000
FXUS62 KKEY 071937
AFDKEY
AREA FORECAST DISCUSSION FOR THE FLORIDA KEYS
NATIONAL WEATHER SERVICE KEY WEST FL
337 PM EDT THU AUG 7 2008
.DISCUSSION...
.CURRENTLY...
A WELL DEFINED CLOUD LINE FORMED ALONG THE ENTIRE KEYS ISLAND CHAIN
FOR A TIME AROUND MIDDAY...BUT HAS ONLY MANAGED TO PRODUCE A FEW
BRIEF ROUNDS OF SHOWERS THUS FAR. AS OF MID AFTERNOON...THE CLOUD
LINE HAS DISSIPATED ACROSS THE UPPER KEYS BUT REMAINS ALONG THE
MIDDLE AND LOWER KEYS. AS NOTED PREVIOUSLY...SIGNIFICANT DRY AIR IN
THE 700-500 MB LAYER (DEWPOINT DEPRESSIONS OF 10-15C) IS LIKELY
BECOMING ENTRAINED WITHIN UPDRAFTS AND RESULTING IN THEIR DEMISE
WITHIN A RELATIVELY SHORT PERIOD OF TIME. RADAR SHOWS A LONE SHOWER
WITHIN THE LOWER KEYS CLOUD LINE...ISOLATED SHOWERS SOUTH OF THE
LOWER KEYS...ACROSS THE NORTHERN OFFSHORE GULF WATERS...AND OVER
SOUTHERN MIAMI-DADE COUNTY...WITH NO PRECIPITATION NOTED ELSEWHERE.
SKIES ALONG THE ISLAND CHAIN REMAIN PARTLY CLOUDY OVERALL...WITH
CONSIDERABLE SPATIAL/TEMPORAL VARIATIONS DUE TO THE CONTINUING
EVOLUTION OF THE CLOUD LINE. TEMPERATURES ARE NEAR 90 DEGREES. WINDS
ARE VARIABLE AND MOSTLY 5 KNOTS OR LESS ACROSS THE ENTIRE SERVICE
AREA.
SURFACE ANALYSIS DEPICTS A VERY WEAK ATLANTIC HIGH PRESSURE RIDGE
EAST OF FLORIDA...WITH ANOTHER PIECE OF THE RIDGE OVER THE EASTERN
AND CENTRAL GULF OF MEXICO...AND WEAK LOW PRESSURE TROUGHING OVER
THE FLORIDA PENINSULA. A WEAK LOW LEVEL CIRCULATION (ASSOCIATED WITH
THE NORTHERN END OF A TROPICAL WAVE) IS NOTED OVER THE NORTHERN
BAHAMAS. WATER VAPOR IMAGERY AND UPPER LEVEL ANALYSIS INDICATES A
BROAD AND DEEP TROUGH (ESPECIALLY FOR EARLY AUGUST) ACROSS THE
EASTERN UNITED STATES. THIS TROUGH IS DISPLACING THE WEAK ATLANTIC
RIDGE SOUTHWARD WITH TIME.
.SHORT TERM (TONIGHT THROUGH SATURDAY NIGHT)...
AT THE SYNOPTIC SCALE...THE DEEP TROUGH IN THE EASTERN STATES WILL
PERSIST THROUGH THE SHORT TERM PERIOD...WITH A SHORTWAVE MOVING FROM
THE OHIO VALLEY DOWN THROUGH THE SOUTHEASTERN STATES. THIS WILL DRIVE
A COLD FRONT INTO NORTH FLORIDA OVER THE WEEKEND...QUITE UNUSUAL FOR
AUGUST. THE ATLANTIC RIDGE WILL BE DISPLACED SOUTHWARD OVER THE KEYS
DURING THIS PERIOD. THIS WILL RESULT IN A VERY WEAK FLOW REGIME
ACROSS THE SERVICE AREA. PRECIPITABLE WATER LEVELS ARE FORECAST TO
HOVER BETWEEN 1.6 AND 1.8 INCHES...NEAR TO SLIGHTLY BELOW NORMAL.
HOWEVER...SIGNIFICANT DRY AIR IS FORECAST TO PERSIST IN THE MID
LEVELS...WHICH SHOULD GENERALLY ACT TO SUPPRESS CONVECTION.
FOR TONIGHT...WILL NEED TO WATCH FOR A LATE AFTERNOON OR EARLY
EVENING REVIVAL OF THE CLOUD LINE...ALTHOUGH WOULD EXPECT ANY
CONVECTION THAT DEVELOPS TO BE RATHER SHORT-LIVED. OTHERWISE...
