FXUS62 KTAE 230835
AFDTAE
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
334 AM EST WED DEC 23 2009
.SYNOPSIS...03 UTC SURFACE ANALYSIS SHOWS A HIGH CENTERED ACROSS THE
OUTER BANKS OF NORTH CAROLINA. THIS HIGH IS PART OF A LARGER SURFACE
RIDGE THAT EXTENDS NORTHWESTWARD FROM THE MID ATLANTIC COAST BACK
INTO NORTHWEST ONTARIO. THE RETURN FLOW IS ALREADY EVIDENT ACROSS
EASTERN TEXAS INTO THE RED RIVER VALLEY.
UPPER AIR DATA AND VAPOR IMAGERY SHOWS A STRONG UPPER LOW CENTERED
ABOUT NEAR LAS VEGAS WITH RIDGING MOVING EASTWARD ACROSS THE MID
MISSISSIPPI RIVER VALLEY. OUR LOCAL AIRMASS CONTINUED TO BE MAINLY
DRY WITH A PRECIPITABLE WATER OF 0.3 INCHES. OVER THE NEXT 48
HOURS...OUR AIRMASS WILL MODIFY CONSIDERABLY AS THE STORM SYSTEM
ACROSS THE PLAINS TAKES SHAPE AND SPREADS EAST.
&&
.SHORT TERM (TODAY THROUGH FRIDAY).
THE SHORT TERM PERIOD WILL BE LARGELY DOMINATED BY THE EVOLUTION OF
A DEEP CYCLONE TAKING SHAPE ACROSS THE INTERMOUNTAIN WEST. THE
GLOBAL MODELS ALL SHOW THIS SYSTEM OCCLUDING ACROSS THE PLAINS BY
THURSDAY NIGHT AS A COLD FRONT MOVES ACROSS THE NORTHERN GULF COAST
STATES. THE MAIN ISSUES FOR THIS FORECAST CYCLE ARE TIMING OF THIS
FRONTAL PASSAGE AND THE POTENTIAL FOR ANY STRONG STORMS.
OVERALL...THE 23/00Z NAM/GFS ARE SIMILAR WITH THE MAIN FEATURES OF
THE SYSTEM...BUT DIFFER ON THE PROGRESSION OF THE FRONTAL BOUNDARY.
THE GFS IS BY FAR THE FASTER OF THE TWO AND IS CLOSER...ALBEIT JUST
A BIT SLOWER THAN THE 22/12Z EURO. THE SYSTEM IS QUITE
PROGRESSIVE...BUT THE GFS JUST APPEARS A LITTLE TOO FAST. OF
COURSE...THE TIMING IS VERY IMPORTANT AS A DEVIATION OF JUST 3 TO 6
HOURS WILL GREATLY AFFECT THE POP FORECAST.
THE PRESSURE GRADIENT WILL BEGIN TO TIGHTEN THIS AFTERNOON AS THE
LOW LEVEL FLOW FINALLY BEGINS TO PICK UP A SOUTHERLY COMPONENT. THIS
WILL GENERALLY AFFORD AROUND 24 HOURS OF RETURN FLOW TO HELP MOISTEN
OUR CURRENTLY DRY AIRMASS. SINCE THIS PROCESS WILL TAKE SOME
TIME...PRECIP WILL BE TOUGH TO GENERATE UNTIL LATE WEDNESDAY
NIGHT...WHEN THE ISENTROPIC ASCENT WILL BEGIN ACROSS THE WESTERN
CWA. THAT BEING SAID...GENERALLY UNDERCUT THE GUIDANCE FOR WEDNESDAY
NIGHT DUE IN LARGE PART TO THE ISENTROPIC ASCENT ON THE 295K SURFACE
GENERALLY OCCURRING IN A RATHER DRY LAYER. MORE MOIST ASCENT
IS INDICATED IN THE 300K LAYER...BUT THE MAGNITUDE OF THIS ASCENT IS
RELATIVELY WEAK. THUS WITH LIMITED FORCED ASCENT COUPLED WITH A
SLOWLY MOISTENING AIRMASS...FEEL SLIGHTLY LOWER POPS ARE JUSTIFIED.
