Area Forecast Discussion
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FXUS62 KKEY 210829
AFDKEY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE KEY WEST FL
429 AM EDT TUE APR 21 2015

.DISCUSSION...
A 33-YEAR OLD WARM MINIMUM TEMPERATURE RECORD WAS BROKEN YESTERDAY AT
KEY WEST...WITH THE AIR TEMPERATURE ONLY DROPPING TO 80F. ANOTHER
1982 WARM MINIMUM TEMPERATURE RECORD MAY DROP TODAY IF THE AIR
TEMPERATURE REMAINS ABOVE 79F THROUGH MIDNIGHT EST. CURRENTLY...IT IS
81F AT KEY WEST INTERNATIONAL AIRPORT...82F AT MARATHON FLORIDA KEYS
AIRPORT...AND 80F HERE AT THE WEATHER OFFICE IN KEY WEST. DEWPOINT
TEMPERATURES REMAIN HIGH AT THIS HOUR AS WELL...IN THE MID 70S. KEY
WEST DOPPLER RADAR SCANS HAVE BEEN MOSTLY VOID OF SIGNIFICANT
PRECIPITATION ACROSS THE SERVICE AREA DURING THE LAST FEW HOURS. A
CLUSTER OF SHOWERS AND THUNDERSTORMS THAT DEVELOPED BETWEEN CUBA AND
THE MARQUESAS KEYS LATE MONDAY EVENING DISSIPATED AS IT MOVED
NORTHEASTWARD. HOWEVER...A FEW PATCHES OF LIGHT RAIN WERE ABLE TO
MAKE IT TO KEY WEST AND VICINITY...WITH LITTLE ACCUMULATION.

THE LARGE-SCALE WEATHER PATTERN FEATURES A BROAD REGION OF FAST MID-
LEVEL FLOW EXTENDING OVER MUCH OF THE EASTERN UNITED STATES AND GULF
OF MEXICO...SOUTH OF A DEEP CYCLONIC GYRE CENTERED OVER THE GREAT
LAKES. WEAK IMPULSES HAVE BEEN EMBEDDED WITHIN THIS FLOW AS FAR SOUTH
AS THE SOUTHERN GULF OF MEXICO...PERIODICALLY DESTABILIZING
TROPOSPHERIC COLUMNS OVER THE FLORIDA PENINSULA. HOWEVER...MOST
SIGNIFICANT LARGE-SCALE ASCENT FIELDS HAVE REMAINED NORTH OF THE
FLORIDA KEYS. A NEARLY MONTH-LONG PERIOD OF ENTRENCHED MARITIME
TROPICAL AIR ACROSS THE FLORIDA KEYS HAS ALLOWED LOCAL SEA SURFACE
TEMPERATURES TO RISE ABOVE SEASONAL AVERAGES...WITH TEMPERATURES
AVERAGING NEAR 85F ACROSS THE SHALLOW NEARSHORE WATERS...AND EVEN 80F
ALONG THE FLORIDA REEF TRACT AND STRAITS OF FLORIDA. THIS COMBINED
WITH EVER-INCREASING APRIL SUN ANGLES HAS FACILITATED ABOVE-AVERAGE
BOUNDARY-LAYER EQUIVALENT POTENTIAL TEMPERATURE VALUES...YIELDING
PERSISTENT MODERATE LEVELS OF CONDITIONAL INSTABILITY. ANY ADDITIONAL
DESTABILIZATION FROM LARGE-SCALE FORCING MECHANISMS WILL EASILY PRIME
THE ATMOSPHERE FOR THUNDERSTORM DEVELOPMENT GIVEN SUFFICIENT SURFACE
LIFTING. A FEW EPISODES OF WEAK LARGE-SCALE LIFT ARE PLAUSIBLE BOTH
TODAY AND WEDNESDAY...ALONG WITH SURFACE CONVERGENCE/LIFTING ALONG
SO-CALLED REVERSE CUMULUS LINES...THAT IS...THOSE WHICH STREAM WEST-
SOUTHWEST TO EAST-NORTHEAST OFF THE LOWER FLORIDA KEYS...CONTRARY TO
THE MORE FREQUENT CLIMATOLOGICAL EASTERLY FLOW REGIMES. THIS PLACES
THE GREATEST SHOWER...THUNDERSTORM...AND WATERSPOUT THREAT FROM THE
CONTENT KEYS TO FLORIDA BAY...NORTH OF THE SEVEN MILE BRIDGE AND
MIDDLE KEYS...AND WE HAVE INDICATED THUS IN OUR DIGITAL FORECAST
DATABASE. AIR TEMPERATURES ONCE AGAIN SHOULD RISE INTO THE 85-90F
RANGE IN MOST KEYS ISLAND COMMUNITIES. A WEAK PRESSURE PATTERN WILL
ALLOW PENINSULA-SCALE AND OTHER MESOSCALE DIFFERENTIAL HEATING
CIRCULATIONS TO DOMINATE LOCAL BREEZES TODAY THROUGH WEDNESDAY...AND
LIKELY BEYOND.

