Aug 21 Afternoon Weather Discussion

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
330 PM EDT SAT AUG 21 2010

.UPDATE (TODAY AND TONIGHT)...
19Z WATER VAPOR AND H4 RUC ANALYSIS SHOWING ELONGATED AND BROAD
RIDGING IN CONTROL OF THE SOUTHERN TIER OF THE COUNTRY...INCLUDING
THE NORTHERN GULF COAST. HOWEVER...WATCHING A WELL DEFINED SHORTWAVE
PASSING OVER THE MIDDLE MS VALLEY THAT IS PROGGED TO MOVE EAST AND
SOUTHEAST TOWARD THE MID-ATLANTIC AND CAROLINA COASTLINES BY SUNDAY
MORNING. AS THIS ENERGY MOVES IT WILL ACT TO BREAK DOWN THE EASTERN
PERIPHERY OF THE RIDGE OVER OUR FORECAST AREA AND TOWARD THE
SOUTHEAST U.S COAST.

AT THE SURFACE...FORECAST AREA RESIDES BETWEEN HIGH PRESSURE
CENTERED SOUTH OF THE MS DELTA...AND A WEAK TROUGH OF LOW PRESSURE
ALONG THE SOUTHEAST U.S. COAST. FURTHER NORTH WE FIND A WEAK FRONT
EXTENDING FROM EASTERN OKLAHOMA INTO SOUTHERN MISSOURI. THIS FRONT
IS ASSOCIATED WITH THE SHORTWAVE MENTIONED ABOVE. AS THE UPPER LEVEL
ENERGY MOVES EASTWARD...THIS FRONT WILL FOLLOW...AND DIG SOUTHEAST
TOWARD NORTHERN AL AND GA BY SUNDAY MORNING.

.SHORT TERM (TONIGHT THROUGH TUESDAY)...

TONIGHT...LAND BASED EVENING SHOWERS AND STORMS WILL BEGIN TO
DECREASE IN COVERAGE AND INTENSITY WITHIN A FEW HOURS OF SUNSET.
HOWEVER...IN TYPICAL FASHION...CONVECTION WILL BEGIN TO RAMP BACK UP
AGAIN OVER THE COASTAL WATERS AFTER MIDNIGHT...AND ESPECIALLY TOWARD
SUNRISE. AS WAS THE CASE THIS PAST MORNING...THE FLOW SHOULD PUSH
THESE SHOWERS EASTWARD TOWARD THE TAYLOR AND DIXIE COUNTY COASTS.

SUNDAY...ANOTHER ACTIVE DAY IN TERMS OF CONVECTION SEEMS LIKELY.
EVOLVING UPPER LEVEL SYSTEM AND ASSOCIATED SURFACE LOW OVER THE
NORTHEASTERN STATES WILL MOVE EAST TOWARD THE COAST...AND GIVE
ANOTHER PUSH TO THE FRONT LAYING ACROSS NORTHERN AL/GA. THE BOUNDARY
WILL SINK SOUTHWARD THROUGH THE DAY REACHING A POSITION IN THE
VICINITY OF MONTGOMERY AND MACON. WILL LIKELY BEGIN THE DAY OFF WITH
SCT-NMRS SHOWERS AND STORM OVER THE COASTAL WATERS...AND IMMEDIATE
COASTAL LAND ZONES. WITH THE ONSET OF GOOD DIURNAL HEATING DURING
THE LATER MORNING/EARLY AFTERNOON HOURS...THE COMBINATION OF THE
RICH ATMOSPHERIC MOISTURE...AND THE LOW LEVEL FOCUS SOUTH OF THE
FRONT SHOULD FORCE A GOOD COVERAGE OF SHOWERS AND STORMS OVER THE
REMAINDER OF THE REGION. HIGH TEMPS RIGHT ABOUT AVERAGE WITH LOWER
90S INLAND AND UPPER 80S TO NEAR 90 AT THE BEACHES.

SUNDAY NIGHT...FRONT/TROUGH BOUNDARY IS PROGGED TO STALL ACROSS
SOUTHERN AL/GA. ALTHOUGH WE SHOULD SEE A DECREASE IN SHOWER COVERAGE
WITH THE LOSS OF DIURNAL HEATING...WILL KEEP AT LEAST LOW END CHANCE
POPS GOING THROUGH THE NIGHT DUE TO CONTINUING LOW LEVEL FOCUS ALONG
THE FRONT. IN FACT...MAY SEE THINGS GET QUITE ACTIVE ALONG THE
IMMEDIATE COAST...AND COASTAL MARINE ZONES DURING THE LATE NIGHT
HOURS.

