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Area Forecast Discussion

115
FXUS62 KFFC 221955
AFDFFC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
255 PM EST MON DEC 22 2014

..SEVERE WEATHER POSSIBLE EARLY TUES THROUGH WEDNESDAY...

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...
SEE PREV DISCUSSION BELOW FOR THOUGHTS ON SVR STORMS TUES THRU
WED. ONLY NEW INFORMATION SINCE LAST UPDATE IS 12Z ECMWF WHICH
SHOWS WARM FRONT LIFTING NORTH TUES BUT INSTABILITY DROPPING IN
THE AFTERNOON AS RAIN MOVES IN...SIMILAR TO 12Z GFS. INSTABILITY
AND 0-1KM SHEAR A LITTLE WEAKER THAN GFS AS WELL.

DERIVED MAX AND MIN TEMPS FROM BLEND OF HOURLY TEMP GUIDANCE WITH
MORE WEIGHT GIVEN TO MET/NAM THAN MAV/GFS.

SNELSON

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
A COLD FRONT WILL PUSH EAST OF THE AREA EARLY WEDNESDAY NIGHT...
WITH RAIN CHANCES OVER THE EASTERN ZONES ENDING BY MIDNIGHT EXCEPT
ACROSS NORTH GA WHERE WRAP AROUND MOISTURE AND A SLIGHT CHANCE OF
LIGHT RAIN SHOWERS OR SNOW SHOWERS WILL REMAIN POSSIBLY THROUGH MID
THURSDAY MORNING. TEMPERATURE PROFILES REMAIN ABOVE FREEZING FOR
MOST OF NORTH GA WEDNESDAY NIGHT-THURSDAY MORNING... EXCEPT MAYBE
SOME AREAS IN THE HIGHER ELEVATIONS. THIS... COUPLED WITH LIMITED
MOISTURE INDICATES THE GREATER SNOW CHANCES WILL BE FOR THE HIGHER
ELEVATIONS WHERE SOME LIGHT DUSTING MAY BE POSSIBLE. NO
SIGNIFICANT ACCUMULATIONS ARE EXPECTED. BEHIND THE FRONT... THE
SURFACE RIDGE MOVES IN FAIRLY QUICKLY... RESULTING IN ONLY MODEST
10 TO 15 MPH WEST WINDS ON THURSDAY. THE TRAILING RIDGE AND
ASSOCIATED AIR MASS IS MORE PACIFIC IN NATURE... SO NOT EXPECTING
A SHARP COOL DOWN... WITH HIGHS ON CHRISTMAS DAY HOLDING IN THE
50S... AND LOWS FRIDAY MORNING ONLY DIPPING INTO THE LOWER TO MID
30S DESPITE EXPECTED CLEAR SKIES AND LIGHT WINDS. LONG RANGE
MODELS DIVERGE CONSIDERABLY ON A WEEKEND COLD FRONT... WITH THE
PREFERRED GFS BRINGING A WEAK COLD FRONT WITH LOW RAIN CHANCES
ACROSS THE AREA SATURDAY AFTERNOON- SATURDAY NIGHT WITH A TRAILING
RIDGE OF COOL HIGH PRESSURE DOMINATING SUNDAY-MONDAY TIMEFRAME.
THE MUCH SLOWER AND MORE AGGRESSIVE ECMWF GENERATES A SURFACE LOW
ALONG THE FRONT THAT TRACKS ACROSS AL AND CENTRAL TN EARLY ON
SUNDAY... DRAGGING THE ASSOCIATED COLD FRONT ACROSS THE AREA
SUNDAY AFTERNOON. WITH MUCH UNCERTAINTY... DECIDED TO MAINTAIN A
BLEND WITH A CHANCE OF POPS SATURDAY-SATURDAY NIGHT... AND SLIGHT
CHANCE ON SUNDAY AND MONDAY. WILL JUST MENTION SHOWERS WITH THIS
SYSTEM FOR NOW... BUT IF THE ECMWF IS RIGHT... MAY HAVE TO ADD
MENTION OF THUNDER ACROSS AT LEAST THE SOUTHERN ZONES SOMETIME
OVER THE WEEKEND.

39

&&

.HYDROLOGY...
LATEST QPF A LITTLE HIGHER OVER NE GA THAN PREV FCSTS. WILL NEED
TO MONITOR THIS AREA FOR POSSIBLE MINOR FLOODING. AT THIS TIME
EXPECT RAIN TO FALL AT MORE STEADY PACE WITH THE HEAVIEST RATE
AROUND 1 INCH FROM 00-06Z TUES EVENING. STORM TOTAL AMOUNTS AROUND
2 TO 3 INCHES TONIGHT THRU WED. WILL CONTINUE TO MONITOR BUT NOT
PLANNING ANY FLOOD WATCHES ATTM.

