624
FXUS62 KFFC 042220
AFDFFC
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
620 PM EDT SAT JUL 4 2009
SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/
UPPER ATMOSPHERIC PATTERN REMAINS VERY PERSISTENT THROUGH THE NEXT
14 DAYS WITH A PREDOMINATE UPPER RIDGE ANCHORED IN THE SOUTH
CENTRAL/SOUTHWEST U.S. AND A PREVAILING UPPER LOW IN THE NE U.S.
THIS PATTERN WILL KEEP NW FLOW ALOFT ACROSS THE E/SE U.S. THE
WESTERLIES ACROSS THE NORTHERN U.S. ON THE NORTH SIDE OF THE UPPER
RIDGE ARE QUITE STRONG FOR JULY. DURING THE NEXT 24-35 HRS...THE
STRONG SUBTROPICAL RIDGE WILL RETROGRADE SLIGHTLY INTO THE SW
DESERTS ALLOWING NW FLOW ALOFT TO STRENGTHEN INTO THE SE STATES.
THIS WILL PUSH A STRONG SHORT WAVE...CURRENTLY LOCATED IN THE
NORTHERN PLAINS/MIDWEST...TOWARD THE SE TONIGHT AND SUN. A RATHER
IMPRESSIVE SFC LOW AND FRONT...FOR JULY...IS EXPECTED TO ACCOMPANY
THE SHORT WAVE AS IT PUSHES INTO THE SE U.S. SUN-MON.
THE MAIN QUESTIONS FOR THIS FORECAST ARE TIMING AND SEVERITY OF
CONVECTION EXPECTED TO ACCOMPANY THE APPROACHING SYNOPTIC SCALE
SYSTEM AND DURATION OF THE CONVECTION ONCE IT ARRIVES. SFC AIR MASS
IS UNUSUALLY DRY FOR THIS TIME OF YEAR...THANKS TO SEVERAL DAYS OF
CYCLONIC FLOW AROUND THE PERSISTENT NE U.S. UPPER LOW AND A LARGE
MODIFIED CANADIAN SFC HIGH THAT SETTLED OVER THE AREA THIS MORNING.
THIS HAS LEFT OUR CWA WITH SOME OF THE COOLEST AND DRIEST AIR ANY
ONE COULD EXPECT IN THIS PART OF THE COUNTRY AT THE FOURTH OF JULY
AND THE FIRST JULY 4TH THAT I HAVE EVER SEEN IN MY OVER TEN YEARS AT
THIS OFFICE WITHOUT AFTERNOON CONVECTION. NUMERICAL MODELS IN GOOD
AGREEMENT THAT MOISTENING OF THE LOWER ATMOSPHERE WILL BE DELAYED
UNTIL LATE TONIGHT/EARLY SUN FROM N-S...BUT WILL OCCUR QUICKLY AND
IMPRESSIVELY ONCE BAROCLINIC ZONE DRIFTS INTO THE AREA. AN
IMPRESSIVE SW LOW-LEVEL JET IS EXPECTED TO DEVELOP ACROSS CENTRAL AL
INTO N GA BY SUN MORNING...QUICKLY BRINGING RICH...GULF MOISTURE
INTO THE CWA. THIS RESULTS IN AN EQUALLY IMPRESSIVE INCREASE IN PWS
AS ADVERTISED BY THE MODELS...RISING FROM LESS THAN 0.75 INCH THIS
MORNING TO WELL OVER 2.0 INCHES BY SUN EVENING.
