FXUS62 KKEY 200750 AFDKEY AREA FORECAST DISCUSSION FOR THE FLORIDA KEYS NATIONAL WEATHER SERVICE KEY WEST FL 350 AM EDT SAT SEP 20 2008 .DISCUSSION... .CURRENTLY... THE RADAR SCOPE IS CONSIDERABLY QUIETER TONIGHT THAN IT WAS 24 HOURS AGO...WITH ONLY ISOLATED SHOWERS OVER THE FLORIDA STRAITS...AND NO PRECIPITATION NOTED ACROSS THE REMAINDER OF THE SERVICE AREA. SKIES ARE MOSTLY CLEAR ALONG THE ISLAND CHAIN...WITH TEMPERATURES AROUND 80 DEGREES. WINDS ARE FROM THE NORTHEAST TO EAST AT 5 TO 10 KNOTS ON LAND...AND 10 TO 15 KNOTS ACROSS THE WATERS. SURFACE ANALYSIS DEPICTS A WEAK LOW PRESSURE TROUGH (THE REMNANTS OF AN OLD FRONTAL BOUNDARY) STRETCHING FROM THE NORTHERN BAHAMAS NORTHEASTWARD ACROSS THE WESTERN ATLANTIC OCEAN...WITH A SPRAWLING HIGH PRESSURE AREA EXTENDING DOWN THE ENTIRE EASTERN SEABOARD AND INTO NORTH/CENTRAL FLORIDA. A WEAK TROPICAL WAVE IS NOTED ALONG 75W... BUT EFFECTS FROM THIS FEATURE ARE CONFINED SOUTH OF CUBA. AT MID AND UPPER LEVELS...A NARROW TROUGH AXIS IS LOCATED OVER THE WESTERN BAHAMAS...WHILE WEAK RIDGING PREVAILS ACROSS THE GULF OF MEXICO. FLORIDA LIES WITHIN WEAKLY ANTICYCLONIC FLOW ALOFT BETWEEN THESE FEATURES. .SHORT TERM (TODAY THROUGH MONDAY)... THE LARGE SCALE PATTERN WILL BE RELATIVELY UNFAVORABLE FOR CONVECTION ACROSS THE KEYS TODAY...WITH ANTICYCLONIC FLOW PREVAILING AT ALL LEVELS AND NO SIGNIFICANT FORCING MECHANISM NOTED. ON THE SMALLER SCALE... THE KEY FORECAST QUESTION FOR TODAY WILL BE HOW MUCH THE MID LEVEL DRY AIR EVIDENT JUST TO OUR NORTH ON MIMIC-TPW SATELLITE LOOPS AND FRIDAY EVENING RAOBS MAKES IT DOWN INTO THE KEYS. MODEL SOUNDINGS AND TIME HEIGHT CROSS SECTIONS INDICATE THAT A VERY DRY AND SOMEWHAT MORE STABLE MID LEVEL AIRMASS WILL MOVE INTO OUR AREA TODAY...WITH DEWPOINT DEPRESSIONS IN THE 700 TO 500 MB LAYER RANGING FROM 15-25C...AND SLIGHT WARMING DEPICTED IN THIS LAYER AS WELL. THIS DRY/STABLE LAYER WAS ALREADY PRESENT IN THE FRIDAY EVENING MIAMI SOUNDING. MOS POPS RESPOND AND ARE ONLY IN THE 15 TO 20 PERCENT RANGE TODAY AND TONIGHT. UNFORTUNATELY THE GEIGER KEY IPW SENSOR IS NOT OPERATING AT PRESENT...SO WE WILL NOT HAVE UPDATED DATA ON OUR ATMOSPHERIC MOISTURE PROFILE LOCALLY UNTIL THE 12Z KEY WEST SOUNDING. THE LOW LEVEL WIND PROFILE MAY BE MARGINALLY FAVORABLE FOR CLOUD LINE DEVELOPMENT TODAY...ALTHOUGH MODEL SOUNDINGS SUGGEST THAT WINDS WILL PROBABLY BE A BIT TOO MUCH FROM THE EAST TO HOLD THE CLOUD LINE ON THE KEYS. THIS EASTERLY STEERING FLOW SHOULD ALSO TAKE ANY MAINLAND CONVECTION THIS AFTERNOON ALMOST STRAIGHT WESTWARD AND KEEP IT MOSTLY NORTH OF OUR AREA. WILL LOWER POPS TO SLIGHT CHANCE (20 PERCENT) FOR TODAY AND TONIGHT...BANKING ON THE FORECASTED DRY/STABLE MID LEVELS. IF THE MORNING SOUNDING DEPICTS OTHERWISE...POPS MAY NEED TO BE RAISED LATER ON TODAY. SUNDAY THROUGH MONDAY...THE OLD TROUGH TO OUR EAST WASHES OUT... WHILE A NEW TROUGH DEVELOPS ACROSS THE