000
FXUS62 KKEY 210822
AFDKEY

AREA FORECAST DISCUSSION FOR THE FLORIDA KEYS
NATIONAL WEATHER SERVICE KEY WEST FL
422 AM EDT TUE MAY 21 2013

.DISCUSSION...
.CURRENTLY...
RADAR DETECTS SCATTERED SHOWERS ACROSS THE FLORIDA STRAITS...
THE UPPER KEYS AND FLORIDA BAY...WITH ISOLATED SHOWERS FURTHER WEST
ACROSS THE GULF WATERS. SKIES ARE PARTLY CLOUDY ACROSS THE AREA...WITH
TEMPERATURES ALONG THE KEYS ISLAND CHAIN HOVERING IN THE UPPER 70S TO
NEAR 80 DEGREES...AND WINDS ON LAND FROM THE EAST TO SOUTHEAST AT 5
TO 10 KNOTS.

SURFACE ANALYSIS DEPICTS A LARGE SUBTROPICAL HIGH PRESSURE AREA OVER
THE WESTERN ATLANTIC...EXTENDING WESTWARD ACROSS CENTRAL FLORIDA INTO
THE CENTRAL GULF OF MEXICO. ALOFT...THE PRIMARY FEATURE OF INTEREST
FOR OUR REGION IS A SMALL BUT WELL-DEFINED MID/UPPER LEVEL TROUGH
WHICH REMAINS NEARLY STATIONARY OVER THE FLORIDA PENINSULA. MIMIC-TPW
SATELLITE IMAGERY SHOWS THE SOUTH FLORIDA AND KEYS REGION ESCONSCED
WITHIN A MOIST AIRMASS...AND THE MONDAY EVENING KEY WEST SOUNDING
DEPICTS A CONDITIONALLY UNSTABLE PROFILE WITH VERY LITTLE CONVECTIVE
INHIBITION...AND PWAT OF 1.64 INCHES.

.SHORT TERM (TODAY THROUGH THURSDAY)...
THE PESKY MID/UPPER LEVEL TROUGH WILL REMAIN PARKED ACROSS THE
FLORIDA PENINSULA TODAY AND TONIGHT...THEN BEGIN TO EDGE SLOWLY
EASTWARD ON WEDNESDAY. AT THE SURFACE THE ATLANTIC RIDGE WILL HOLD
IN PLACE THROUGH TONIGHT...THEN WEAKEN AND SHIFT EASTWARD ON
WEDNESDAY AS A MID LATITUDE SHORTWAVE TROUGH AND ASSOCIATED COLD
FRONT ADVANCE INTO THE MISSISSIPPI VALLEY. THE TROUGH ALOFT SHOULD
WEAKEN AND SHIFT NORTHEASTWARD WEDNESDAY NIGHT AND THURSDAY...AS IT
BECOMES ENTRAINED INTO THE LARGER MID LATITUDE TROUGH. A VERY WEAK
SURFACE PRESSURE PATTERN WILL PREVAIL ACROSS THE KEYS DURING THIS
TIME AS THE FRONTAL BOUNDARY STALLS ACROSS THE NORTHERN GULF.

FOR THE FORECAST...SCATTERED SHOWER ACTIVITY ALONG VARIOUS BOUNDARIES
MAY PERSIST AT LEAST INTO THE EARLY MORNING HOURS TODAY...THEN THIS
AFTERNOON THERE WILL BE SOME POTENTIAL FOR A CLOUD LINE TO DEVELOP AS
MODEL SOUNDINGS SHOW A MARGINALLY FAVORABLE WIND PROFILE WITH DECENT
LOW LEVEL MOISTURE AND LAPSE RATES. OUTFLOWS FROM MAINLAND CONVECTION
MAY INVADE THE WATERS NORTH OF THE KEYS LATER THIS AFTERNOON...POSSIBLY
INTERACTING WITH A CLOUD LINE LATE THIS AFTERNOON INTO THIS EVENING.
WILL NUDGE POPS UP TO 40 PERCENT FOR TODAY...AND MAINTAIN 30 PERCENT
TONIGHT.

ON WEDNESDAY...A DRIER AIRMASS MAY BEGIN TO FILTER IN ALOFT AS THE
MID/UPPER TROUGH BEGINS TO EDGE SLOWLY EASTWARD...BUT MODEL SOUNDINGS
INDICATE DECENT LOW LEVEL MOISTURE REMAINING IN PLACE WITH THE
POTENTIAL FOR A REVERSE CLOUD LINE TO DEVELOP AS LOW/MID LEVEL FLOW
BECOMES VERY LIGHT AND MOSTLY FROM THE WEST/SOUTHWEST. WILL NUDGE POPS
UP TO 30 PERCENT.

WEDNESDAY NIGHT AND THURSDAY...A SOMEWHAT DRIER AIRMASS COMBINED WITH
A VERY WEAK FLOW REGIME...INCREASING STABILITY ALOFT...AND POSSIBLY
SOME DEVELOPING LOW LEVEL INHIBITION (AS SHOWN IN FORECAST
SOUNDINGS) SHOULD RESULT IN JUST ISOLATED CONVECTIVE COVERAGE. WILL
MAINTAIN 20 PERCENT POPS FOR THOSE PERIODS.