MONDAY THROUGH TUESDAY...GLOBAL GUIDANCE HAS COME INTO BETTER
AGREEMENT WITH RESPECT TO THE SOUTHWARD SURGE OF DRIER MID-LEVEL AIR
DURING THE EARLY PORTION OF THE WEEK. EARLIER GFS RUNS HAD KEPT THE
MOISTURE IN PLACE OVER THE ENTIRE FORECAST AREA...WHILE THE ECMWF
WAS MORE AGGRESSIVE AT BRINGING THIS AIRMASS DOWN TO AT LEAST THE
I-10 CORRIDOR. THE LATEST 12Z OPER GFS HAS NOW COME INTO BETTER
AGREEMENT WITH THE ECMWF. THIS SOLUTION WOULD ARGUE FOR A POP
GRADIENT FAVORING HIGHER RAIN CHANCES OVER THE SOUTHERN HALF OF THE
AREA. WITH THE LOW LEVEL BOUNDARY/TROUGH STILL IN PLACE...CAN NOT
COMPLETELY REMOVE POPS FROM ANY LOCATION...BUT WILL CERTAINLY FAVOR
HIGHER PERCENTAGES ALONG AND SOUTH OF THE I-10 CORRIDOR.

&&

.LONG TERM (TUESDAY THROUGH NEXT SATURDAY)...
THE LARGE SCALE LONG TERM PERIOD BEGINS IN THE NRN STREAM WITH WEAK
RIDGING OVER PAC NW...TROUGHING EWD THRU PLAINS...THEN AMPLIFIED
RIDGING ACROSS GREAT LAKES FOLLOWED BY DEEP TROUGH FROM SRN CANADIAN
MARITIMES SWD TO GA WITH 575DM LOW CLOSING OFF OVER PENN. SRN STREAM
SHOWS FLAT FLOW FROM CA EWD TO FL WITH PERSISTENT MEAN H5 HIGH OVER
THE SOUTHWEST US. THE ERN UPPER LOW IS FORECAST TO WEAKEN INTO AN
OPEN WAVE BY WED AND LIFT TO THE NORTHEAST ON THURS ALLOWING GREAT
LAKES MID LEVEL RIDGE TO SPREAD ESE AND BUILD DOWN EAST COAST AS
DRIER AIR ATTEMPTS TO WORK IN FROM NW-SE. MODELS SHOW A SIGNIFICANT
DECREASE IN ATMOSPHERIC MOISTURE ABOVE 850MB WED THRU SAT.

AT SURFACE...COMPARED TO YESTERDAY...GFS AND ECMWF ARE IN BETTER
AGREEMENT. IN BOTH CASES PERIOD BEGINS WITH CLOSED LOW IN WRN ATLC
JUST OFF PENN/DE WITH TRAILING FRONTAL BOUNDARY SWWD INTO NE GULF.
INITIAL POSITION OF UPPER TROUGH ALLOWS FRONT TO SAG SWD. THEN AS
UPPER TROUGH AND THEN CLOSED LOW LIFTS NEWD..FRONT STALLS ALONG
FL/GA BORDER WITH A SMALL AMOUNT OF DRIER AIR FILTERING INTO NRN
AL/GA COUNTIES. THIS SCENARIO WOULD KEEP MUCH OF CWA IN WARM SECTOR
WITH FRONTAL TROUGH/MOISTURE/HIGH PWATS DRAPED ACROSS CWA AT LEAST
UNTIL THURS THEN IN WAKE OF DEPARTING LOW...UPSTREAM RIDGE BUILDS
SEWD INTO REGION ALLOWING THE SURFACE RIDGE AXIS OVER S/CNTRL FL TO
LIFT NORTH YIELDING A TRANSITION INTO A MORE TYPICAL LATE MORNING
COASTAL AND AFTERNOON AND EVE INLAND PATTERN OF SCATTERED
THUNDERSTORMS.

