Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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FXUS62 KTBW 260927 AAA
AFDTBW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE TAMPA BAY RUSKIN FL
525 AM EDT TUE AUG 26 2014

...UPDATED AVIATION SECTION FOR LOWER STRATUS POTENTIAL EARLY THIS
MORNING AT KLAL AND KTPA...

.SYNOPSIS...
08Z WATER VAPOR AND H4 RAP ANALYSIS SHOWS A PATTERN THAT HAS NOT
BEEN ALL THAT COMMON THIS SUMMER. IT IS HARD TO BELIEVE...BUT THE
MID/UPPER LEVEL PATTERN CONSISTS OF LONGWAVE TROUGHING OVER THE
WESTERN CONUS...AND DOWNSTREAM AMPLIFIED RIDGING EXTENDING FROM THE
SOUTHERN PLAINS TO THE EASTERN SEABOARD. MUCH OF THIS SUMMER SEASON
HAS BEEN THE OPPOSITE WITH A STUBBORN TROUGH OVER THE EAST. THE
FLORIDA PENINSULA RESIDES MAINLY SOUTH OF THIS LARGE RIDGE IN A DEEP
LAYER NORTHEAST FLOW REGIME BETWEEN THIS FEATURE AND WHAT IS NOW
HURRICANE CRISTOBAL SPINNING TO THE EAST OF THE BAHAMAS. THIS
NORTHEAST FLOW IS DELIVERING A VERY ANOMALOUS MID-LEVEL AIRMASS
ABOVE OUR HEADS IN TERMS OF TEMPERATURES AND DRY AIR FOR THE NEXT
24-36 HOURS. THIS AIRMASS IS ALREADY ARRIVING PER WV IMAGERY AND THE
LATEST KTBW SOUNDING PROFILE...AND WILL ONLY FURTHER DRY OUT
OVERNIGHT INTO TUESDAY MORNING. THE KTBW EVENING SOUNDING SHOWED A
PW VALUES ALREADY DOWN TO 1.27"...WITH DECENT DEWPOINT DEPRESSIONS
ABOVE THE DIURNAL MIXED LAYER ALL THE WAY UP THROUGH THE TROP
COLUMN. THIS PW VALUE IS SLIGHTLY MORE THAN 2 STANDARD DEVIATIONS
BELOW CLIMATOLOGY FOR THE LAST WEEK OF AUGUST...AND EXPECT PW VALUES
DURING THE DAY ON TUESDAY TO DROP EVEN FURTHER. MINIMUM ALL TIME
VALUES FOR LATE AUGUST ARE AROUND 0.8". WE WILL NOT GET DOWN THAT
MUCH...BUT STILL AN INCREDIBLY DRY COLUMN FOR THIS TIME OF YEAR.

THE OTHER AND EVEN MORE IMPRESSIVE FEATURE OF THIS AIRMASS IS THE
MID-LEVEL TEMPS. FOR SIMPLICITY LETS FOCUS IN ON THE 500MB / H5
TEMPERATURES. THE KTBW SOUNDING MEASURED A TEMPERATURE OF -2C.
TEMPERATURES THIS WARM ARE ALMOST UNHEARD OF IN AUGUST...AND IT IS
FORECAST BY THE RELIABLE GUIDANCE TO WARM ANOTHER DEGREE OR 2 TODAY.
USING THE NAEFS (NORTH AMERICAN ENSEMBLE FORECAST SYSTEM) COMPARED
TO THE CFSR (CLIMATE FORECAST SYSTEM REANALYSIS 1979-2009)
CLIMATOLOGY FOR A 3 WEEK PERIOD CENTERED ON TODAY...WE FIND THAT THE
FORECAST H5 TEMPS ABOVE THE FL PENINSULA ARE ALL ABOVE THE 99TH
PERCENTILE OF VALUES...WITH MUCH OF THE AREA AT H5 EXPERIENCING
TEMPS HIGHER THAN ANYTHING SEEN IN THIS 30 YEAR CLIMATOLOGICAL
RECORD. SUCH ANOMALOUS VALUES ARE JUST INTERESTING FROM A CLIMATE
AND METEOROLOGICAL PERSPECTIVE. SO...GUESS WHAT? DO NOT SEE MUCH IN
THE WAY OF DEEP CONVECTION AROUND TODAY (IF ANY).

