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000 FXUS62 KMFL 210524 AFDMFL AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MIAMI FL 124 AM EDT SUN SEP 21 2008 .AVIATION...MOSTLY VFR CONDITIONS CAN BE EXPECTED THROUGH THE PERIOD, HOWEVER A FEW SHOWERS WILL OCCUR ALONG THE EAST COAST THIS MORNING AND A FEW TSRA WILL OCCUR ALONG THE SEA BREEZE BOUNDARIES THIS AFTERNOON. WINDS WILL GENERALLY REMAIN EASTERLY AT AROUND 12 KTS OR LESS AT ALL TERMINALS EXCEPT FOR AT APF WHERE THE SEA BREEZE IS EXPECTED TO CREATE A WEST BREEZE DURING THE AFTERNOON. && .PREV DISCUSSION... /ISSUED 838 PM EDT SAT SEP 20 2008/ UPDATE... MOST OF THE SHOWERS AND THUNDERSTORMS THAT DEVELOPED OVER THE INTERIOR AREAS HAVE DISSIPATED EARLY THIS EVENING. THERE WAS STILL SOME SHOWERS AND THUNDERSTORMS OVER THE WEST COAST METRO AREAS OF SOUTH FLORIDA THIS EVENING ALONG THE WEST COAST SEA BREEZE...AND A FEW SHOWERS OVER THE ATLANTIC WATERS. THE SHOWERS AND THUNDERSTORMS OVER THE WEST COAST METRO AREAS SHOULD SLOWLY DISSIPATE THROUGH REST OF THE EVENING HOURS AND BE GONE BY MIDNIGHT TONIGHT...AS THE HEATING OF THE DAY IS LOST. THE FEW SHOWERS OVER THE ATLANTIC WATERS SHOULD CONTINUE THROUGH THE OVERNIGHT HOURS AND COULD MOVE INTO THE EAST COAST METRO AREAS OVERNIGHT...AS THE WINDS WILL REMAIN FROM THE EASTERLY DIRECTION FROM HIGH PRESSURE OVER THE SOUTHEAST UNITED STATES. SO WILL END THE SHOWER AND THUNDERSTORM ACTIVITY OVER THE INTERIOR AND WEST COAST METRO AREAS BY MIDNIGHT...AND LOWER THE EAST COAST POPS DOWN TO 20 PERCENT ALONG WITH REMOVING THE THUNDER WORDING. THE ONLY OTHER CHANGE IS TO ALSO END THE SHOWERS AND THUNDERSTORMS FOR LAKE OKEECHOBEE AND THE GULF WATERS AFTER MIDNIGHT...AND TO REMOVE THE THUNDER WORDING FOR THE ATLANTIC WATERS AND BISCAYNE BAY WATERS FOR TONIGHT. REST OF THE FORECAST LOOKS GOOD AT THIS TIME. UPDATE...54/BNB PREV UPDATE... /ISSUED 732 PM EDT SAT SEP 20 2008/ AVIATION...AFTERNOON ISOLD CONVECTION OVER CENTRAL COLLIER COUNTY SHOULD BEGIN TO DISSIPATE SOON AFTER SUNSET AND THUS IT SHOULD NOT HAVE A SIGNIFICANT IMPACT ON KAPF OPERATIONS. MOSTLY VFR CONDITIONS ARE EXPECTED FOR KAPF OVERNIGHT AS WELL WITH ONLY VCSH INDICATED AFTER 18Z. LIKEWISE, VFR CONDITIONS ARE EXPECTED FOR ALL THE ERN TERMINALS OVERNIGHT, EVEN THOUGH ONE OR TWO SHOWERS COULD MOVE CLOSE TO THE TERMINALS. DECIDED TO MENTION VCTS FOR ERN