MODELS SHOW THAT THE AMPLE DAILY CONVECTION CURRENTLY EXPERIENCED
WILL BE DISRUPTED BY THE ARRIVAL OF DRIER MID-LEVEL DURING THE
MIDDLE OF THE WEEK...AND THEN RETURN TO A MORE TYPICAL SUMMERTIME
PATTERN WITH SEABREEZE INDUCED MID-SCT POPS EACH AFTERNOON AND EVE
FOR THE END OF THE WEEK. WITH FRONTAL CLOUDS IN AREA...MINIMUM
TEMPS WILL REMAIN ABOVE CLIMO (INLAND CLIMO AROUND 69) WITH LOWS
DROPPING FROM MID TO LOW 70S THRU PERIOD. HIGHS WILL PERSIST AT OR
SLIGHTLY ABOVE CLIMO (90 DEGREES) THRU THE PERIOD.

&&

.AVIATION...
IT WILL BE AN INTERESTING FCST AT THE TERMINALS THIS AFTERNOON AS
SHOWERS AND THUNDERSTORMS ARE DEVELOPING AND INCREASING IN COVERAGE
IN THE MOIST NW FLOW OUT AHEAD OF A WEAK COLD FRONT OVER SE
AL...WESTERN SECTIONS OF SW GA...AND THE FL PANHANDLE...AS WELL AS
ALONG THE SEA BREEZE FRONT OVER THE FLORIS BIG BEND. THE FAIRLY
STRONG LOW LEVEL NW FLOW IS RETARDING THE INLAND PROGRESS OF THE SEA
BREEZE FRONT...WHICH IS FINALLY PUSHING ITS WAY NORTHEASTWARD OUT OF
GULF COUNTY AND INTO FRANKLIN...LIBERTY...AND WAKULLA COUNTIES. HOW
THIS CONVECTIVE ACTIVITY WILL TRANSLATE TO THE TAF SITES IS A BIT
UNCERTAIN....BUT BELIEVE THE BEST CHANCES FOR HEAVIER RAINFALL AND
MORE RESTRICTIVE FLYING CONDITIONS WILL BE AT TLH...ECP...AND
DHN...WITH WEAKER AND FASTER MOVING STORMS MORE LIKELY FOR ABY AND
VLD. FOR THE OVERNIGHT HOURS...USED A BLEND OF GUIDANCE AND
PERSISTENCE...WHICH WOULD PUT ABY AND VLD AT THE HIGHEST RISK OF IFR
CONDITIONS.

&&

.MARINE...
HIGH PRESSURE CENTERED JUST WEST OF THE FORECAST AREA HOLD IN PLACE
THROUGH TONIGHT...BEFORE RETREATING WESTWARD IN RESPONSE TO AN
APPROACHING COLD FRONT. THIS FRONT IS FORECAST TO STALL JUST NORTH
OF THE FORECAST WATERS SUNDAY NIGHT AND REMAIN IN PLACE THROUGH THE
EARLY PORTION OF THE WEEK. IN GENERAL WINDS AND SEAS WILL REMAIN
BELOW CAUTIONARY LEVELS...HOWEVER MARINERS SHOULD EXPECT PLENTY OF
SHOWER AND THUNDERSTORM ACTIVITY...ESPECIALLY DURING THE LATE NIGHT
AND MORNING HOURS EACH DAY.

&&

.FIRE WEATHER...
WITH THE COLD FRONTAL BOUNDARY STILL TO OUR NORTH... AFTERNOON
RELATIVE HUMIDITIES WILL REMAIN VERY HIGH THROUGH MONDAY AFTERNOON.
IF THE ABOVE MENTIONED COLD FRONT IS ABLE TO MAKE A CLEAN PASSAGE
THROUGH THE REGION ON TUESDAY...WE MAY SEE A SIGNIFICANT DROP IN
AFTERNOON RELATIVE HUMIDITIES FOR THE MIDDLE PART OF NEXT WEEK.

&&

.PRELIMINARY POINT TEMPS/POPS...
TALLAHASSEE 92 76 94 76 94 / 60 20 60 30 50
PANAMA CITY 91 78 91 79 93 / 60 30 60 40 50
DOTHAN 93 75 95 77 95 / 60 20 60 40 40
ALBANY 93 76 95 76 95 / 60 30 60 40 40
VALDOSTA 92 74 94 74 94 / 60 30 60 30 50
CROSS CITY 92 75 94 76 92 / 70 40 50 40 60
APALACHICOLA 90 77 92 78 92 / 60 40 60 50 50

&&

.TAE WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
GA...NONE.
FL...NONE.
GM...NONE.


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