Atlanta forecast discussion for major ice storm


000
FXUS62 KFFC 120839
AFDFFC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
339 AM EST WED FEB 12 2014

.SHORT TERM /TODAY THROUGH THURSDAY/...
WE ARE WATCHING A WINTER STORM OF HISTORICAL SIGNIFICANCE BEGINNING
TO UNFOLD ON THE MIDNIGHT SHIFT TONIGHT. REPORTS OF SNOW AND SLEET
ALREADY COMING IN ACROSS THE METRO AREA AS CAD WEDGE STRENGTHENS AND
PUSHES FURTHER SOUTHEAST. SURFACE HIGH HAS JUST CROSSED THE
APPALACHIANS SO THE AGEOSTROPHIC ADJUSTMENT IS KICKING IN...AND CAD
SHOULD BE FIRMLY ENTRENCHED WITHIN THE NEXT FEW HOURS. WITH THAT...
IT WILL BE A MATTER OF COMPETING EFFECTS OF HORIZONTAL THERMAL
ADVECTION /CAA IN SURFACE LAYER AND WAA ALOFT/ WITH LATENT HEAT
EFFECTS. IN A NUTSHELL...SNOW FALLING INTO AN ABOVE-FREEZING LAYER
WILL COOL THE LAYER FROM LATENT HEAT OF MELTING...AND RAIN FALLING
INTO A SUB-FREEZING SURFACE LAYER WILL WARM THE LAYER FROM LATENT
HEAT OF FREEZING. SHOULD BE ENOUGH ADVECTION TO COMPENSATE FOR THIS
AND AT TIMES EVEN SURPASS THE DIABATIC EFFECTS.

NO SURPRISE ON OVERALL SYNOPTIC FIELDS. SURFACE CAD...SOUTHWEST FLOW
ALOFT INITIALLY AS SHORTWAVE PUSHES EAST ACROSS THE SOUTHERN TIER OF
THE COUNTRY AND CUTS OFF TOWARD THE END OF THE SHORT TERM. SURFACE
LOW DEVELOPING IN THE NORTHERN GULF WILL CROSS EXTREME SOUTHEAST
GEORGIA BEFORE PUSHING UP THE ATLANTIC COAST AND CAUSING MORE
PROBLEMS UP THAT WAY. ECMWF AND GFS IN PRETTY GOOD AGREEMENT BUT
ECMWF REMAINS ON THE WEST SIDE. VERY IMPRESSIVE SYNOPTIC LIFT
ESPECIALLY TOWARD THE END OF THE PERIOD AS WE ARE SOLIDLY IN THE
RIGHT ENTRANCE REGION OF THE UPPER JET.

THINK THERE IS STILL SOME CONVECTIVE FEEDBACK IN QPF FIELDS ON
NAM/GFS BUT OVERALL SOLUTION IS PRETTY SIMILAR TO WHAT WE HAVE SEEN
THE PAST FEW RUNS WITH AROUND 2 INCHES QPF ACROSS CENTRAL GEORGIA
WITH AROUND AN INCH LIQUID ALONG THE NORTHERN TIER. AS THE WEATHER
ENTERPRISE HAS BEEN ADVERTISING FOR DAYS NOW...WE ARE LOOKING AT
MAINLY SNOW EXTREME NORTH...SNOW/SLEET/FZRA MIX ACROSS MOST OF THE
METRO AREA...WITH MAINLY FZRA ON THE SOUTH SIDE. AS MENTIONED
ABOVE...THIS IS A STORM OF HISTORICAL PROPORTIONS WITH POTENTIALLY
CATASTROPHIC...CRIPPLING IMPACTS.

