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000
FXUS62 KKEY 200633
AFDKEY

AREA FORECAST DISCUSSION FOR THE FLORIDA KEYS
NATIONAL WEATHER SERVICE KEY WEST FL
230 AM EDT MON MAY 20 2013

.DISCUSSION...IN THE MID AND UPPER LEVELS...LATEST AVAILABLE IR
SATELLITE IMAGERY OVERLAID WITH MODEL STREAMLINES AS OF 200 AM
DEPICT A ROBUST COLD CORE MID AND UPPER CYCLONE POSITIONED OVER THE
ROCKY MOUNTAINS AND PLAINS STATES. JUST DOWNSTREAM OF THIS
FEATURE...EXCELLENT UPPER DIVERGENCE IS INDICATED OVER
IOWA...ILLINOIS AND MISSOURI THIS MORNING. AND WELL SOUTH OF THIS
FEATURE...A NEARLY ZONAL APPROX 50 KNOT JET AT 250 MB ROUNDS THE
BASE OF THE GYRE ACROSS NORTHERN OLD MEXICO AND TEXAS...WITH A WEAK
UPPER TROUGH AXIS IDENTIFIED DOWNSTREAM IN WITHIN THIS FLOW...WHICH
IS SITUATED ACROSS THE EASTERN GULF OF MEXICO...AND JUST UPSTREAM OF
THE FLORIDA PENINSULA.

AT THE SURFACE...LATEST AVAILABLE TROPICAL SURFACE ANALYSIS COMBINED
WITH AVAILABLE LAND AND MARINE OBSERVATIONS INDICATE THE WESTERN
PERIPHERY OF A 1020 MB SURFACE RIDGE EXTENDS TO ACROSS CENTRAL
FLORIDA FROM THE WEST CENTRAL ATLANTIC OCEAN. LAST EVENINGS 00Z
SOUNDING REVEALED A GENTLE MOSTLY EASTERLY FLOW FROM THE SURFACE TO
ABOUT 800 MB...WITH A MODERATELY DRY LOWER TO MID TROPOSPHERE AND
COLUMNAR PWAT AT 1.2 INCHES.

.CURRENTLY...AS OF 200 AM...SKIES ARE PARTLY CLOUDY GIVEN A
COMBINATION OF SOME HIGH CIRRUS AND SOME SCATTERED LOW CUMULUS
CLOUDS. KEY WEST RADAR DETECTS THE LEADING EDGE OF SMALL LOW TOPPED
SHOWERS ADVANCING WESTWARD IN THE EASTERN STRAITS. TEMPERATURES
ACROSS THE ISLANDS REMAIN NEAR 80 DEGREES WITH DEWPOINTS NEAR 70
DEGREES. WINDS ALONG THE FLORIDA REEF ARE EAST NEAR 10 KNOTS...WITH
ISLAND SENSORS RECORDING EAST WINDS NEAR 10 MPH.

.SHORT TERM (TODAY THROUGH TUESDAY NIGHT)...SLIGHTLY LOWER HEIGHTS
(AT 500 MB) WILL BE ASSOCIATED WITH THE AFOREMENTIONED APPROACHING
WEAK REGIONAL SCALE MID-LEVEL TROUGH...(NOW OVER THE EASTERN GULF OF
MEXICO). THIS FEATURE IS PROGGED TO LINGER NEAR THE KEYS TODAY
THROUGH AT LEAST TOMORROW NIGHT. BOTH AVAILABLE 00Z NAM AND GFS
MODEL FORECAST SOUNDINGS DO ILLUSTRATE SOME MOISTENING OF THE COLUMN
IN THE LOW TO MID TROPOSPHERE...WITH PWAT (PRECIPITABLE WATER)
RISING TO ABOVE 1.50 INCHES BY 12Z TODAY THROUGH TOMORROW EVENING
(06Z WED).

BUT DESPITE THIS UPSWING IN THE MOISTURE...WEAK SURFACE/LOW LEVEL
RIDGING SHOULD REMAIN NORTH OF THE KEYS...WHICH USUALLY MAINTAINS A
GENTLE EAST TO SOUTHEASTERLY FLOW ACROSS THE ISLANDS AND IS
GENERALLY NOT FAVORABLE FOR SHOWERS. HOWEVER...THE LATEST AVAILABLE
00Z MODEL SOUNDINGS HINT AT THE POSSIBLE DEVELOPMENT OF A CLOUDLINE
DURING THE LATER AFTERNOON AND EVENING HOURS LATE. AND THIS IS TRUE
TO A LESSER EXTENT AGAIN DURING TUESDAY AND TUESDAY EVENING. SO
THEREFORE WILL CONTINUE TO ADVERTISE SLIGHTLY ABOVE NORMAL CHANCES
FOR RAIN...OR A LOW CHANCE...(30 PERCENT) FOR SHOWERS WITH ISOLATED
THUNDERSTORMS THIS AFTERNOON AND EVENING THROUGH TUESDAY NIGHT.

