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000 FXUS62 KMFL 201904 AFDMFL AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MIAMI FL 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 76 88 77 88 / 30 50 30 50 FORT LAUDERDALE 79 89 80 89 / 30 40 30 30 MIAMI 77 89 78 89 / 30 40 30 30 NAPLES 74 91 75 91 / 20 40 30 40 && .MFL WATCHES/WARNINGS/ADVISORIES... FL...NONE. AM...NONE. GM...NONE. && $$ LONG TERM...70/DD SHORT TERM/AVIATION...30/KOB