOING ACTIVITY IS HOW MUCH CLOUD DEBRIS WILL REMAIN AND
HINDER OR DELAY ANY DAYTIME HEATING ACROSS GA AND THE ONSET OF
THUNDERSTORM DEVELOPMENT.  HRRR AND HIGH RES WRF CONTINUE TO SHOW
SHOWERS THROUGH MORNING WITH MORE SCATTERED CONVECTION DEVELOPING
THIS AFTERNOON.  AS A RESULT...HAVE OPTED TO MAINTAIN SOME
CONSISTENCY WITH THE PREVIOUS FORECAST WHICH SHOWS HIGHEST POPS
ACROSS NORTHERN GA THROUGH SAT AND LESSER POPS ACROSS THE SOUTHERN TIER.
WILL HAVE TO WATCH THE SOUTHERN TIER OF COUNTIES AS DIFFERENTIAL
HEATING MAY HAVE A BIGGER THAN CURRENT MODELS FORECASTING GIVEN THE
LOCATION OF THE CURRENT ACTIVITY.

WILL HAVE TO WATCH AREA SAT NIGHT INTO SUNDAY AS SEVERAL MODELS ARE
HINTING AT SOME TYPE OF MCS DEVELOPMENT IN NORTHERN AL AND TN.  GIVEN
THE NORTHWEST FLOW AND THESE DISTURBANCES...CANT RULE IT OUT.  GFS
INSTABILITY FORECAST FOR SUNDAY IS IMPRESSIVE WITH 500 TO 700 MB
LAPSE RATES OVER 8 DEG C/KM FOR NORTHWEST GA.  ALONG WITH CAPE
VALUES 2000 TO AS MUCH AS 4000 J/KG ... THERE WILL BE SOME SEVERE
POTENTIAL...HOWEVER SHEAR VALUES ARE LESS THAN MARGINAL SO
CONFIDENCE ON A MORE WIDESPREAD SEVERE EVENT IS LESS.  MAIN IMPACTS
WILL BE DAMAGING WINDS AND HAIL. TIMING OF STORMS ON SUNDAY IS A
CRAP SHOOT AT THIS POINT AND MODEL TIMING IS ALL OVER THE PLACE
GIVEN THE POTENTIAL MCS DEVELOPMENT EARLY ON SUN.  NEEDLESS TO
SAY...WILL NEED TO WATCH THE FORECAST CLOSELY THROUGH THE WEEKEND AS
IT COULD CHANGE SIGNIFICANTLY FOR BETTER OR WORSE.

HIGH PW VALUES AND THE SLOW MOVING NATURE OF THESE DISTURBANCES HAVE
THE POTENTIAL TO PRODUCE SOME ISOLATED FLASH FLOODING...PARTICULARLY
ACROSS NORTH GA BOTH SAT AND SUN.  WILL NEED TO ALSO MONITOR THIS AS
WELL AS THE SEVERE THREAT.

30

LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
HAVE ADJUSTED POPS UPWARD FOR SUNDAY NIGHT ACROSS THE CWFA. MODELS
ARE SHOWING SIGNS OF AN MCS DEVELOPING IN THE NW FLOW AND AFFECTING
THE CWFA. STRONG SURFACE INSTABILITY AND VERY STEEP LAPSE RATES WILL
HELP SUSTAIN CONVECTION AND CONTINUE THE POSSIBILITY OF SEVERE
STORMS AT LEAST THROUGH MIDNIGHT.

FOR MONDAY...A 500MB TROUGH/SHEAR AXIS SETS UP RIGHT ALONG THE SE
COAST. THIS FEATURE SHOULD SERVE TO FOCUS ANY CONVECTIVE ACTIVITY
FOR MONDAY AND MONDAY EVENING. DO THINK SHRA/TSRA WILL BE MORE
DIURNAL IN NATURE. KEPT THE TREND OF HIGHEST POPS IN THE EAST...WITH
ONLY ISOLATED ACTIVITY TO THE WEST.

WEAK SHEAR AXIS/500MB TROUGH MOVES OFF THE COAST ON TUESDAY WITH
RIDGING BUILDING ALOFT ACROSS THE CWFA. ANY CONVECTION SHOULD BE
ISOLATED IN NATURE...SURFACE INSTABILITY WILL BE PRESENT BUT NO
DISCERNIBLE SURFACE MECHANISM TO FOCUS PRECIP IS NOTED. PRECIP
SHOULD BE DIURNAL.