THE DEVELOPING LOW-LEVEL JET AND INCREASING PVA/UPPER DYNAMICS MAY
ALLOW UPSTREAM TSRA TO DRIFT INTO THE FAR NORTHERN PART OF THE CWA
EARLY SUN. THE MODELS ARE FAIRLY CONSISTENT IN ADVERTISING A
RELATIVELY NARROW BAND OF WEST-EAST CONVECTION ALONG THE SEWD MOVING
FRONTAL BOUNDARY...WHICH MAY DRIFT INTO OUR FAR NORTHERN ZONES
TOWARD 12Z SUN. THIS CONVECTION WOULD THEN REDEVELOP FURTHER
SOUTHWARD INTO THE CWA DURING THE AFTERNOON/EVENING SUN AS THE
ENTIRE SYSTEM CONTINUES SAGGING SOUTHWARD. WITH RESPECT TO THE
SEVERE THREAT...THE ADVERTISED LLJ CONCERNS ME SOMEWHAT...ALTHOUGH
IT IS OUT OF PHASE WITH THE BEST THERMODYNAMICS...THE FORMER OF
WHICH REACHES THE CWA EARLY IN THE DAY SUN AND THE LATER DURING THE
AFTERNOON. AFTERNOON CONVECTION AND ASSOCIATED INTENSITY WILL ALSO
BE LARGELY DEPENDENT ON CONVECTIVE CLOUD DEBRIS THAT OVERSPREADS THE
AREA EARLIER IN THE DAY...WHICH SHOULD BE LIMITED INTO CENTRAL GA.
ANOTHER CONCERN IS WARM MID-LEVEL TEMPS OF -4 TO -6C AT
500MB...LEADING TO WEAK LAPSE RATES. MOREOVER...THE SYSTEM APPEARS
SIMILAR TO THAT OF LAST WEEK THAT PRODUCED ONLY ISOLD SEVERE
CONVECTION AND SCT CONVECTION AT BEST. I AM STILL NOT CONFIDENT THAT
THIS IS GOING TO BRING WIDESPREAD RAIN TO THE CWA.
OVERALL QPF SHOULD BE IN THE 0.50 TO 1.50 INCH RANGE ACROSS THE CWA
THROUGH THE SHORT TERM. THE THREAT FOR SHRA/TSRA WILL DIMINISH FROM
N-S MON...AND SHOULD GENERALLY BE CONFINED TO THE SOUTHERN HALF OF
THE CWA MON. DIMINISHING DYNAMICS SUGGEST THAT SEVERE THREAT MON
WILL BE LESS THAN EXPECTED SUN AFTERNOON/EVENING...BUT ISOLD SEVERE
DURING MAX HEATING MON ACROSS SOUTHERN COUNTIES NOT OUT OF THE
QUESTION.
FOR TEMPS...A COUPLE OF MILD DAYS ARE IN ORDER...CERTAINLY COOLER
THAN RECENT WEEKS...WITH INCREASED CLOUD COVER AND PCPN CHANCES.
WITH THE EXCEPTION OF LAST NIGHT MINS WHERE THE MAV MOS BEAT THE MET
MOS...THE MET HAS SHOWN OVERALL LESS ERROR THE PAST SEVERAL DAYS.
GIVEN THAT THE AIR MASS IS RETURNING TO A MUCH MORE HUMID AND
CLIMATOLOGICALLY CONSISTENT AIR MASS FOR JULY...WILL LEAN HEAVILY
TOWARD THE MET MOS WITH THIS PACKAGE FOR THE SHORT TERM TEMPS.
LONG TERM /TUESDAY THROUGH SATURDAY/
AS NOTED ABOVE...LITTLE CHANGE IN THE OVERALL PATTERN IS NOTED INTO
THE EXTENDED PERIODS. NW FLOW ALOFT CONTINUES ACROSS THE SE U.S. AS
THE SW U.S. UPPER RIDGE PERSISTS. THE EARLY WEEK FRONTAL BOUNDARY IS
PUSHED SOUTH INTO S GA/N FL TUE AND STALLS IN THAT REGION. THIS IS
VIRTUALLY IDENTICAL TO THE PATTERN WE EXPERIENCED LAST WEEK. THE GFS
IS SLIGHTLY FASTER IN PUSHING THE FRONT SOUTH OF THE CWA THAN THE
ECMWF...BUT ANY CHC SHRA/TSRA TUE SHOULD BE LIMITED TO OUR FAR S
COUNTIES. FROM WED AND BEYOND...THE GFS/ECMWF ARE FAIRLY CONSISTENT
IN EXPANDING THE UPPER RIDGE FROM THE SOUTHERN PLAINS INTO THE
MID-SOUTH AND WESTWARD FROM THE ATLANTIC TOWARD THE SE U.S.