SNELSON

&&

.PREV DISCUSSION... /ISSUED 125 PM EST MON DEC 22 2014/

UPDATE... NEAR TERM FCST STILL ON TRACK. WEDGE STILL VERY STRONG
THIS AFTERNOON SO LOW CLOUDS HERE TO STAY TODAY.

12Z LOCAL WRF MODEL RUNS INDICATING CONVECTION POSSIBLY FIRING UP
AS EARLY AS MIDDAY TUESDAY OVER MIDDLE GA. BY LATE TUES AFTERNOON
AND EVENING...PRECIP BECOMES MORE WIDESPREAD ESP NORTH OF THE
WARM FRONT OVER NORTH GA. LINES OF CONVECTION POSSIBLE FROM
MIDNIGHT ON. DID NOT SEE ANYTHING TOO OMINOUS LOOKING IN
REFLECTIVITY FORECASTS...BUT CAN NEVER LET YOUR GUARD DOWN WITH
HIGH SHEAR LOW CAPE CONVECTION. EXPERIMENTAL SHERB INDEX WHICH
HAS BEEN SHOWN TO CORRELATE WELL WITH TORNADO AND WIND DAMAGE
REPORTS IN HIGH SHEAR LOW CAPE ENVIRONMENTS ONLY INDICATING MAX
VALUES OF 0.5 TO 0.9 TUES THRU WED. TYPICALLY NEED VALUES OVER
1.0 TO SEE MUCH CHANCE OF TORNADOES OR WIND DAMAGE.

12Z GFS SIG TOR PARAMETER /STP/ VALUES HIGHEST 18Z TUES /AROUND
2-2.5/ WHEN GFS HAS HIGHER MLCAPE VALUES...HOWEVER GFS ALSO SHOWS
WEDGE ERODING WAY TO FAST WITH SFC WINDS MORE SLY AND SELY THAN IS
LIKELY. NAM STP VALUES DO NOT COME UP AS MUCH AS GFS TONIGHT OR
TUES MORNING.

CIPS ANALOG FORECASTS COURTESY ST LOUIS UNIV VALID 36 AND 48HRS
SHOW ONLY 3 TO 4 OUT OF TOP 15 MATCHING EVENTS /ABOUT 20-25PCT/
HAD *ANY* SEVERE WX IN THE AREA. BUT OF THOSE 3 OR 4...THERE WERE
TORNADOES.

HEAVY RAIN OR MINOR FLOODING THREAT SHOULD BE CONTAINED TO AREAS
SOUTH OF THE FALL LINE. BASED ON FLASH FLOOD GUIDANCE AND ANY EXPECTED
6HR QPF NO MORE THAN 2-2.5 INCHES MAINLY OVER MIDDLE GA THRU THE
EVENT...DO NOT EXPECT ANY SIGNIFICANT FLOODING.

WILL CONTINUE TO MONITOR.

SNELSON

&&

AVIATION...
18Z UPDATE...
SLOW IMPROVEMENT TO CIGS/VSBYS TODAY BEFORE GOING BACK DOWN TO
LIFR AFTER 03Z. WEDGE PATTERN NOT EXPECTED TO ERODE FOR ANOTHER 36
HOURS AT LEAST. SFC WINDS WILL GRADUALLY VEER FROM E TO SE
TUESDAY AFTERNOON WITH SPEEDS 5-10KTS THRU THE ENTIRE FCST PERIOD.

//ATL CONFIDENCE...18Z UPDATE...
HIGH CONFIDENCE ON ALL ELEMENTS.

SNELSON

&&

.PRELIMINARY POINT TEMPS/POPS...
ATHENS          42  51  50  62 /  70 100 100  90
ATLANTA         44  54  53  63 /  70 100 100  90
BLAIRSVILLE     41  52  50  58 /  60 100 100  90
CARTERSVILLE    45  54  52  63 /  60 100 100  80
COLUMBUS        51  65  61  67 /  60 100 100  80
GAINESVILLE     40  49  47  59 /  70 100 100  90
MACON           48  63  62  66 /  60 100 100  90
ROME            46  54  52  63 /  70 100 100  80
PEACHTREE CITY  45  58  57  63 /  60 100 100  90
VIDALIA         51  68  63  67 /  50 100 100  90

&&

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

$$

SHORT TERM...SNELSON
LONG TERM....39
AVIATION...SNELSON



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