UNCERTAINTY BEGINS TO CREEP INTO THE FORECAST BY WEDNESDAY. EARLIER
MODEL RUNS PROGGED A COLD FRONT TO MOVE THROUGH THE CWFA DURING THE
LATTER HALF OF THE WEEK. THE ECMWF NOW KEEPS THE FRONT TO THE
NORTH...WHILE THE GFS DOES MOVE THE BOUNDARY THROUGH SLOWLY FRIDAY
THROUGH SATURDAY. HAVE OPTED TO GO PERSISTENCE FOR THIS PERIOD DUE
TO LOW CONFIDENCE.

NLISTEMAA

AVIATION...
06Z UPDATE...
SHOWERS AND A FEW EMBEDDED TSTORMS WORKING EAST OUT OF AL INTO GA
TONIGHT. TIME OF ARRIVAL ON MAIN CELLS FOR ATLANTA AREA SITES
BETWEEN 07Z AND 08Z BUT AM NOT CONFIDENT THEY WILL HOLD TOGETHER
AS AIRMASS IS A LITTLE MORE STABLE ACROSS GA. THIS IS EVIDENT IN THE
LARGER CELL WEST OF CSG WHICH HAS BEEN WEAKENING AS IT MOVES EAST.
DID INCLUDE TSRA IN A TEMPO GROUP AT ATL OVERNIGHT FOR SOME OF THE
STRONGER CELLS. HRRR AND HIGH RES MODELS CONTINUE TO SHOW THE EAST WEST
BANDING OF STORMS THROUGH MID MORNING BUT PRIMARILY OVER AL AND
EXTREME WESTERN GA. WILL NEED TO MONITOR THIS FOR ANY SLIGHT
EASTWARD EXTENSION AND THE NEED TO AMD WITH THUNDER.
OTHERWISE...BELIEVE CLOUD DEBRIS WILL HAMPER EARLY TS DEVELOPMENT
ON SAT BUT STILL THINK TSTORMS WILL DEVELOP IN THE AFTERNOON
PARTICULARLY ACROSS NORTH GA. MVFR CIGS AND VSBYS ARE POSSIBLE
OVERNIGHT PARTICULARLY IN SHOWERS MAINLY BETWEEN 08Z AND 12Z AND
WINDS ARE EXPECTED TO REMAIN GENERALLY LIGHT OUT OF THE SOUTHWEST
BUT INCREASING TO AROUND 10KTS AFTER 16Z.

//ATL CONFIDENCE...06Z UPDATE...
MEDIUM CONFIDENCE ON ALL ELEMENTS

30

&&

.AVIATION...
12Z UPDATE...
MADE SOME CHANGES IN THE TAF BASED ON THE SHORT TERM TRENDS TO
OPEN UP A MORE RAIN FREE WINDOW FROM AROUND 15Z THROUGH EARLY
AFTERNOON. EXPECT THE RAIN COOLED AIR TO STABILIZE THE ATMSPHERE
FOR A FEW HOURS BEFORE STORMS BEGIN TO REDEVELOP THIS AFTERNOON.
HRRR AND WRF SUPPORT NEW DEVELOPMENT AROUND 20Z ACROSS NORTH GA
HOWEVER COVERAGE IS IN QUESTION AT THIS POINT. WINDS AND CIGS
CONSISTENT WITH PREVIOUS DISCUSSION.

//ATL CONFIDENCE...12Z UPDATE...
CONFIDENCE MEDIUM ON CIGS...VSBY.
CONFIDENCE LOW ON PRECIP TIMING.

30

&&

.PRELIMINARY POINT TEMPS/POPS...
ATHENS          80  63  79  65 /  60  60  60  60
ATLANTA         78  65  79  66 /  70  70  60  60
BLAIRSVILLE     73  60  76  60 /  70  70  70  70
CARTERSVILLE    78  63  79  65 /  70  70  60  50
COLUMBUS        83  67  85  67 /  90  50  40  50
GAINESVILLE     76  63  76  63 /  70  70  70  60
MACON           85  64  84  66 /  50  50  60  60
ROME            79  63  81  65 /  70  70  60  40
PEACHTREE CITY  79  64  79  65 /  60  60  60  60
VIDALIA         88  69  87  68 /  40  40  60  60

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...30
LONG TERM....NLISTEMAA
AVIATION...NLISTEMAA