AT THE SURFACE...WE ARE EXPERIENCING A TIGHT GRADIENT AND ELEVATED
NORTHEAST FLOW BETWEEN 1020+MB HIGH PRESSURE TO OUR NORTH...AND
HURRICANE CRISTOBAL TO OUR EAST. EXPERIENCED A GUSTY DAY ON MONDAY
WITHIN THIS GRADIENT...HOWEVER THE REAL IMPACTS HAVE BEEN OVER THE
COASTAL WATERS OF THE EASTERN GULF...WHERE A RARE SUMMER ADVISORY
EVENT IS UNDERWAY EARLY THIS MORNING. THIS ADVISORY SHOULD BE
BRIEF IN DURATION WITH WINDS DROPPING SOMEWHAT DURING THE DAY ON
TUESDAY...AND THEN IMPROVING CONDITIONS THROUGH THE MIDDLE OF THE
WEEK.

&&

.SHORT TERM (TODAY THROUGH WEDNESDAY)...
TODAY...
JUST ABOUT AS QUIET A SUMMER DAY IN TERMS OF DEEP CONVECTION AS WE
SEE DURING THE SUMMER MONTHS. THE ATMOSPHERE ABOVE 850MB IS VERY
HOSTILE TOWARD CONVECTIVE COLUMNS THROUGHOUT THE FORECAST AREA AND
WILL REMAIN SO THROUGH THE DAY. MINIMUM THETAE VALUES BETWEEN 850MB
AND 500MB ARE BELOW 320K...AND IN SOME GUIDANCE MEMBERS DOWN BELOW
315K. THIS IS ABOUT AS LOW AS WE SEE THIS TIME OF YEAR AND THE DEPTH
OF THESE LOW VALUES SUGGESTS MAINLY A SHALLOW CU FIELD WITH LITTLE
IN THE WAY OF VERTICAL GROWTH BEFORE SIGNIFICANT ENTRAINMENT OF THIS
LOW THETAE AIR BEGINS. WITH THIS IN MIND...HAVE RAIN CHANCES DOWN AT
10% OR LESS TODAY FOR ALL AREAS. THERE IS SOME ATLANTIC MOISTURE IN
THE LOWEST LEVELS OF THE ATMOSPHERE TO SUPPORT THE SHALLOW CU
FIELD...BUT CAN NOT SEE ANYTHING MORE THAN A FEW ISOLATED AND
SHALLOW SPEED CONVERGENCE SPRINKLES MAKING THERE WAY INTO THE
FORECAST AREA WITHIN THE NE FLOW FROM THE ATLANTIC COAST. SOME OF
THE HIRES GUIDANCE MEMBERS TRY TO PRODUCE A FEW SHOWERS RIGHT AT THE
GULF COAST VERY LATE IN THE DAY AS THE LOCAL GRADIENT DIURNALLY
WEAKENS OVER THE NEARSHORE WATERS AND PROVIDES A BRIEF WINDOW OF
SURFACE FOCUS/CONVERGENCE FOR UPWARD MOTION. HOWEVER...THESE MODELS
ARE GENERALLY A BIT AGGRESSIVE...AND WILL HOLD WITH THE IDEA THAT
THE COLUMN IS SIMPLY TO HOSTILE FOR ANY OF THESE SHOWERS TO GET
ESTABLISHED.

SO...LOOK FOR A GENERALLY DRY DAY UNDER A FEW-SCT CU FIELD AND
TEMPERATURES CLIMBING INTO THE LOWER TO MIDDLE 90S. WINDS WILL
BECOME A LITTLE GUSTY WITHIN A FEW HOURS OF SUNRISE...BUT LIKELY
WILL NOT BE AS PRONOUNCED AS THEY WERE ON MONDAY.

TONIGHT...
NOT MUCH SENSIBLE WEATHER TO TALK ABOUT. THE DRY CONDITIONS WILL
CONTINUE OTHER THAN AN ISOLATED SPEED CONVERGENCE SPRINKLE MAKING IT
FAR ENOUGH INLAND FROM THE EAST COAST TO REACH EASTERN
POLK/HIGHLANDS COUNTIES. THE LOSS OF DIURNAL MIXING WILL END THE
GUSTY NATURE TO THE WINDS QUICKLY AFTER SUNSET OVER THE LANDMASS.
WINDS WILL ACTUALLY INCREASE AT THIS POINT OVER THE COASTAL WATERS
ONCE AGAIN...AND MORE CAN BE READ ABOUT THIS NEXT EASTERLY NOCTURNAL
SURGE IN THE MARINE DISCUSSION BELOW.