TERMINALS AFTER 18Z SUNDAY AS EXPECT SEA BREEZE AND DIURNAL HEATING TO INITIATE CONVECTION, BUT NO SIGNIFICANT IMPACT TO TERMINALS EXPECTED EITHER. PREV DISCUSSION... /ISSUED 304 PM EDT SAT SEP 20 2008/ DISCUSSION... WIDELY SCATTERED SHOWER ACTIVITY AFFECTING THE LOCAL ATLANTIC WATERS THIS AFTERNOON SHOULD PERSIST FOR A FEW MORE HOURS...WITH ISOLATED SHOWERS EXPECTED ACROSS THE REMAINDER OF THE FORECAST AREA. THE LATEST WATER VAPOR ANIMATION INDICATES A FAIRLY BROAD AREA OF DRY AIR ALOFT EMBEDDED WITHIN THE BASE OF A LOW-AMPLITUDE SHORTWAVE TROUGH LOCATED JUST OFF THE EAST COAST OF FLORIDA. THIS AIRMASS IS STILL IN PLACE ACROSS THE SOUTHEASTERN FLORIDA PENINSULA AND ADJACENT PORTIONS OF THE ATLANTIC...AND WAS SAMPLED VERY WELL IN THE 12Z KMFL RAOB...WITH PRECIPITABLE WATER SLIGHTLY BELOW 1.5 INCHES. MODELS AGREE THAT THIS MID-LEVEL SHORTWAVE TROUGH WILL GRADUALLY SHEAR APART OVER THE NEXT 24 HOURS...WITH ITS REMNANTS EXPECTED TO LIFT NORTHEASTWARD AROUND THE WESTERN PERIPHERY OF A STRENGTHENING MID-TROPOSPHERIC ANTICYCLONE OVER THE CENTRAL NORTH ATLANTIC. IN THE WAKE OF THIS FEATURE...A SUBSIDENT AIRMASS WILL SPREAD ACROSS SOUTH FLORIDA...COURTESY OF CENTRAL ATLANTIC RIDGE BUILDING OVERHEAD...AND THIS WILL RESULT IN A PERIOD OF SLIGHTLY BELOW CLIMATOLOGICALLY NORMAL POPS...EXTENDING THROUGH MUCH OF THE SHORT TERM PORTION OF THIS FORECAST. AN EAST TO NORTHEASTERLY COMPONENT TO FLOW IN THE BOUNDARY LAYER WILL PERSIST THROUGHOUT THE PERIOD...AND AT LEAST SCATTERED SHOWER ACTIVITY IS ANTICIPATED ACROSS THE ATLANTIC WATERS. ASSUMING NO LAND BREEZE DEVELOPMENT... IT SEEMS REASONABLE TO ASSUME THAT HIGHEST NOCTURNAL POPS WILL OCCUR ALONG THE IMMEDIATE EAST COAST AND ADJACENT WATERS...WITH ACTIVITY SPREADING INLAND DURING THE DAY. GUIDANCE FROM THE NGM/NAM...AS WELL AS FROM THE VARIOUS GLOBAL MODELS INDICATE THAT BACKGROUND FLOW WILL LIKELY BE LIGHT ENOUGH FOR THE FORMATION OF A GULF COAST SEA BREEZE...AND WILL PLACE SLIGHTLY HIGHER AFTERNOON POPS OVER THE WESTERN HALF OF THE PENINSULA. NO MAJOR CHANGES ARE ANTICIPATED IN THE TEMPERATURE FORECAST...WITH A ROUGHLY 10 DEGREE DIURNAL VARIATION EXPECTED ALONG THE EAST COAST...AND A SLIGHTLY LARGER SPREAD ALONG THE WEST COAST. MEDIUM RANGE GUIDANCE FROM BOTH THE GFS/ECMWF SUGGEST THAT A TRANSITION TO A WETTER PATTERN WILL OCCUR JUST BEYOND THE END OF THE SHORT TERM...AS