THE LARGE-SCALE SUBTROPICAL RIDGE SHOULD BUILD SLOWLY NORTHWARD OVER
THE GULF BASIN AND FLORIDA PENINSULA TONIGHT THROUGH FRIDAY...
ALLOWING LOWER/MID-TROPOSPHERIC FLOW TO VEER WEST-NORTHWESTERLY.
LARGE-SCALE SUBSIDENCE SHOULD HELP DRY THE ATMOSPHERE OUT A BIT...
AND THE BEST RAIN CHANCES MAY SHIFT TO UPPER KEYS COMMUNITIES BY
THURSDAY...SINCE THEY LIKELY WILL BE DOWNWIND OF THE GREATER SHOWER
COVERAGE OVER SOUTHERN MIAMI-DADE COUNTY. THE FORTIFIED RIDGE WILL BE
SHORT-LIVED...HOWEVER...AND SKY COVER AND RAIN CHANCES SHOULD SPIKE
AGAIN BY SUNDAY OR MONDAY.
&&

.MARINE...
A WEAK HIGH PRESSURE CELL WILL MOVE FROM THE LOWER MISSISSIPPI RIVER
VALLEY TO THE WESTERN NORTH ATLANTIC...TODAY THROUGH THURSDAY.
THEREAFTER...A WEAK PRESSURE PATTERN IS EXPECTED TO PERSIST ACROSS
THE FLORIDA KEYS AND ADJACENT COASTAL WATERS...FROM THURSDAY NIGHT
THROUGH SATURDAY NIGHT.
&&

.AVIATION...
A FEW SHORT PERIODS OF MVFR CEILINGS FROM FL015-020 ARE STILL
POSSIBLE PRIOR TO SUNRISE AT EYW AND MTH...OTHERWISE VFR TO PREVAIL.
SOME CUMULUS/TOWERING CUMULUS BUILDUPS ARE EXPECTED OVER THE LOWER
KEYS...GENERALLY EAST/NORTHEAST OF EYW AND IN THE NORTHERN VICINITY
OF MTH. SOME VCSH WAS INSERTED FOR MTH ALTHOUGH SIGNIFICANT IMPACTS
ARE NOT EXPECTED AT THIS TIME. LIGHT SOUTH TO SOUTHWEST WINDS EARLY
THIS MORNING ARE EXPECTED TO BECOME WEST THIS AFTERNOON ABOUT 4 TO 7
KNOTS. LIGHT NORTHEAST TO EAST WINDS ARE EXPECTED TO RESUME LATE
TUESDAY EVENING.
&&

.CLIMATE...
THE LOW TEMPERATURE AT THE KEY WEST INTERNATIONAL AIRPORT ON MONDAY
APRIL 20TH WAS 80 DEGREES. THIS BROKE THE PREVIOUS DAILY RECORD FOR
WARM MINIMUM TEMPERATURE OF 79 DEGREES OBSERVED IN 1982.

ON THIS DATE IN FLORIDA KEYS WEATHER HISTORY...APRIL 21...ANOTHER
WARM MINIMUM TEMPERATURE RECORD OF 79 DEGREES...ALSO SET IN 1982...
COULD BE TIED OR BROKEN UNLESS ANY RAINFALL AT THE KEY WEST
INTERNATIONAL AIRPORT TODAY BRINGS TEMPORARILY COOLER TEMPERATURES.
&&

.PRELIMINARY POINT TEMPS/POPS...
KEY WEST  86  78  85  79 / 30 30 30 20
MARATHON  90  77  89  79 / 30 30 30 20
&&

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

$$

PUBLIC/MARINE.........KASPER
AVIATION/NOWCASTS.....RIZZO
DATA COLLECTION.......RIZZO

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