SNOW RATIOS UP NORTH RUN BETWEEN 6:1 AND 10:1 WHICH LINES UP WITH
WITH OUR CLIMATOLOGICAL 8:1. OVERALL A WET SNOW /VERY GOOD FOR
SNOWBALLS FOR THE KIDS HOME FOR SCHOOL...AND OF COURSE SOME PARENTS
HOME FROM WORK/. IN ADDITION TO WHAT HAS ALREADY FALLING...EXPECTING
A WIDESPREAD AREA OF GREATER THAN 4 INCHES OF SNOW AND SOME SLEET
FROM GENERALLY ATLANTA TO ATHENS AND POINTS NORTH. A LARGE SWATH OF
GREATER THAN 6 INCHES EXPECTED ACROSS THE EXTREME NORTHERN TIER.
CANNOT RULE OUT SOME TOTALS UPWARDS OF A FOOT IN EXTREME NORTHEAST
ZONES.

AS SIGNIFICANT AS THE SNOWFALL IS...AND MAKE NO MISTAKE THESE ARE
SOME IMPRESSIVE TOTALS...IT IS THE ICE THAT WILL HAVE THE
CATASTROPHIC IMPACTS. A LITTLE DIFFICULT TO DETERMINE THE NORTHERN
EDGE OF THE ICE. FOR EXAMPLE AT AHN...THE GFS WARM NOSE IS A LITTLE
COOLER THAN THE NAM AND WOULD RESULT IN MORE SLEET THAN FREEZING
RAIN. HOWEVER...BECAUSE OF THE DEPTH OF THE WARM NOSE...AM STILL
LEANING TOWARD KEEPING QUITE A BIT OF FZRA IN THAT AREA. SO WITH
THAT...ICE TOTALS REMAIN VERY SIMILAR TO WHAT WAS PREVIOUSLY OUT BUT
THINK THE 1/4 INCH LINE MAY BE A LITTLE FURTHER SOUTH AND MAX TOTALS
ALONG THE EAST I-20 CORRIDOR ARE BUMPED UP A LITTLE TO OVER AN INCH.
ISOLATED HIGHER AMOUNTS CANNOT BE RULED OUT. IN CASE IT HASNT BEEN
MADE CLEAR ALREADY...THESE ARE CATASTROPHIC AND CRIPPLING TOTALS
WHICH COULD RESULT IN WIDESPREAD POWER OUTAGES THAT MAY LAST FOR
DAYS. IF RESIDENTS HAVE NOT COMPLETED THEIR PREPARATIONS IT MAY BE
TOO LATE.

NORTHEAST WINDS WILL BE PRETTY BREEZY...GENERALLY 15-20MPH WITH
GUSTS TO 25 OR POSSIBLY 30. ADD THESE WINDS TO THE AMOUNT OF ICE
EXPECTED AND THAT WILL JUST EXACERBATE THE POWER LOSS ISSUES.

ALONG OUR SOUTHERN TIER OF COUNTIES WE EXPECT MAINLY RAIN BUT CANNOT
RULE OUT SOME LIGHT FZRA RESULTING IN A GLAZE...SO WILL LEAVE THE
FREEZING RAIN ADVISORY IN PLACE. WILL START THE ADVISORY A LITTLE
EARLIER JUST TO COVER THE POTENTIAL FOR A LIGHT GLAZE THIS MORNING.

AS IF ALL THAT WASNT ENOUGH...STRENGTH OF FRONTOGENETICAL FORCING
ALONG THE WEDGE FRONT PRETTY IMPRESSIVE...AND WITH THAT THERE IS
SOME NEGATIVE SATURATED EQUIVALENT POTENTIAL VORTICITY ON TOP OF IT.
THOSE ARE BIG GEEKY WORDS BUT BASICALLY IT MEANS THERE COULD BE SOME
EMBEDDED /SYMMETRIC/ INSTABILITY TO DEAL WITH AND SO CANNOT RULE OUT
SOME EMBEDDED CLAPS OF THUNDER. NOT ENOUGH OF A CONCERN TO ADD TO
THE GRIDS BUT IF ANY CONVECTION DEVELOPS THAT COULD EASILY AND
QUICKLY CHANGE THE P-TYPE. TRYING TO CATCH HOURLY P-TYPE IS NEARLY
IMPOSSIBLE IN AN EVENT LIKE THIS SO IT IS BEST TO JUST FOCUS ON
PREDOMINANT P-TYPE.