A WEAK UPPER-LOW OVER THE MID-MISSISSIPPI VALLEY REGION...WHICH IS CURRENTLY CUT OFF FROM THE POLAR BRANCH OF THE WESTERLIES...BEGINS TO DRIFT EASTWARD IN RESPONSE TO RIDGING ALOFT OVER THE SOUTHERN PLAINS. THIS FEATURE WILL BEGIN TO INFLUENCE THE WEATHER ACROSS SOUTH FLORIDA BY MID-WEEK...WITH AN INCREASE IN DIURNAL SHOWER/THUNDERSTORM ACTIVITY FOR THE EASTERN HALF OF THE AREA...AS STEERING FLOW BACKS TO THE WEST/NORTHWEST. THE LATEST GFS SOLUTION IS SLOWLY COMING INTO BETTER AGREEMENT WITH THE ECMWF IN GRADUALLY DEEPENING THIS MID-LEVEL TROUGH INTO A CUTOFF LOW AS IT DRIFTS OVER WARM OCEAN WATERS OFF THE COAST OF THE CAROLINAS BY THE END OF THE WEEK...WITH THE POTENTIAL FOR SEVERAL WEAK SURFACE LOWS TO SPIN UP OFF THE SOUTHEAST COAST OF THE U.S. A PROLONGED PERIOD OF NORTHERLY FLOW WILL LIKELY BE THE RESULT FOR SOUTH FLORIDA...AND THIS PATTERN WILL CERTAINLY HAVE THE POTENTIAL TO BRING AT LEAST A BRIEF PERIOD OF SLIGHTLY COOLER/DRIER AIR TO SOUTH FLORIDA NEXT WEEKEND...BUT IT IS FAR TOO EARLY TO INDICATE SPECIFIC DETAILS IN THE FORECAST. MARINE... THERE ARE NO IMMINENT MARINE WEATHER CONCERNS...AS DATA FROM LOCAL WEB CAMS SUGGESTS THAT LONG PERIOD NORTH/NORTHEAST SWELL IN THE PALM BEACH WATERS IS LIKELY NOT MATERIALIZING AS EARLIER THOUGHT. GIVEN THIS...AND EXPECTATIONS OF A WEAKER SURFACE LOW OFF THE COAST OF NORTHEAST FLORIDA...HAVE DECIDED TO TRIM BACK SWELL HEIGHT...BUT DID NOT REMOVE ENTIRELY. ANY SWELL WILL ENHANCE COMBINED SEAS IN THE PALM BEACH WATERS...WITH 2-4 FOOT SEAS POSSIBLE THROUGH THE END OF THE WEEKEND. OTHERWISE...EXPECT EAST FLOW TO PERSIST THROUGH EARLY NEXT WEEK...WITH SPEEDS IN THE 10-15 KNOT RANGE OVER THE ATLANTIC WATERS/BISCAYNE BAY/LAKE OKEECHOBEE... AND 5-10 KNOTS OVER THE LOCAL GULF WATERS. PERHAPS A GREATER THREAT FOR LONG PERIOD SWELL WILL COME AT THE END OF THE WEEK...AS A DEEPER CYCLONE MAY DEVELOP OFF THE COAST OF THE CAROLINAS. FIRE WEATHER... THERE ARE NO FIRE WEATHER CONCERNS THROUGHOUT THE FORECAST PERIOD...AS MINIMUM AFTERNOON RH VALUES WILL REMAIN SUFFICIENTLY HIGH. && .PRELIMINARY POINT TEMPS/POPS... WEST PALM BEACH 88 77 88 77 / 50 30 50 30 FORT LAUDERDALE 89 80 89 80 / 40 30 30 30 MIAMI 89 78 89 78 / 40 30 30 30 NAPLES 91 75 91 75 / 40 30 40 30 && .MFL WATCHES/WARNINGS/ADVISORIES... FL...NONE. AM...NONE. GM...NONE. && $$ LONG TERM...04/AT SHORT TERM/AVIATION...04/AT