Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 280522
AFDFFC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
1222 AM EST SUN DEC 28 2014



.PREV DISCUSSION... /ISSUED 746 PM EST SAT DEC 27 2014/

UPDATE...CORRECTION...

CORRECTION FOR FLOOD WATCH WORDING.

RAIN SHOWERS ACROSS THE AREA THIS EVENING WILL CONTINUE THROUGH
SUNDAY. HAVE ISSUED A FLOOD WATCH FOR A LARGE PORTION OF NORTH
GEORGIA THROUGH EARLY MONDAY TO ACCOUNT FOR THE RAINFALL FROM TWO
WAVES OF PRECIPITATION. ANTICIPATE TOTAL RAINFALL AMOUNTS OF 1-3
INCHES ACROSS THE WATCH AREA...WITH LOCALLY HIGHER AMOUNTS. GIVEN
THE RAIN EARLIER IN THE WEEK...SOILS ARE WET...AND FLOODING IS
POSSIBLE IN FLOOD PRONE OR POOR DRAINAGE AREAS ALONG WITH AREA
CREEKS AND RIVERS. MODELED PWS ARE ANTICIPATED TO INCREASE TO
1.3-1.5 ACROSS NORTH GEORGIA THROUGH SUNDAY...APPROACHING THE 99TH
PERCENTILE. GIVEN THE MODELED ISENTROPIC LIFT...ANOMALOUS
PWS...AND EXPECTED QPF...HAVE GONE AHEAD WITH THE WATCH. WPC
CONTINUES TO ADVERTISE 2-3 INCHES OF RAINFALL THROUGH MONDAY
MORNING FOR PORTIONS OF THE ATL METRO AND WESTERN GREATER METRO
AREAS.

31

PREV DISCUSSION... /ISSUED 222 PM EST SAT DEC 27 2014/

SHORT TERM /TONIGHT THROUGH MONDAY/...

SHORT TERM BEGINS WITH STRONG SOUTHWEST FLOW ALOFT...WITH A FRONT
STRETCHING FROM THE MID-LEVEL TROUGH AND SURFACE LOW IN THE GREAT
LAKES DOWN TO THE SURFACE LOW IN THE LOWER MISSISSIPPI VALLEY. DEEP
GULF MOISTURE BEING ADVECTED INTO THE SOUTHEAST THIS AFTERNOON. WE
HAD A SLUG OF MOISTURE MOVE ACROSS ALABAMA THIS MORNING INTO NORTH
GEORGIA EARLY THIS AFTERNOON WITH SOME ISENTROPIC LIFT AT THE 300-
305K SURFACE...BUT MAIN PRECIPITATION SHIELD REMAINS DOWN NEAR THE
GULF AND WILL APPROACH THE AREA THIS EVENING. CONTINUED TREND OF
CATEGORICAL POPS ESPECIALLY UP NORTH AS THE HEAVIEST PRECIP IS
SHUNTED NORTHEAST WITH THE FLOW ALOFT. NEW 18Z WPC QPF HAS OVER 2.5
INCHES IN SOME AREAS ESPECIALLY WEST OF THE ATLANTA METRO BUT
GENERALLY AROUND 2 INCHES FOR MOST OF NORTH GEORGIA. CONSIDERED
ISSUING A FLOOD WATCH BUT IT SEEMS FOR NOW THAT THE QPF WILL BE
SPREAD OUT ENOUGH THROUGH THE SHORT TERM TO AVOID TOO MANY RAPID
RISES. WILL HAVE THE MIDNIGHT SHIFT REEVALUATE IF TRENDS ARE
DIFFERENT.

AS FAR AS THUNDER POTENTIAL GOES...MUCAPE VALUES TOMORROW AND MONDAY
ARE REALLY ON THE LOW SIDE...LIKE 50 J/KG OR LESS. COULD POSSIBLY
ARGUE FOR MORE THUNDER CHANCES SOUTHEAST MONDAY AFTERNOON BUT ONLY
ONE MODEL IS GIVING US MUCH MUCAPE TO WORK WITH. THAT SAID...IT
COULD VERY WELL END UP NOT BEING UPRIGHT INSTABILITY BUT MORE
SLANTWISE INSTABILITY THAT RESULTS IN ISOLATED CLAPS OF THUNDER...
BUT NOT CONFIDENT ENOUGH TO REALLY BE CONCERNED...SO KEPT WORDING AT
JUST SHOWERS FOR NOW.