AS THE UPPER LOW MOVES ACROSS...LAPSE RATES INCREASE AND
TEMPERATURES DECREASE ENOUGH TO CHANGE P-TYPE TO ALL SNOW WITH SOME
LIGHT ACCUMULATIONS POSSIBLE ON THE BACK SIDE. LUCKILY ON THURSDAY
THE TEMPS SHOULD REBOUND INTO THE 40S ALL BUT EXTREME NORTHEAST
MOUNTAINS. I WANT TO ADD A CAVEAT THAT GUIDANCE IS HORRIBLE AT
TEMPERATURES WITH SNOWPACK...SO WHERE THE SNOW COVER IS TONIGHT IT
IS ENTIRELY POSSIBLE THAT LOW TEMPS MIGHT BE SIGNIFICANTLY LOWER
THAN CURRENT FORECASTS ARE ADVERTISING /LIKE WHAT HAPPENED A COUPLE
OF WEEKS AGO/...AND HIGHS ON THURSDAY MAY BE KEPT LOWER. WILL
REEVALUATE THIS WITH LATER FORECASTS.

PRETTY SURE THAT THERE ARE SOME THINGS I HAVENT COVERED HERE BUT
HOPEFULLY THIS LIGHT READING MATERIAL IS GOOD ENOUGH TO GET
EVERYONES DAY STARTED. THE BOTTOM LINE...AS THE ENTIRE WEATHER
ENTERPRISE HAS BEEN ADVERTISING FOR DAYS NOW...
THIS-IS-AN-EVENT-OF-HISTORICAL-PROPORTIONS!!
CATASTROPHIC...CRIPPLING...PARALYZING...CHOOSE YOUR ADJECTIVE. THIS
IS A VERY VERY BIG DEAL ESPECIALLY FROM METRO ATLANTA EAST ALONG THE
I-20 CORRIDOR...GENERALLY ANYWHERE SOUTH OF I-85 AND NORTH OF I-16
WHERE THE MOST SIGNIFICANT ICE TOTALS ARE FORECAST. STAY SAFE OUT
THERE!

TDP

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
NEXT CONCERN WILL BE A FAST MOVING COLD FRONT AND ASSOCIATED UPPER
TROUGH FORECAST TO MOVE ACROSS THE AREA FRIDAY AFTERNOON AND NIGHT.
ASSOCIATE MOISTURE BRINGS PRECIP CHANCES MAINLY INTO N GA FRIDAY
AFTERNOON AND ACROSS THE AREA FRIDAY NIGHT. TEMPERATURES APPEAR COLD
ENOUGH FOR A RAIN SNOW MIX OR A CHANGE OVER TO SNOW FRIDAY EVENING
FOR PORTIONS OF N GA WITH A DUSTING POSSIBLE FOR FAR N FULTON COUNTY
AND NE TO THE MOUNTAINS.

MODELS RATHER INCONSISTENT BRINGING MOISTURE ACROSS N GA SATURDAY NIGHT
ASSOCIATED WITH A WEAK UPPER TROUGH. TEMPERATURES COLD ENOUGH FOR
LIGHT RAIN OR SNOW WITH NO SIGNIFICANT ACCUMULATIONS EXPECTED AT
THIS TIME.

GFS AND EUROPEAN DIFFERING WITH A COLD FRONT EARLY NEXT WEEK. GFS MUCH
FASTER BRINGIMG A COLD FRONT ACROSS THE AREA MONDAY NIGHT AND
TUESDAY AND THE EUROPEAN BRINGING THE FRONT ACROSS THE AREA
TUESDAY AFTERNOON AND NIGHT. AT THIS TIME FAVORING THE WPC POP
GRID FOR MONDAY NIGHT WITH CHANCE POPS ACROSS NORTH AND PORTIONS
OF CENTRAL GA. THIS COULD CHANGE GREATLY WITH OTHER MODEL RUNS. AT
THIS TIME AIRMASS SEEMS TO SUPPORT LIGHT RAIN OR LIGHT SHOWERS
ONLY.