WEDNESDAY LOOKS TO BE ANOTHER GENERALLY QUIET DAY FOR MUCH OF THE
FORECAST AREA AS DRY AND WARM AIR REMAIN OVERHEAD. SOME UNCERTAINLY
IN THE THUNDERSTORM CHANCES DOWN TOWARD THE FORT MYERS FOR THE
SECOND HALF OF THE AFTERNOON AND EVENING AS THERE IS SOME
DISAGREEMENT IN THE GUIDANCE AS TO THE RATE OF MOISTURE RETURN AND
LOW LEVEL FOCUS RETURN FROM THE SOUTH. ALL THE RELIABLE GUIDANCE
MEMBERS HAVE A WEAK TROUGH EXTENDING ACROSS THE SOUTHEAST GULF AND
FLORIDA STRAITS ON WEDNESDAY. THIS TROUGH AXIS MARKS THE LEADING
EDGE OF THE DEEP LAYER MOISTURE AND NOSE OF AN EXPANDING LOW LEVEL
THETAE RIDGE. THE 00Z NAM IS THE MOST AGGRESSIVE WITH THE NORTHWARD
PROGRESSION OF THIS THETAE RIDGE AND BRINGS A DECENT SWATH OF
MOISTURE RETURN AND RESULTING VERTICAL MOTION BELOW 700MB INTO THE
SOUTHERN FL PENINSULA...INCLUDING LEE/CHARLOTTE COUNTIES LATE IN THE
DAY. THE GFS IS IN THE MIDDLE OF THE GUIDANCE ENVELOPE...WITH THE
00Z ECMWF THE SLOWEST AND LEAST AGGRESSIVE TO BRING THE MOISTURE
BACK TO OUR SOUTHERN ZONES. THE NAM UPGRADE FROM EARLIER THIS MONTH
IS STILL FAIRLY RECENT AND THERE HAS NOT BEEN ENOUGH TIME TO REALLY
FIGURE OUT ALL ITS BIASES ETC. NOT GOING TO SAY ITS SOLUTION IS NOT
CORRECT...BUT FOR THIS FORECAST CYCLE...A MORE CONSERVATIVE
PHILOSOPHY IN LINE WITH THE GFS SEEMS MORE APPROPRIATE. WILL BRING A
CHANCE FOR A FEW LATE DAY STORMS INTO THE FORT MYERS AREA...BUT KEEP
LIKELY WORDING OUT OF THE FORECAST (AS THE NAM WOULD SUGGEST).
FURTHER NORTH...TOWARD TAMPA BAY AND THE NATURE COAST...CONFIDENCE
IS HIGHER IN NO SIGNIFICANT MOISTURE RETURN DURING THE DAYLIGHT
HOURS. TOUGH THIS TIME OF YEAR AS A FORECASTER TO GO YET ANOTHER DAY
WITH NO STORM CHANCES...BUT WILL STILL KEEP RAIN CHANCES WELL BELOW
CLIMO...WITH JUST A 15-20% POP FOR MOST AREAS. NORTH OF BROOKSVILLE
IS THE ONLY PLACE CONFIDENCE IS HIGH ENOUGH TO LEAVE RAIN MENTION
OUT OF THE FORECAST. THURSDAY LOOKS TO BE THE TRUE TRANSITION DAY
FOR MOST AREAS...WITH DEEPER LAYER MOISTURE AND INCREASING STORMS
CHANCES EXPANDING NORTHWARD ACROSS THE PENINSULA.

&&

.LONG TERM (THURSDAY THROUGH NEXT MONDAY)...
A COMPACT UPPER RIDGE INITIALLY ACROSS SOUTH GA AND NORTH FL BUILDS
AND ELONGATES...EXTENDING FROM THE NORTHERN GULF TO THE ATLANTIC
WATERS OFF THE SOUTHEAST AND MID-ATLANTIC COASTS. THIS RIDGE
DOMINATES THE AREA ALTHOUGH A WEAK UPPER LOW/TROUGH APPROACHES THE
BAHAMAS BY THE END OF THE PERIOD. MEANWHILE AT THE SURFACE CRISTOBAL
EXITS AS HIGH PRESSURE BEGINS TO RIDGE IN ACROSS SOUTH FL. THIS
LIFTS INTO CENTRAL FL FRI AND CONTINUES NORTHWARD...RESIDING ACROSS
NORTHERN FL THROUGH THE HOLIDAY WEEKEND WITH A BROAD AND RELAXED
PRESSURE PATTERN.

THE UPPER RIDGE WILL HELP KEEP TEMPERATURES ON THE WARM SIDE OF
NORMAL...WITH THE HIGHS ABOVE NORMAL BY A COUPLE OF DEGREES. THE
SURFACE RIDGE SLIDING NORTH PROVIDES ENOUGH MOISTURE...MODEL PWAT
VALUES RUN IN THE 1.9 TO 2 INCH RANGE...FOR ISOLATED TO SCATTERED
SHOWERS AND THUNDERSTORMS. SOUTHEAST BECOMING SOUTHERLY LOW LEVEL
FLOW...BUT WITH ONSHORE FLOW IN THE AFTERNOONS...WILL RESULT IN THE
MOST COVERAGE INLAND IN THE AFTERNOON AND EVENING HOURS. ISOLATED
OVERNIGHT ACTIVITY ON THE GULF MAY DRIFT OVER SOME COASTAL LOCATIONS
IN THE MORNINGS.

