Area Forecast Discussion
Issued by NWS Atlanta, GA

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FXUS62 KFFC 280046
AFDFFC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
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...
00Z UPDATE...
RAIN ALREADY IMPACTING TAF SITES THIS EVENING...AND EXPECT IT TO
LARGELY CONTINUE THROUGH THE TAF PERIOD. MID LEVEL CLOUDS WILL
DEGRADE TO MVFR...THEN IFR BY 10-12Z. LITTLE IMPROVEMENT EXPECTED
THROUGH SUNDAY MORNING...WITH POSSIBLE CIG LIFTING TO MVFR BY
21-23Z. CIGS WILL DEGRADE AGAIN BY 02-03Z SUNDAY NIGHT. WINDS WILL
SHIFT FROM THE SSE TO THE SW BY 12-13Z FOR THE METRO
SITES...15-17Z ELSEWHERE...THEN CONTINUE TO SHIFT TO THE NW BY
02Z MONDAY. SPEEDS OF 7KT OR LESS EXPECTED.

//ATL CONFIDENCE...00Z UPDATE...
MEDIUM-HIGH ON TIMING OF ALL ELEMENTS.
HIGH ON OCCURRENCE OF ELEMENTS.

31

&&

.PRELIMINARY POINT TEMPS/POPS...
ATHENS          49  63  53  60 / 100  90  80  40
ATLANTA         51  62  52  61 / 100  90  90  30
BLAIRSVILLE     46  57  47  57 / 100 100 100  30
CARTERSVILLE    50  61  48  59 / 100 100 100  20
COLUMBUS        53  70  57  66 / 100  60  70  30
GAINESVILLE     48  58  50  59 / 100 100  90  30
MACON           51  70  58  66 / 100  50  70  50
ROME            49  60  47  58 / 100 100 100  20
PEACHTREE CITY  51  64  53  63 / 100  80  80  30
VIDALIA         53  73  60  69 /  30  20  40  40

&&

.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...31
LONG TERM....30
AVIATION...31



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