COAST...KEEPING A WEAKNESS IN THE SUBTROPICAL RIDGE ACROSS THE SE
U.S. SFC MOISTURE REMAINS TRAPPED NEAR THE DISSIPATING FRONTAL
BOUNDARY/SFC TROUGH ACROSS FAR S GA/N FL. THE GFS...AND TO A LESSER
EXTENT THE ECMWF...BEGIN TO PULL SOME OF THIS MOISTURE BACK INTO OUR
CENTRAL ZONES...LEADING TO A TYPICAL PATTERN OF DIURNAL CONVECTION
EXPECTED AT THIS TIME OF YEAR. FOR WED WILL SHOW AFTN/EVE DIURNAL
POPS C/S ZONES AND ACROSS THE ENTIRE CWA FOR THU. BEYOND THU...THE
GFS/ECMWF BEGIN TO DIVERGE...WITH THE GFS EXPANDING THE NEARLY 600DM
MAMMOTH SUBTROPICAL RIDGE EASTWARD INTO THE SE STATES...WHILE THE
ECMWF ALLOWS YET ANOTHER NW FLOW UPPER TROUGH TO PENETRATE THE SE
U.S. GIVEN THE UNCERTAINTIES AND TIME-OF-YEAR...WILL CONTINUE TO
SHOW LOW CHC DIURNAL POPS. TEMPS WILL BE ON THE INCREASE THROUGH THE
EXTENDED AS THE UPPER RIDGE EXPANDS EASTWARD. 850MB TEMPS BY LATE
WEEK ARE PROGGED TO RISE TO AROUND +22C ACROSS MUCH OF THE SE
U.S...SO WE WILL LIKELY BE RETURNING TO WIDESPREAD 90S OUTSIDE OF
THE MOUNTAINS.
AVIATION
VFR OVERNIGHT WITH ONLY PATCHES OF MID/HIGH CLOUDS. AROUND
SUNRISE...MVFR CEILINGS INCREASING OVER SOUTHWEST GA...MOVING INTO
NORTH GA BUT LIFTING ABOVE 3 THSD FT AFTER 15Z. VISIBILITY
UNRESTRICTED. WINDS LIGHT SOUTHWEST OVERNIGHT...INCREASING TO 10 TO
15KT AFTER 15Z. A FEW THUNDERSTORMS DEVELOPING NORTH OF ATLANTA TO
ATHENS AFTER 12Z SUNDAY...INCREASING OVER THE REMAINDER OF NORTH AND
CENTRAL GA AFTER 16Z.
PRELIMINARY POINT TEMPS/POPS
ATHENS 65 89 72 87 66 / 10 60 60 30 10
ATLANTA 70 86 72 86 68 / 10 60 60 30 10
BLAIRSVILLE 63 80 65 83 61 / 20 60 50 10 10
CARTERSVILLE 65 84 69 87 62 / 10 60 60 20 10
COLUMBUS 71 91 74 88 70 / 0 50 60 50 20
GAINESVILLE 68 83 70 85 67 / 10 60 60 20 10
MACON 67 92 74 88 69 / 0 50 60 50 30
ROME 66 84 70 87 64 / 20 60 50 20 10
PEACHTREE CITY 64 87 70 86 65 / 5 60 60 40 10
VIDALIA 73 92 75 89 69 / 0 30 50 60 40
FFC WATCHES/WARNINGS/ADVISORIES
NONE.
19/41
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