THE WEAK LOW LEVEL CIRCULATION IN THE NORTHERN BAHAMAS APPEARS LIKELY
TO BE STRETCHED AND DRAWN NORTHWARD INTO THE DEVELOPING EAST COAST
TROUGH...AND SHOULD NOT HAVE ANY EFFECT ON OUR LOCAL WEATHER. HAVE
CONTINUED WITH SLIGHT CHANCE POPS FOR THE FORECAST.
FRIDAY THROUGH SATURDAY NIGHT...MODEL SOUNDINGS INDICATE LITTLE
CHANGE IN THE VERTICAL PROFILE...WITH MODERATE INSTABILITY AND LOW
LEVEL MOISTURE...WEAK FLOW THROUGH A DEEP LAYER...AND A MARKED DRY
LAYER IN THE MID LEVELS. MESOSCALE INTERACTIONS SUCH AS DIURNAL
EFFECTS...CLOUD LINES AND OUTFLOW BOUNDARIES WILL LARGELY CONTROL THE
DISTRIBUTION OF PRECIP. IF ANYTHING THE LOW LEVEL STEERING FLOW WILL
BE FROM THE SOUTHWEST TO WEST...WHICH MIGHT ALLOW FOR A REVERSE CLOUD
LINE BUT IS GENERALLY UNFAVORABLE FOR PRECIP IN THE KEYS. HAVE
MAINTAINED JUST SLIGHT CHANCE POPS THROUGH THE PERIOD...DUE TO THE
LIMITED MOISTURE AND WEAK FLOW REGIME.
.LONG TERM (SUNDAY THROUGH THURSDAY)...
GLOBAL MODELS SHOW LITTLE CHANGE TO THE LARGE SCALE PATTERN DURING
THE LONG TERM PERIOD...WITH SOUTH FLORIDA REMAINING NEAR THE BASE OF
A DEEP LONGWAVE TROUGH IN THE EASTERN UNITED STATES. THIS SHOULD
KEEP THE ATLANTIC RIDGE SUPPRESSED SOUTHWARD NEAR SOUTH FLORIDA OR
THE KEYS INTO NEXT WEEK. THIS PATTERN WOULD MAKE ANY INTRUSIONS OF
DEEP TROPICAL MOISTURE INTO OUR AREA QUITE UNLIKELY...AND INDEED THE
LONG RANGE GFS KEEPS PWATS GENERALLY BELOW NORMAL IN OUR AREA THROUGH
NEXT WEEK. DUE TO THE INHERENT UNCERTAINTIES IN THE LATER FORECAST
PERIODS...WILL CONTINUE WITH NEAR CLIMO POPS OF 30 PERCENT THROUGHOUT
THE LONG TERM PERIOD.
&&
.MARINE...
WINDS HAVE REMAINED VARIABLE AND MOSTLY 5 KNOTS OR LESS ACROSS THE
COASTAL WATERS TODAY. EXPECT SIMILAR CONDITIONS TO CONTINUE THROUGH
THE WEEKEND AS A VERY WEAK ATLANTIC SURFACE RIDGE SITS ALMOST
DIRECTLY OVER THE KEYS. THE GULF WATERS MAY BE JUST FAR ENOUGH NORTH
TO REMAIN WITHIN PREVAILING SOUTHWEST TO WEST FLOW...BUT IN ANY CASE
THE LARGER SCALE WINDS WILL BE SO WEAK THAT THEY CAN BE OVERWHELMED
BY SMALL SCALE EFFECTS SUCH AS OUTFLOW BOUNDARIES AND DIURNAL
VARIATIONS. THE RIDGE MAY START TO EASE SLOWLY BACK NORTHWARD INTO
SOUTH FLORIDA EARLY NEXT WEEK...ALLOWING FOR A RETURN OF LIGHT TO
GENTLE SOUTHEAST BREEZES ACROSS THE KEYS WATERS.
&&
.AVIATION...
VFR CONDITIONS WILL PREVAIL THIS EVENING AT THE KEY WEST AND
MARATHON TERMINALS. ANY SHOWERS THAT DEVELOP WILL HAVE SHORT
LIFETIMES AND SLOW ERRATIC MOTION...BOTH OF WHICH REDUCE
PREDICTABILITY. WE WILL CONTINUE WITH OUR "CLEAN" TAFS AND ISSUE A
TIMELY AMENDMENT IF THUNDER THREATENS.
&&
.PRELIMINARY POINT TEMPS/POPS...
KEY WEST 79 90 79 90 / 20 20 20 20
MARATHON 79 92 79 93 / 20 20 20 20
&&
.KEY WATCHES/WARNINGS/ADVISORIES...
FL...NONE.
GM...NONE.
&&
$$
PUBLIC/MARINE.........JACOBSON
AVIATION/NOWCASTS.....DFM
DATA COLLECTION.......PARKE
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