FIRST WAVE OF DEEP MOISTURE PUSHES THROUGH TONIGHT INTO SUNDAY
AFTERNOON...THEN A BIT /NOT MUCH/ OF A LULL BEFORE A SECOND ROUND
WORKS ITS WAY UP THE SOUTHWEST FLOW ALOFT...DRIVEN BY THE TROUGH
ITSELF. THIS SHOULD AFFECT THE AREA SUNDAY NIGHT INTO MONDAY
MORNING. SHORTWAVE REALLY DAMPS BY THIS TIME AS THE SURFACE FRONT
ELONGATES...BUT IT FINALLY PUSHES THROUGH MOST OF THE AREA ON MONDAY
USHERING A DRIER AND COOLER PERIOD BEHIND IT. OVERALL FOR THE SHORT
TERM...EXPECT TEMPERATURES TO BE ABOVE-NORMAL FOR THIS TIME OF YEAR
DUE TO THE SOUTHWEST FLOW ALOFT AND PLENTIFUL MOISTURE.

TDP

LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...

NOT MUCH CHANGE TO THE PREVIOUS LONG TERM PORTION WITH THE
LOADING IN OF 12Z GRID DATA. STILL LOOKS LIKE FOR THE NEXT STORM
SYSTEM THURSDAY NIGHT INTO FRIDAY THAT THERE WILL BE SOME COLD AIR
TRAPPED IN THE VALLEYS TO START FRIDAY MORNING. CHANCES ARE THIS
COLD AIR WILL RETREAT PRIOR TO ONSET OF PRECIPITATION BUT ENOUGH
OF A CLOSE CALL TO WARRANT KEEPING SLIGHT CHANCE FREEZING RAIN IN
THERE FOR NOW. OTHERWISE...DID INCREASE POPS FOR THE EVENT WITH
LIKELY NOW FOR MOST AREAS.

WITH UPPER LEVEL LOW FORECAST TO LIFT ACROSS THE UPPER MISSISSIPPI
VALLEY...THERE WILL BE AN OPPORTUNITY FOR A SURFACE SOUTHERLY FLOW
TO DEVELOP LOCALLY FROM THE GULF OF MEXICO FRIDAY NIGHT INTO
SATURDAY. DEWPOINTS FROM THE GFS INCREASE TO MID TO UPPER 60S AS
A RESULT AND ASSOCIATED CAPES GO TO ALMOST 1000 J/KG OVER THE
EXTREME SOUTHEAST CORNER OF THE AREA. BEST SHEAR REMAINS NORTH OF
THE GREATEST INSTABILITY BUT THERE IS A SECTION OF OVERLAP OVER
CENTRAL GEORGIA WHERE STRONG STORMS WOULD BE POSSIBLE SHOULD THIS
FORECAST TREND CONTINUE. NO ADDITIONAL CHANGES WERE MADE TO THE
PREVIOUS EXTENDED.