NORTHERN GULF OF MEXICO. THE SUBTROPICAL RIDGE WILL BUILD IN TEMPORARILY FROM THE EAST DURING THIS TIME. GENTLE TO MODERATE EASTERLY FLOW IS FORECAST TO CONTINUE IN THE LOW LEVELS...ALONG WITH A SLOW MOISTENING OF THE MID LEVELS. MODEL SOUNDINGS SHOW A PERSISTENT STABLE LAYER IN THE MID LEVELS DURING THIS TIME...WHICH MAKES SENSE GIVEN THE INCREASED INFLUENCE OF THE ATLANTIC RIDGE. THERE IS SOME HINT THAT THE LOW LEVEL REMNANTS OF THE BAHAMAS TROUGH MAY SLIDE WESTWARD INTO SOUTH FLORIDA ON MONDAY... BRINGING A MODEST INCREASE IN MOISTURE AND LOW LEVEL CONVERGENCE. WILL CONTINUE WITH NEAR CLIMO POPS OF 30 PERCENT FOR THESE PERIODS AT THIS POINT...ALTHOUGH THEY COULD END UP BEING LOWER FOR SUNDAY AND HIGHER FOR MONDAY IF THE AFOREMENTIONED FEATURES DEVELOP AS ADVERTISED. .LONG TERM (MONDAY NIGHT THROUGH FRIDAY)... NO CHANGE TO THE FORECAST THINKING FOR THE LONG TERM PERIOD. THE LATEST GLOBAL MODELS CONTINUE TO DEVELOP A SIGNIFICANT MID AND UPPER LEVEL TROUGH NEAR THE MID ATLANTIC REGION BY THE MIDDLE OF NEXT WEEK...WHICH MAY EVENTUALLY INDUCE CYCLOGENESIS SOMEWHERE OFF THE SOUTHEASTERN U.S. COAST. THIS LARGE SCALE REGIME WILL RESULT IN INCREASING WESTERLY FLOW AT UPPER LEVELS ACROSS THE AREA...WHILE AT LOW LEVELS MODERATE EASTERLY FLOW CONTINUES ALONG THE SOUTHERN PERIPHERY OF A VERY LARGE AND PERSISTENT RIDGE ALONG THE EASTERN SEABOARD. WITH MOISTURE LEVELS FORECAST TO HOVER NEAR NORMAL FOR LATE SEPTEMBER...AND A LACK OF SUPPRESSING MID LEVEL SUBSIDENCE DUE TO THE ABSENCE OF A DOMINANT ATLANTIC SUBTROPICAL RIDGE...WE SHOULD HAVE AMPLE OPPORTUNITY FOR MESOSCALE EFFECTS SUCH AS CLOUD LINES... OUTFLOW BOUNDARIES AND MAINLAND INTERACTIONS TO YIELD PRECIP ACROSS THE KEYS. HAVE MAINTAINED NEAR CLIMO POPS AND TEMPS THROUGHOUT THE LONG TERM PERIOD. && .MARINE... EASTERLY FLOW HAS DECREASED ACROSS MOST OF THE KEYS COASTAL WATERS DURING THE OVERNIGHT HOURS...AND IS NOW RUNNING IN THE 10 TO 15 KNOT RANGE. EXPECT SIMILAR CONDITIONS WITH GENTLE TO OCCASIONALLY MODERATE NORTHEAST TO EAST FLOW TO PREVAIL THROUGH THE WEEKEND AND INTO NEXT WEEK. NO ADVISORIES OR CAUTIONARY STATEMENTS ARE EXPECTED FOR TODAY THROUGH AT LEAST NEXT WEDNESDAY. && .AVIATION... VFR CONDITIONS WILL PREVAIL AT THE KEYS ISLAND TERMINALS. THERE IS A SLIGHT CHANCE THAT AN ISOLATED SHOWER MAY IMPACT THE ISLAND TERMINALS BEFORE 14Z. DUE TO TIMING ISSUES AND THE LOW POSSIBILITY OF OCCURRENCE...MENTION OF SHOWERS IN THE TAF HAS BEEN OMITTED. SURFACE WINDS WILL BE BETWEEN 8 AND 12 KNOTS FROM 070 TO 090 DEGREES. BRIEF MVFR/IFR CEILINGS AND VISIBILITIES ARE POSSIBLE WITH ANY SHOWERS THAT PASS NEAR THE TERMINALS. && .PRELIMINARY POINT TEMPS/POPS... KEY WEST 88 79 88 79 / 20 20 30 30 MARATHON 89 79 89 79 / 20 20 30 30 && .KEY WATCHES/WARNINGS/ADVISORIES... FL...NONE. GM...NONE. && $$ PUBLIC/MARINE.........JACOBSON AVIATION/NOWCASTS.....BWC VISIT US ON THE WEB AT WWW.WEATHER.GOV/KEYWEST