BDL

&&

.AVIATION...
06Z UPDATE...
NO MAJOR CHANGES TO THE TAF EXCEPT FOR THE MCN AREA WHERE IT
APPEARS IT WILL GET TO FREEZING EARLIER THAN EXPECTED AND WILL
SWITCH OVER THE RAIN TO FREEZING RAIN AROUND NOON TODAY. FREEZING
RAIN AND SLEET WILL BEGIN EARLY THIS MORNING IN ATL/AHN AREAS AND
CONTINUE INTO THIS EVENING...SWITCHING OVER TO SNOW LATE THIS
EVENING WHILE CSG WILL REMAIN MOSTLY RAIN UNTIL THIS EVENING WHEN
IT WILL BECOME MIXED WITH SNOW. EXPECT IFR CONDITIONS ALL AREAS
THROUGH THE FORECAST. WINDS WILL BECOME GUSTY OVER THE NEXT FEW
HOURS AS THE WEDGE STRENGTHENS. WINDS WILL REMAIN GUSTY THROUGH
MOST OF THE FORECAST.

//ATL CONFIDENCE...06Z UPDATE...
HIGH CONFIDENCE ALL ELEMENTS.

17

&&

.PRELIMINARY POINT TEMPS/POPS...
ATHENS 29 28 40 31 / 100 100 40 0
ATLANTA 31 27 41 34 / 100 100 20 0
BLAIRSVILLE 32 29 39 25 / 100 100 40 5
CARTERSVILLE 32 29 42 30 / 100 100 20 0
COLUMBUS 36 30 48 36 / 100 100 10 0
GAINESVILLE 29 27 41 32 / 100 100 40 0
MACON 33 29 45 32 / 100 100 20 0
ROME 34 29 43 29 / 100 100 20 5
PEACHTREE CITY 32 27 43 30 / 100 100 20 0
VIDALIA 34 31 48 35 / 100 100 20 0

&&

.FFC WATCHES/WARNINGS/ADVISORIES...
ICE STORM WARNING UNTIL 1 PM EST THURSDAY FOR THE FOLLOWING
ZONES: BALDWIN...BIBB...BLECKLEY...BUTTS...CARROLL...CLARKE...
CLAYTON...COWETA...CRAWFORD...DOUGLAS...EMANUEL...FAYETTE...
GLASCOCK...GREENE...HANCOCK...HEARD...HENRY...HOUSTON...JASPER...
JEFFERSON...JOHNSON...JONES...LAMAR...LAURENS...MADISON...
MERIWETHER...MONROE...MORGAN...NEWTON...OCONEE...OGLETHORPE...
PEACH...PIKE...PUTNAM...ROCKDALE...SOUTH FULTON...SPALDING...
TALIAFERRO...TREUTLEN...TROUP...TWIGGS...UPSON...WALTON...
WARREN...WASHINGTON...WILKES...WILKINSON.

FREEZING RAIN ADVISORY UNTIL 1 PM EST THURSDAY FOR THE FOLLOWING
ZONES: CHATTAHOOCHEE...CRISP...DODGE...DOOLY...HARRIS...MACON...
MARION...MONTGOMERY...MUSCOGEE...PULASKI...SCHLEY...STEWART...
SUMTER...TALBOT...TAYLOR...TELFAIR...TOOMBS...WEBSTER...
WHEELER...WILCOX.

WINTER STORM WARNING UNTIL 1 PM EST THURSDAY FOR THE FOLLOWING
ZONES: BANKS...BARROW...BARTOW...CATOOSA...CHATTOOGA...
CHEROKEE...COBB...DADE...DAWSON...DEKALB...FANNIN...FLOYD...
FORSYTH...GILMER...GORDON...GWINNETT...HALL...HARALSON...
JACKSON...LUMPKIN...MURRAY...NORTH FULTON...PAULDING...PICKENS...
POLK...TOWNS...UNION...WALKER...WHITE...WHITFIELD.

&&

$$

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