Special Marine Warning 21, NWS Key West, FL

FLORIDA KEYS AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE KEY WEST FL
502 AM EDT THU APR 5 2007

.DISCUSSION...
OBSERVATIONS/ANALYSIS -- A WEAK RIDGE OF HIGH PRESSURE WAS ANALYZED
OVER THE BAHAMAS...EXTENDING WEST TO ABOUT THE MARQUESAS KEYS WEST OF
KEY WEST. A COL REGION WAS JUST WEST OF THE RIDGE OVER THE
SOUTHEASTERN GULF OF MEXICO...AND A COLD FRONT WAS MOVING SOUTH OVER
NORTH FLORIDA AND THE NORTHEASTERN GULF OF MEXICO. AIR TEMPERATURES
EARLY THIS MORNING HAVE BEEN HOLDING STEADY IN THE MID 70S ACROSS THE
FLORIDA KEYS...WITH DEWPOINTS IN THE MID 60S. KEY WEST DOPPLER RADAR
IMAGES WERE VOID OF PRECIPITATION ECHOES FOR MOST OF THE NIGHT.
HOWEVER...THE MOST RECENT SCANS ARE SHOWING ISOLATED SHOWERS
DEVELOPING ALONG A CLOUD LINE OVER THE STRAITS OF FLORIDA BEYOND 20
NAUTICAL MILES SOUTH OF THE FLORIDA REEFS.

HEIGHT FALLS HAVE OCCURRED THROUGH MUCH OF THE TROPOSPHERE OVER THE
REGION DURING THE LAST 24 HOURS...AS EXPECTED. NOAA GPS-MET SENSORS
IN THE KEYS SHOW INCREASING VALUES OF TOTAL PRECIPITABLE WATER SINCE
WEDNESDAY EVENING...INDICATIVE OF LAYER MOISTENING.

FORECASTS -- A LOW-LATITUDE MID-TROPOSPHERIC SHORT WAVE TROUGH...NOW
EVIDENT IN MOISTURE CHANNEL IMAGERY OVER THE WESTERN GULF OF
MEXICO...WILL MOVE EAST TODAY AND TONIGHT...INDUCING ADDITIONAL
HEIGHT FALLS AND SYNOPTIC-SCALE ASCENT ACROSS SOUTH FLORIDA AND THE
KEYS. AS A RESULT...FURTHER DESTABILIZATION AND SOME MOISTENING OF
THE LOWER TROPOSPHERE WILL OCCUR. ISLAND CUMULUS LINES WILL LIKELY
DEVELOP OVER THE KEYS IN THE INCREASINGLY UNSTABLE...BUT WEAK LOW-
LEVEL FLOW BY THIS AFTERNOON...PRODUCING SOME SHOWERS. HOWEVER...
COVERAGE AND UPDRAFT STRENGTH WILL BE LIMITED DUE TO DRY POCKETS
PERSISTING NEAR THE CONDENSATION LEVEL AND A WARM LAYER BETWEEN
10000-15000FT AGL.

THE BEST CHANCE FOR RAIN AND POSSIBLY EVEN A FEW THUNDERSTORMS IN THE
KEYS WILL BE TONIGHT AND EARLY FRIDAY...AS THE MID-LEVEL TROUGH AXIS
APPROACHES AND A LOW-LEVEL BOUNDARY...A COLD FRONT...APPROACHES FROM
THE NORTH. WE HAVE INCREASED 12-HOUR RAIN CHANCES BY 10 PERCENT FOR
TODAY...TONIGHT...AND FRIDAY...AND VALUES FOR TONIGHT AND FRIDAY MAY
HAVE TO BE INCREASED FURTHER LATER TODAY. IT DOES LOOK AS THOUGH THE
COLD FRONT WILL MAKE A CLEAN SWEEP...ALBEIT SHORT-LIVED...THROUGH THE
KEYS ON FRIDAY AND FRIDAY NIGHT...WITH A SLIGHTLY COOLER AND DRIER
AIR MASS ARRIVING WITH FRESHENING NORTH BREEZES.

FORECAST CONFIDENCE GOES DOWN FAST AFTER SATURDAY BECAUSE OUR WEATHER
PATTERN WILL BE MODULATED BY BOTH RAPIDLY-MOVING JET STREAKS IN
STRENGTHENING FLOW ALOFT AND THE POSITION AND NATURE OF THE LOW-LEVEL
FRONTAL ZONE MOVING THROUGH THE KEYS FRIDAY. NEVERTHELESS...A
SLIGHTLY MORE ACTIVE WEATHER PATTERN IN THE FORM OF HIGHER CLOUD
COVER AND RAIN CHANCES...AND MORE FREQUENT CHANGES IN WINDS AND
TEMPERATURES WILL BE IN STORE FOR NEXT WEEK...AS A STRONG SUBTROPICAL
JET STREAM TAKES SHAPE ALOFT OVER THE GULF BASIN.
&&