&&

.AVIATION...
VFR CONDITIONS PREVAIL ACROSS MUCH THE FORECAST AREA EARLY THIS
MORNING. HAVE SEEN AN AREA OF LOWER STRATUS WITH CEILINGS IN THE IFR
RANGE DEVELOP ALONG AND NORTH OF THE I-4 CORRIDOR IN PAST COUPLE OF
HOURS. HAVE INCLUDED TEMPOS FOR BRIEF IFR CIGS AT KLAL AND KTPA
THROUGH EARLY THIS MORNING. ANY CIG RESTRICTIONS ALONG THE I-4
CORRIDOR SHOULD LIFT WITHIN A FEW HOURS OF SUNRISE. ELSEWHERE VFR
CONDITIONS WILL PREVAIL. CHANCES FOR THUNDERSTORMS TODAY ARE VERY
LOW FOR ALL THE TERMINALS. ONLY AVIATION CONCERNS WILL BE GUSTY
NORTHEAST WINDS FROM MID MORNING THROUGH THE AFTERNOON HOURS. WINDS
WILL BECOME LESS GUSTY AFTER SUNSET THIS EVENING.

&&

.MARINE...
A TIGHT GRADIENT BETWEEN HIGH PRESSURE TO OUR NORTH AND HURRICANE
CRISTOBAL TO THE EAST OF FLORIDA ARE RESULTING IN AN UNSEASONABLY
STRONG EAST TO NORTHEAST FLOW ACROSS THE EASTERN GULF OF MEXICO.
ADVISORY LEVEL WINDS WILL CONTINUE EARLY THIS MORNING OVER THE
COASTAL WATERS AND THEN SLOWLY SUBSIDE BELOW HEADLINE CRITERIA
THROUGH THE MORNING HOURS. ANOTHER PERIOD OF CAUTIONARY LEVEL WINDS
IS EXPECTED TONIGHT INTO EARLY WEDNESDAY MORNING BEFORE ONCE AGAIN
SUBSIDING DURING THE THE DAYLIGHT HOURS. WINDS AND SEAS SHOULD THEN
REMAIN BELOW HEADLINE CRITERIA FOR THE REMAINDER OF THE FORECAST
PERIOD.

&&

.FIRE WEATHER...
DESPITE DRIER CONDITIONS AND LITTLE TO NO RAIN CHANCES TODAY...
RELATIVE HUMIDITY VALUES WILL REMAIN ABOVE CRITICAL LEVELS. WINDS
HOWEVER WILL BE GUSTY OUT OF THE NORTHEAST AND EAST. MOISTURE AND
CHANCES FOR AFTERNOON STORMS WILL BEGIN TO INCREASE ONCE AGAIN
DURING THE MIDDLE PORTION OF THE WEEK AND BE BACK TO NORMAL LATE
AUGUST CONDITIONS BY THE END OF THE WEEK.

&&

.PRELIMINARY POINT TEMPS/POPS...
TPA  94  75  94  78 /  10  10  20  10
FMY  95  74  94  78 /  10  10  40  20
GIF  93  72  94  74 /  10  10  20  10
SRQ  94  74  94  77 /  10  10  20  10
BKV  94  70  94  71 /  10  10  20  10
SPG  94  79  93  80 /  10  10  20  10

&&

.TBW WATCHES/WARNINGS/ADVISORIES...
FL...NONE.
GULF WATERS...SMALL CRAFT ADVISORY UNTIL 10 AM EDT THIS MORNING FOR
     BONITA BEACH TO ENGLEWOOD OUT 20 TO 60 NM-ENGLEWOOD
     TO TARPON SPRINGS OUT 20 TO 60 NM-TARPON SPRINGS TO
     SUWANNEE RIVER OUT 20 TO 60 NM.

     SMALL CRAFT ADVISORY UNTIL 8 AM EDT THIS MORNING FOR
     BONITA BEACH TO ENGLEWOOD OUT 20 NM-ENGLEWOOD TO
     TARPON SPRINGS OUT 20 NM-TARPON SPRINGS TO SUWANNEE
     RIVER OUT 20 NM.

&&

$$

SYNOPSIS/SHORT TERM/AVIATION/MARINE/FIRE WX...MROCZKA
LONG TERM...RUDE







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