DEESE

PREV DISCUSSION... /ISSUED 303 AM EST SAT DEC 27 2014/

LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... GFS/ECMWF IN PRETTY
GOOD AGREEMENT WITH THE EXITING SYSTEM EARLY IN THE LONG TERM. AS
THE NEXT LOW DIGS OVER THE SOUTHWEST U.S. A NEARLY ZONAL FLOW
DEVELOPS WITH A LOW LEVEL NORTH TO NORTHEAST FLOW. THIS SHOULD
KEEP A STEADY STREAM OF UPPER LEVEL CLOUDS MOVING ACROSS THE
SOUTHERN U.S. THIS WILL ALSO KEEP TEMPS AT OR JUST SLIGHTLY BELOW
NORMAL THROUGH MUCH OF THE WEEK. DIFFERENCES DEVELOP IN THE MODELS
WITH THE TIMING OF THE NEXT SYSTEM. THE ECMWF IS FASTER WITH
BRINGING MOISTURE INTO THE CWA THURSDAY NIGHT BUT IT ALSO HAS A
DEEPER LOW OVER THE SOUTHERN ROCKIES. THIS BRINGS IN AMPLE
MOISTURE THURSDAY NIGHT WITH A WARMER SOUTHERLY FLOW WHILE THE GFS
IS SLOWER AND A LITTLE COOLER AS IT HOLDS ON TO THE SURFACE HIGH
OVER THE CWA. WILL GO WITH A COMPROMISE ON TIMING OF POPS BUT AT
LEAST BOTH INDICATE MOISTURE MOVING INTO THE CWA SO WILL GO WITH
LOW CHANCE THURSDAY NIGHT...AND HIGH CHANCE TO LIKELY BEGINNING
FRIDAY. THERE STILL REMAINS THE QUESTION OF WINTER PRECIP THURSDAY
NIGHT. BOTH MODELS PUSH THE 0C 850 TEMP NORTH OF THE CWA WITH THE
ECMWF PUSHING THE 0C LINE THE FURTHEST NORTH...INTO KY. LOW LEVEL
THICKNESSES INDICATE ALL LIQUID PRECIP. AT THIS TIME...WILL GO
WITH A CHANCE OF R-/ZR- IN THE MOUNTAINS LATE THURSDAY NIGHT.
CONFIDENCE IN THIS HAPPENING IS LOW.

17


&&

.AVIATION...
06Z UPDATE...
MIX OF VFR/MVFR SHOULD BECOME ALL MVFR AND IFR BY EARLY THIS
MORNING. DO NOT EXPECT MUCH IN THE WAY OF IMPROVEMENT DURING THE
DAY WITH CIGS RISING TO MVFR OVER THE FORECAST AREA. CIGS SHOULD
DROP BACK TO IFR THIS EVENING. WINDS CALM TO LIGHT SOUTHEAST
SHOULD BECOME LIGHT SOUTHWEST TO WEST BY MID MORNING AND REMAIN
WESTERLY THROUGH THIS EVENING. RAIN WILL CONTINUE ACROSS THE
ATL/AHN AREA...POSSIBLY LIFTING NORTH OF THESE AREAS THIS
AFTERNOON...ONLY TO RETURN THIS EVENING AND SPREAD INTO CSG/MCN
LATE THIS EVENING.

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

17

&&

.PRELIMINARY POINT TEMPS/POPS...
ATHENS          63  53  60  45 /  90  90  40  10
ATLANTA         62  52  61  47 /  90 100  20  10
BLAIRSVILLE     57  47  57  42 / 100 100  30  30
CARTERSVILLE    61  48  59  41 / 100 100  20  10
COLUMBUS        70  57  66  47 /  50  90  30  10
GAINESVILLE     58  50  59  44 / 100 100  30  20
MACON           70  58  66  46 /  30  80  50  10
ROME            60  47  58  39 / 100 100  20  10
PEACHTREE CITY  64  53  63  41 /  80  90  20  10
VIDALIA         73  60  69  56 /  10  50  50  20

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
FLOOD WATCH THROUGH MONDAY MORNING FOR THE FOLLOWING ZONES:
BANKS...BARROW...BARTOW...CARROLL...CATOOSA...CHATTOOGA...
CHEROKEE...CLAYTON...COBB...COWETA...DADE...DAWSON...DE KALB...
DOUGLAS...FANNIN...FAYETTE...FLOYD...FORSYTH...GILMER...GORDON...
GWINNETT...HALL...HARALSON...HEARD...HENRY...JACKSON...LUMPKIN...
MERIWETHER...MURRAY...NEWTON...NORTH FULTON...PAULDING...
PICKENS...PIKE...POLK...ROCKDALE...SOUTH FULTON...SPALDING...
TOWNS...TROUP...UNION...WALKER...WALTON...WHITE...WHITFIELD.

&&

$$

SHORT TERM...17
LONG TERM....BDL
AVIATION...17


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