LITTLE CHANGE TO TEMPERATURES THROUGH THE PERIOD...WITH HIGHS MOSTLY
IN THE UPPER 80S...AND LOWS IN THE UPPER 70S TO NEAR 80.

&&

.LONG TERM (THURSDAY NIGHT THROUGH MONDAY)...
A DRIER AIRMASS WILL MOVE INTO THE KEYS AND SOUTH FLORIDA THURSDAY
NIGHT AND FRIDAY...AS THE AREA COMES UNDER NORTHWEST FLOW ALOFT DUE
TO A FAIRLY DEEP MID LATITUDE TROUGH ADVANCING TO THE EASTERN
SEABOARD. MODEL TIME-HEIGHT CROSS SECTIONS SHOW SIGNIFICANT DRY AIR
INVADING DOWN INTO THE LOWER/MID LEVELS...WITH DEWPOINT DEPRESSIONS
OF 15-20C BETWEEN 850 AND 700 MB DURING THIS TIME. THE EXISTING 20
PERCENT POPS WITH A THUNDER MENTION MAY BE A BIT GENEROUS FOR
THURSDAY NIGHT THROUGH FRIDAY NIGHT...BUT WILL LEAVE THEM UNCHANGED
FOR NOW.

AS WE HEAD INTO THE WEEKEND...HIGH PRESSURE BUILDING ACROSS THE
EASTERN CONUS WILL INDUCE INCREASING NORTHEAST/EAST FLOW ACROSS THE
KEYS. THE AIRMASS WILL REMAIN RATHER DRY INITIALLY...BUT LOW LEVEL
MOISTURE SHOULD INCREASE SATURDAY NIGHT AND BEYOND AS THE FLOW
BECOMES QUITE BRISK. SEE NO REASON TO ADJUST THE LATER FORECAST
PERIODS EITHER...WITH CHANCE POPS RETURNING SUNDAY THROUGH MEMORIAL
DAY DUE TO INCREASINGLY MOIST EASTERLY FLOW. NO SIGNIFICANT CHANGE TO
TEMPS.

&&

.MARINE...
OVERNIGHT OBSERVATIONS AND THE EDGE OF A NEARBY ASCAT PASS INDICATE
A MODEST EAST-SOUTHEAST WIND SURGE IN PROGRESS ACROSS THE KEYS
WATERS...WITH SPEEDS UP AROUND 15 KNOTS AT TIMES AT SOME OF THE
PLATFORMS. WINDS SHOULD DECREASE DURING THE MORNING HOURS...BUT WILL
PROBABLY SURGE A BIT ONCE AGAIN THIS EVENING. SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS WILL REMAIN A CONCERN AS WELL. A FEW
WATERSPOUTS WILL BE POSSIBLE THIS AFTERNOON AND AGAIN ON WEDNESDAY
NEAR SHORE AS THE LOW LEVEL WIND PROFILE SHOULD BE FAVORABLE. WINDS
SHOULD BECOME LIGHT FROM THE SOUTHEAST ON WEDNESDAY...AND A PERIOD OF
LIGHT/VARIABLE WINDS IS STILL EXPECTED FROM THURSDAY THROUGH FRIDAY.
NORTHEAST FLOW SHOULD BECOME ESTABLISHED ON SATURDAY...THEN INCREASE
SATURDAY NIGHT AND SUNDAY. NO HEADLINES ARE EXPECTED THROUGH
SATURDAY...BUT EXERCISE CAUTION OR EVEN A SMALL CRAFT ADVISORY MAY BE
NEEDED SATURDAY NIGHT AND BEYOND.

&&

.AVIATION...
THROUGH 22/06Z...THE SOUTHEASTERLY MEAN FLOW HAS FAVORED A NOCTURNAL
PEAK IN SHOWER ACTIVITY...WHICH SHOULD DWINDLE BY DAYBREAK. FORECAST
SOUNDINGS INDICATE THE DEVELOPMENT OF AN AFTERNOON CLOUD
LINE...MAINLY AFTER 18Z. ANY CONVECTIVE TRIGGERS ACTING ON THIS
CLOUDLINE COULD ENHANCE THE CHANCE FOR PRECIPITATION AT EITHER
TERMINAL...BUT FORECAST CONFIDENCE REMAINS POOR UNTIL THE 12Z
SOUNDING. EAST SOUTHEASTERLY SURFACE WINDS AT EYW AND MTH WILL
AVERAGE UNDER 10 KNOTS.

&&

.PRELIMINARY POINT TEMPS/POPS...
KEY WEST  87  78  87  77 / 40 30 30 20
MARATHON  90  78  90  77 / 40 30 30 20

&&

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

$$

PUBLIC/MARINE.........JACOBSON
AVIATION/NOWCASTS.....CLR

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