BIG CHANGES OCCUR ON CHRISTMAS EVE AS THE WARM FRONTAL BOUNDARY
BEGINS TO LIFT TOWARD THE GULF COAST. THERE IS SOME DIFFERENCES IN
THE MODELS AS TO HOW ACTIVE THIS BOUNDARY WILL BE. THE AIRMASS
SHOULD BE SUFFICIENTLY MOIST BY THIS TIME TO SUPPORT SCATTERED TO
NUMEROUS SHOWERS...ESPECIALLY WITH BETTER DYNAMICAL FORCING
APPROACHING. INSTABILITY WILL BE CONSIDERABLY LIMITED...THUS WILL
HOLD OFF ANY MENTION OF THUNDER UNTIL AFTER 18Z THURSDAY. POPS WILL
RAMP UP QUICKLY ON THURSDAY FROM WEST TO EAST AS THE MAGNITUDE OF
THE LIFT INCREASES. WILL INDICATE CATEGORICAL POPS IN THE WEST...IN
PART DUE TO THE FRONT BEING VERY NEAR THESE AREAS BY 00Z
FRIDAY...TAPERING DOWN TO 40 TO 50 PERCENT POPS IN THE EAST. THESE
HIGHER POPS GENERALLY REFLECT THE BETTER RAIN CHANCES IN THE
AFTERNOON TIME PERIOD.
BY THURSDAY EVENING...THE COLD FRONT WILL BE APPROACHING MOBILE AL.
THE TIMING NOW BETWEEN THE GFS/NAM BECOMES A LITTLE DIFFERENT WITH
THE GFS PREFERRING A FASTER SOLUTION THROUGH THE FORECAST AREA.
OPTED FOR A BLEND OF THE TWO SOLUTIONS HERE. THE NAM MAY INDEED BE
ON TO SOMETHING SINCE THE MID LEVEL FORCING WILL BE LIFTING OUT
NORTHEASTWARD AS THE STRONG MID LEVEL CYCLONE OCCLUDES OVER THE
CENTRAL PLAINS. INCREASED POPS INTO THE 80 TO 90 PERCENT RANGE
OVERNIGHT GIVEN ENOUGH AGREEMENT IN THIS PERIOD OF THE FORECAST TO
JUSTIFY THESE HIGH NUMBERS.
THE OVERALL POTENTIAL FOR STRONG TO SEVERE WEATHER DOES EXIST WITH
THIS SYSTEM THOUGH THERE ARE SOME CONCERNS WITH THE QUALITY OF THE
AIRMASS. KINEMATICS DO NOT APPEAR TO BE A PROBLEM. DEEP LAYER WIND
SHEAR OF 50 TO 60 KNOTS IS PREDICTED BY BOTH THE NAM AND GFS AHEAD
OF THE COLD FRONT. THE NAM ALSO SHOWS A RATHER IMPRESSIVE 50 KT JET
AT 850 MB ACROSS SOUTHEAST ALABAMA AND SW GEORGIA AHEAD OF THE
FRONT. WHILE THESE VALUES ARE GOOD...BETTER VALUES ARE LOCATED NORTH
AND WEST OF THE REGION IN AN AREA OF STRONGER HEIGHT FALLS. IN
FACT...WE ARE GENERALLY ONLY SEEING HEIGHT FALLS OF 30 TO 40 METERS
AT 500 MB OVER A 24 HOUR PERIOD AHEAD OF THIS SYSTEM IN OUR
REGION...WHICH DOES CAST SOME DOUBT ON THE SEVERE WEATHER POTENTIAL.
