000
FXUS62 KMFL 211921
AFDMFL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MIAMI FL
321 PM EDT SUN SEP 21 2008

.DISCUSSION...
THE CURRENT WATER VAPOR SATELLITE ANIMATION CONTINUES TO INDICATE
AN INCREASING TREND IN MID AND HIGH-LEVEL MOISTURE ACROSS SOUTH
FLORIDA...WITH THIS CONFIRMED BY DATA FROM RECENT UPPER AIR
OBSERVATIONS ACROSS THE AREA. THUS...DESPITE WEAK MID-LEVEL
SUBSIDENCE PROVIDED BY RIDGE AXIS ALOFT...EXPECT AT LEAST WIDELY
SCATTERED SHOWERS/THUNDERSTORMS TO DEVELOP ALONG COASTAL
CONVERGENCE BOUNDARIES AND AFFECT MUCH OF THE CENTRAL/WESTERN
PORTIONS OF THE FORECAST AREA THIS AFTERNOON. SHORT TERM MODEL
SOLUTIONS INDICATE THAT MODERATE EASTERLY FLOW WILL PERSIST IN THE
BOUNDARY LAYER OVERNIGHT...NECESSITATING SLIGHT CHANCE POPS OVER
THE ATLANTIC WATERS AND ADJACENT PORTIONS OF THE MAINLAND.
HOWEVER...SUBSIDENT EFFECTS OF MID-LEVEL RIDGE WILL LIKELY LIMIT
COVERAGE COMPARED TO WHAT WAS SEEN SUNDAY MORNING...AS INDICATED
BY RUC/WRF-NMM MODEL SOLUTIONS.

THE GLOBAL MODELS UNANIMOUSLY SUGGEST THAT A LARGE MID-
TROPOSPHERIC HIGH OVER THE CENTRAL NORTH ATLANTIC WILL WEAKEN
AND SHIFT EASTWARD THROUGH THE REMAINDER OF THE SHORT TERM
PERIOD...AS A POTENT VORT MAX TRANSLATES THROUGH BASE OF LONG WAVE
TROUGH CURRENTLY POSITIONED OVER THE DAVIS STRAIT. THIS WILL
SLOWLY ERODE THE RIDGE CENTERED ACROSS SOUTH FLORIDA ON
MONDAY...AND THIS...WHEN COMBINED WITH GRADUALLY INCREASING DEEP-
LAYER MOISTURE...SHOULD RESULT IN A SLIGHTLY GREATER COVERAGE OF
AFTERNOON CONVECTION OVER THE MAINLAND. WILL STILL CARRY HIGHEST
POPS ALONG THE CORRIDOR FROM WEST PALM BEACH SOUTHWESTWARD TO
NAPLES...WITH LOWEST CHANCES ALONG THE IMMEDIATE SOUTHEAST COAST.
EXPECT THE COVERAGE OF NOCTURNAL SHOWERS AND THUNDERSTORMS TO
INCREASE OVER THE ATLANTIC WATERS ON MONDAY NIGHT...AS WELL.
HOWEVER...THE HIGHEST CHANCES FOR RAIN WILL LIKELY COME ON
TUESDAY...AS MODELS REMAIN VERY CONSISTENT IN FORECASTING A WEAK
UPPER LOW CURRENTLY CENTERED OVER WESTERN ILLINOIS TO DRIFT EAST-
SOUTHEASTWARD OVER THE NEXT 48 HOURS...IN RESPONSE TO RISING MID-
LEVEL HEIGHTS OVER THE SOUTHERN PLAINS. THIS FEATURE IS PROGGED TO
DRIFT SLOWLY SOUTHWARD OVER THE CAROLINAS ON TUESDAY...AND DEEPEN
INTO A CUTOFF LOW...AS A BLOCKING SHORTWAVE RIDGE AXIS BUILDS
ACROSS THE GREAT LAKES AND NORTHEAST U.S. AS THIS OCCURS...THE
COMBINATION OF STRENGTHENING MID-LEVEL DIVERGENCE...STEEPENING
LAPSE RATES...AND INCREASING DEEP-LAYER MOISTURE WILL LEAD TO A
VERY FAVORABLE ENVIRONMENT FOR SHOWERS/THUNDERSTORMS TO DEVELOP.
REGARDING TEMPERATURES...WILL MAINTAIN A PERSISTENCE FORECAST FOR
THE NEXT 48 HOURS...AND COOL BOTH MAXIMUM/MINIMUM VALUES BY A 1-3
DEGREES ON TUESDAY/TUESDAY NIGHT...WITH EXPECTATIONS OF A HIGHER
COVERAGE OF CLOUDS AND PRECIPITATION...AS WELL AS NORTHEAST FLOW
OFF THE ATLANTIC.