.MARINE...
GENTLE TO OCCASIONALLY MODERATE SOUTH BREEZES WILL PREVAIL OVER
WATERS ADJOINING THE FLORIDA KEYS...INCLUDING FLORIDA BAY TODAY.
LOCATIONS ADJOINING THE UPPER KEYS MAY COME UNDER THE INFLUENCE OF
THE MAINLAND SEA BREEZES THIS AFTERNOON...KEEPING MODERATE SOUTH
BREEZES GOING UNTIL LATE. MEANWHILE...LIGHT AND VARIABLE BREEZES ARE
EXPECTED FOR MOST OF THE DAY OVER GULF WATERS NORTH AND WEST OF THE
LOWER KEYS...INCLUDING AT DRY TORTUGAS NATIONAL PARK. LIGHT AND
VARIABLE BREEZES SHOULD AFFECT MOST OF THE SERVICE AREA FOR A FEW
HOURS THIS EVENING...BEFORE A LIGHT NORTH WIND FLOW SETS IN
OVERNIGHT. SCATTERED SHOWERS AND EVEN A FEW THUNDERSTORMS ARE
EXPECTED AS AN UPPER-AIR DISTURBANCE APPROACHES FROM THE WEST WITH
THE MOST ACTIVE PERIOD PROBABLY FROM MIDNIGHT TO FRIDAY MID-MORNING.
THEN...FRESHENING NORTH BREEZES WILL DEVELOP WITH SMALL CRAFT
EXERCISE CAUTION STATEMENTS LIKELY BY FRIDAY NIGHT AND SATURDAY.
THE WIND FORECAST BECOMES VERY UNCERTAIN SUNDAY AND BEYOND AS A
COMPLEX PRESSURE PATTERN DEVELOPS OVER THE GULF OF MEXICO.
&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL TODAY ALONG THE ROUTE FROM MIAMI TO KEY
WEST...AND AT THE ISLAND TERMINALS.

SHOWERS WILL PROBABLY DEVELOP ALONG THE MAINLAND LEG OF THE ROUTE
THIS AFTERNOON. COVERAGE SHOULD REMAIN ISOLD-WDLY SCT. THUNDER WILL
BE POSSIBLE.

CELL MOTION WILL BE APPROX 25010KT WITH MOST TOPS AOB FL350 AND A
FEW MAX TOPS TO FL450.
&&

.PRELIMINARY POINT TEMPS/POPS...
KEY WEST  83  69  78  67 / 30 40 40 -
MARATHON  83  69  78  67 / 30 40 40 -

&&

.KEY WATCHES/WARNINGS/ADVISORIES...
FL...NONE.
GM...NONE.
&&

PUBLIC/MARINE.........12/KASPER
AVIATION/SHORT TERM...DFM

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE KEY WEST FL
910 AM EDT THU APR 5 2007

FLZ076-077-078-GMZ031-032-033-052-053-054-072-073-074-075-061000-
MONROE UPPER KEYS-MONROE MIDDLE KEYS-MONROE LOWER KEYS-
FLORIDA BAY-GULF SIDE OF THE LOWER KEYS OUT 20 NM-
GULF WATERS FROM EAST CAPE SABLE TO CHOKOLOSKEE 20 TO 60 NM OUT-
OCEAN REEF TO CRAIG KEY OUT 20 NM-
CRAIG KEY TO THE WEST END OF THE 7 MILE BRIDGE OUT 20 NM-
WEST END OF THE 7 MILE BRIDGE TO KEY WEST OUT 20 NM-
OCEAN REEF TO CRAIG KEY 20 TO 60 NM OUT-
CRAIG KEY TO THE WEST END OF THE 7 MILE BRIDGE 20 TO 60 NM OUT-
WEST END OF THE 7 MILE BRIDGE TO KEY WEST 20 TO 60 NM OUT-
KEY WEST TO 20 NM WEST OF DRY TORTUGAS OUT 60 NM...INCLUDING THE
FLORIDA KEYS NATIONAL MARINE SANCTUARY-
910 AM EDT THU APR 5 2007

...HIGH RISK OF WATERSPOUTS TODAY...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE FLORIDA KEYS AND
ADJACENT COASTAL WATERS.

.DAY ONE...TODAY AND TONIGHT

AMPLE LOW LEVEL ATMOSPHERIC MOISTURE COUPLED WITH A LIGHT AND
VARIABLE WIND PROFILE WILL ALLOW CLOUD LINES TO FORM ALONG AND NEAR
THE ISLAND CHAIN TODAY. WATERSPOUTS ARE POSSIBLE WITHIN THESE CLOUD
LINES.

WATERSPOUTS TEND TO FORM UNDER LARGE...TOWERING CUMULUS CLOUDS WITH
DARK...FLAT BASES. WINDS IN AND NEAR WATERSPOUTS CAN EASILY OVERTURN
SMALL BOATS. IF A WATERSPOUT IS SIGHTED...MOVE AWAY FROM IT AS
QUICKLY AND SAFELY AS POSSIBLE. REPORT WATERSPOUT SIGHTINGS DIRECTLY
TO THE NATIONAL WEATHER SERVICE BY CALLING 3 0 5...2 9 5...1 3 1
6...EXTENSION 3.

THERE IS A SLIGHT CHANCE OF THUNDERSTORMS TONIGHT.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

THERE IS A SLIGHT CHANCE OF THUNDERSTORMS FRIDAY.

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME FRIDAY NIGHT THROUGH
WEDNESDAY.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NEEDED TODAY.

BS