WHAT FURTHER CLOUDS THE PICTURE IS THE QUALITY OF THE AIRMASS ACROSS
THE REGION. BOTH THE NAM AND GFS STRUGGLE TO GENERATE MEANINGFUL
SURFACE-BASED INSTABILITY AHEAD OF THE FRONT...AND ANY GENERATED
(GENERALLY 500 J/KG OR LESS) IS CONFINED TO AN AREA MAINLY ALONG AND
SOUTH OF A LINE FROM DOTHAN TO MOULTRIE. THUS...THE OVERLAP AREA OF
BEST INSTABILITY AND KINEMATICS IS ACROSS THE FLORIDA PANHANDLE AND
WESTERN FLORIDA BIG BEND. WHICH IS WHY SPC HAS OUTLOOKED OUR FLORIDA
COUNTIES IN A SLIGHT RISK FOR THURSDAY NIGHT. IT SHOULD BE NOTED
THAT IF BETTER INSTABILITY CAN BE REALIZED AND SPREAD FURTHER
INLAND...THE SEVERE THREAT WOULD INCREASE FOR OUR ALABAMA/GEORGIA
COUNTIES.
GIVEN THAT THE BETTER DYNAMICS WILL BE LIFTING OUT NORTH OF THE AREA
AFTER 06Z...SHOULD SEE THE SEVERE THREAT TAPER OFF AS THIS SYSTEM
MOVES EAST ACROSS THE FORECAST AREA...ESPECIALLY AS IT NEARS THE
I-75 CORRIDOR.
GIVEN THE SLOWER PROGRESSION OF THE NAM...OPTED TO KEEP SCATTERED
SHOWERS IN THE FORECAST GENERALLY ALONG AND EAST OF A LINE FROM
ALBANY TO TALLAHASSEE ON CHRISTMAS DAY. THESE SHOWERS SHOULD END BY
MIDDAY WITH CLEARING SKIES ARRIVING AS THE COLD FRONT MOVES ACROSS
NORTHEAST FLORIDA. THE AIRMASS BEHIND THIS SYSTEM IS NOT
SIGNIFICANTLY COOL OR DRY AS IT HAS ITS ORIGINS IN THE MARITIME
PACIFIC.
.LONG TERM (SATURDAY THROUGH WEDNESDAY).
A POTENT COLD FRONT WILL SWEEP ACROSS THE AREA BY SATURDAY BRINGING
IN ANOTHER RUSH OF COLD WEATHER FOR THE HOLIDAY WEEKEND. A DRY
AIRMASS WILL MOVE IN BEHIND THE FRONT MAKING FOR PLEASANT WEATHER
THROUGH THE BEGINNING OF NEXT WEEK. THE GFS SHOWS THE DEVELOPMENT OF
A WEAK DISTURBANCE IN THE WESTERN GULF SUNDAY NIGHT INTO MONDAY.
THIS MAY BRING IN SOME MID TO UPPER-LEVEL CLOUDS BUT RAIN CHANCES
APPEAR LOW AT THIS TIME...THUS SILENT 10% POPS HAVE BEEN INTRODUCED
FROM THIS TIME THROUGH WEDNESDAY. THE FORECAST BECOMES A BIT TRICKY
BY WEDNESDAY NIGHT REGARDING AN APPROACHING FRONTAL SYSTEM. THE GFS
SOLUTION DEVELOPS A SURFACE LOW OVER THE CENTRAL GULF WEDNESDAY
NIGHT AND BRINGS IT OVER OUR AREA THROUGH 1200Z THURSDAY. THE ECMWF
DEPICTS A SURFACE LOW FORMING OVER THE NORTHWESTERN GULF BY
WEDNESDAY MORNING AND ACCELERATES THE SYSTEM NORTH OF OUR AREA WITH
A TRAILING COLD FRONT PUSHING THROUGH BY THURSDAY MORNING. BOTH
SOLUTIONS WOULD SUGGEST THAT POPS MAY NEED TO BE RAISED IN FUTURE
FORECASTS...BUT DUE TO UNCERTAINTY AND THE LONG-TERM NATURE OF THE
PREDICTION...WILL MAINTAIN SILENT 10% POPS FOR NOW. TEMPS WILL
AVERAGE BELOW CLIMO THROUGH MID-WEEK WITH LOWS APPROACHING THE
FREEZING MARK SEVERAL NIGHTS NEXT WEEK AND HIGHS AVERAGING AROUND 60
DEGREES. DEPENDING ON THE TIMING OF THE NEXT FRONTAL
SYSTEM...TEMPERATURES WILL BEGIN INCREASING BY THURSDAY AHEAD OF THE
NEXT DISTURBANCE.