THE PRIMARY FEATURE OF INTEREST IN THE EXTENDED FORECAST WILL BE
THE CUTOFF LOW...WHICH MODELS INDICATE WILL REMAIN LOCATED
EITHER OVER COASTAL PORTIONS OF GEORGIA/S.C./N.C. OR JUST OFF
SHORE THROUGH THE END OF THE WEEK. THE INFLUENCE OF THIS FEATURE
ON THE WEATHER ACROSS SOUTH FLORIDA WILL BE DIRECTLY RELATED TO
HOW DEEP THE SYSTEM BECOMES...AND HOW CLOSE IT ACTUALLY COMES TO
THE LOCAL AREA. THE LATEST ECMWF MAINTAINS THE CORE OF THE CYCLONE
OVER WATER...ALLOWING IT TO INITIALLY BECOME SIGNIFICANTLY DEEPER
THAN THE GFS WOULD SUGGEST...AND DEEPENS THE SYSTEM EVEN FURTHER
AS ENERGY FROM A POTENT MID-LATITUDE TROUGH BEGINS TO PHASE WITH
IT LATE NEXT WEEKEND. AS A RESULT OF THIS...THE ECMWF ALSO
INDICATES A MUCH DRIER PATTERN FOR SOUTH FLORIDA...AND WITH THIS
AMOUNT OF UNCERTAINTY...WILL MAKE NO CHANGES TO THE EXTENDED
FORECAST AT THIS TIME.

&&

.MARINE...
THERE ARE NO MARINE WEATHER CONCERNS THROUGH THE FIRST 24-36 HOURS
OF THIS FORECAST...WITH EAST TO NORTHEAST WINDS EXPECTED TO
REMAIN IN THE 10-15 KNOT RANGE OVER THE SOUTHERN LEG OF THE
ATLANTIC WATERS...AND SLIGHTLY LOWER ACROSS THE REMAINDER OF THE
COASTAL ZONES. A SMALL RESIDUAL SWELL IN THE PALM BEACH COUNTY
WATERS WILL RESULT IN SLIGHTLY HIGHER COMBINED SEAS IN THIS
REGION. MARINE WEATHER CONDITIONS ACROSS THE ATLANTIC WILL LIKELY
DETERIORATE BY TUESDAY AFTERNOON...AS NORTHEAST FLOW INCREASES IN
RESPONSE TO DEVELOPING LOW PRESSURE OFF THE COAST OF NORTH/CENTRAL
FLORIDA. NORTH/NORTHEAST WINDS WILL LIKELY INCREASE INTO THE 15-20
KNOT RANGE...WITH SEAS IN THE GULF STREAM APPROACHING MARGINAL SCA
CRITERIA. WINDS/SEAS ARE FORECAST TO RELAX LATER IN THE WEEK...BUT
THE DEVELOPMENT OF A DEEPER CYCLONE OFF THE COAST OF THE CAROLINAS
WILL GENERATE NORTH/NORTHEAST SWELL WHICH WILL AFFECT THE ATLANTIC
WATERS BY NEXT WEEKEND.

&&

.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  77  88  76  87 / 20 50 40 60
FORT LAUDERDALE  82  90  79  88 / 20 40 40 50
MIAMI            80  90  79  88 / 20 40 40 50
NAPLES           75  90  74  87 / 30 50 40 60

&&

.MFL WATCHES/WARNINGS/ADVISORIES...
FL...NONE.
AM...NONE.
GM...NONE.

&&

$$

LONG TERM...70/DD
SHORT TERM/AVIATION...30/KOB