DURING WEDNESDAY AND WEDNESDAY NIGHT...MODELS NOW INDICATE GREATER
MID LEVEL DRYING...WITH 00Z NAM AND GFS MODEL FORECAST SOUNDINGS
ILLUSTRATING PWAT ONLY BETWEEN 1.25 AND 1.50 INCHES. ON THE OTHER
HAND...THE SURFACE RIDGE IS ALSO PROGGED TO MOVE NEAR THE
KEYS...WHICH RESULTS IN LIGHT AND VARIABLE WINDS IN THE LOWER LEVELS
AND THE SURFACE. AND GIVEN THE POSITION OF THIS RIDGING
NEARBY...THERE IS A HINT THAT A REVERSE CLOUD LINE MAY DEVELOP. SO
FOR NOW WILL HOLD WITH 12 HOUR POPS NEAR CLIMO...WHICH IS AT 20
PERCENT.

EXTENDED...THURSDAY THROUGH SUNDAY...A WEAKLY CYCLONIC/MORE ZONAL
MID AND UPPER FLOW PATTERN IS INDICATED IN THE AVAILABLE
MODELS...(WITH LIGHT WINDS EXPECTED UP TO AT LEAST 500 MB). AT THE
SURFACE...WINDS WILL REMAIN LIGHT...GIVEN THE POSITION OF THE RIDGE
AXIS NEAR THE KEYS. HOWEVER...AVAILABLE 00Z MODEL FORECAST SOUNDINGS
ILLUSTRATE COLUMNAR PWAT ONLY AROUND 1.25 INCHES THROUGH THE PERIOD.
GIVEN THIS SETUP...ERRANT SOUTH FLORIDA MAINLAND AND CUBAN
BOUNDARIES CAN STILL IMPACT THE KEYS. SO WILL HOLD WITH THE 20
PERCENT WE ALREADY HAVE IN THE GRIDS FOR NOW...SLIGHT CHANCE FOR
ISOLATED SHOWERS AND THUNDERSTORMS.

&&

.MARINE...EXPECT GENTLE TO OCCASIONALLY MODERATE EAST/SOUTHEAST
BREEZES TO CONTINUE TODAY THROUGH TUESDAY NIGHT. WEAK RIDGING IS
ALSO PROGGED TO BE NEAR THE KEYS WEDNESDAY THROUGH THE REST OF THE
WEEK...RESULTING IN VARIABLE WINDS LESS THAN 10 KNOTS. THIS WILL
RESULT IN GENERALLY EXCELLENT BOATING CONDITIONS. SOME SCATTERED
SHOWERS AND ISOLATED THUNDERSTORMS WILL BE POSSIBLE WITHIN CUMULUS
CLOUD LINES NEAR THE SHORE THIS AFTERNOON AND EVENING...AND AGAIN ON
TUESDAY...WITH ONLY ISOLATED SHOWER AND THUNDERSTORM ACTIVITY
EXPECTED THEREAFTER.

&&

.AVIATION...VFR CONDITIONS WILL PREVAIL AT THE KEY WEST AND MARATHON
TERMINALS THROUGH EARLY THIS AFTERNOON. SCATTERED SHOWERS WILL
LIKELY BE PRESENT OVER THE FLORIDA STRAITS...BUT CONFIDENCE IN THIS
ACTIVITY DIRECTLY IMPACTING THE TERMINALS IS TOO LOW TO MENTION IN
THE TAFS AT THIS POINT. FROM MID AFTERNOON THROUGH EARLY
EVENING...FORECAST SOUNDINGS SUGGEST THE POTENTIAL FOR A CLOUD LINE
TO DEVELOP OVER OR JUST TO THE NORTH OF THE KEYS ISLAND CHAIN. HAVE
INCLUDED A VCSH FOR BOTH TERMINALS AFTER 20Z TO ACCOUNT FOR THIS
POSSIBILITY...BUT THERE REMAINS MUCH UNCERTAINTY AS TO WHETHER
CONVECTION WILL DIRECTLY IMPACT EITHER TERMINAL. SURFACE WINDS WILL
REMAIN FROM THE EAST TO SOUTHEAST AT 5 TO 10 KNOTS.

&&

.CLIMATE...ON THIS DATE IN KEYS WEATHER HISTORY...MAY 20TH IN
1932...THE DAILY RECORD RAINFALL OF 3.83" WAS RECORDED.

&&

.PRELIMINARY POINT TEMPS/POPS...
KEY WEST  87 78 87 78 87 / 30 30 30 30 20
MARATHON  90 78 90 78 90 / 30 30 30 30 20

&&

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

&&

$$

PUBLIC/MARINE/FIRE/DIGITAL/CLIMATE...FUTTERMAN
AVIATION/NOWCASTS....................JACOBSON

VISIT US ON THE WEB AT WWW.WEATHER.GOV/KEYWEST