&&
.AVIATION...ANOTHER NIGHT OF CALM WINDS AND MOSTLY CLEAR SKIES IS IN
SESSION ACROSS OUR AREA AS HIGH PRESSURE SLIDES TO THE NORTHEAST.
WHILE VFR CONDITIONS ARE EXPECTED AT MOST TERMINALS FOR MUCH OF THE
TAF CYCLE...A FEW TERMINALS...NAMELY TLH AND VLD...MAY SEE FOG
DEVELOPMENT AS TEMPERATURES DROP TO NEAR THE DEWPOINT EARLY
WEDNESDAY MORNING. WHILE PATCHY FOG MAY YIELD MVFR VSBYS AT
TLH...VLD COULD BRIEFLY SEE VSBYS DROP TO BELOW IFR CRITERION
EARLY WEDNESDAY MORNING. A CIRRUS DECK WILL MOVE INTO THE REGION BY
EARLY MORNING WITH CLOUDINESS EXPECTED TO INCREASE BY WEDNESDAY
AFTERNOON. VFR CONDITIONS WILL PREVAIL THROUGH WEDNESDAY NIGHT WHEN
FOG FORMATION WILL BE POSSIBLE INTO THURSDAY MORNING. MVFR
CONDITIONS WILL BE LIKELY THURSDAY AFTERNOON AND EVENING AHEAD OF A
POTENT COLD FRONT EXPECTED TO PASS THROUGH EARLY CHRISTMAS DAY.
&&
.MARINE...CONDITIONS WILL BEGIN TO WORSEN OVER THE MARINE AREA AS
WINDS BEGIN TO INCREASE OUT OF THE SOUTHEAST AHEAD OF AN APPROACHING
STORM SYSTEM. WINDS SHOULD INCREASE TO ADVISORY LEVELS BY WEDNESDAY
NIGHT AND REMAIN ELEVATED THROUGH FRIDAY MORNING BEFORE DECREASING
IN THE AFTERNOON. SEAS WILL BUILD WELL OFFSHORE TO AS HIGH AS 10
FEET WITH THIS SYSTEM. HIGH PRESSURE WILL BUILD OVER THE MARINE AREA
BY THE WEEKEND KEEPING WINDS AND SEAS RELATIVELY LOW THROUGH SUNDAY.
&&
.FIRE WEATHER...WITH THE STUBBORN HIGH PRESSURE SYSTEM EXITING OUR
AREA AND A NEW FRONTAL SYSTEM COMING INTO THE PICTURE...WINDS WILL
SHIFT TO EAST THEN SOUTHEAST. THIS WILL ALLOW FOR QUICK MOISTENING
OF THE LOWER ATMOSPHERE THROUGH THURSDAY AS RICH GULF MOISTURE
STREAMS IN AHEAD OF AN APPROACHING COLD FRONT. WITH AMPLE
MOISTURE...RELATIVE HUMIDITY IS NOT EXPECTED TO REACH CRITICAL
LEVELS AND NO FIRE WEATHER CONCERNS ARE ANTICIPATED.
&&
.PRELIMINARY POINT TEMPS/POPS...
TALLAHASSEE 67 49 66 58 64 / 0 10 60 90 30
PANAMA CITY 68 57 65 56 62 / 5 20 70 90 10
DOTHAN 64 51 63 50 60 / 5 20 80 90 10
ALBANY 64 46 64 55 63 / 0 10 60 90 30
VALDOSTA 67 47 68 58 67 / 0 0 40 90 40
CROSS CITY 71 46 70 62 69 / 0 10 50 80 40
APALACHICOLA 65 55 64 57 63 / 5 10 70 80 10
&&
.TAE WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
GA...NONE.
FL...NONE.
GM...SMALL CRAFT ADVISORY FROM DESTIN TO THE OCHLOCKONEE RIVER
ENTRANCE OUT TO 60 NAUTICAL MILES FROM 1 AM EST THURSDAY TO
1 PM EST FRIDAY.
&&
$$
AVIATION...GOULD
LONG TERM...AUSTIN
REST OF DISCUSSION...GODSEY
