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HURRICANE ISIDORE DISCUSSION NUMBER  23
NATIONAL WEATHER SERVICE MIAMI FL
5 PM EDT SAT SEP 21 2002
 
AIR FORCE RECON DATA INDICATE MAJOR HURRICANE ISIDORE HAS CONTINUED 
TO RAPIDLY STRENGTHEN AND THE CENTRAL PRESSURE HAS DROPPED 21 MB IN 
THE PAST 13 HOURS. A PRESSURE OF 946 MB GENERALLY CORRESPONDS TO A 
MAXIMUM WIND OF ABOUT 117 KT. HOWEVER...THE EYE IS OPEN TO THE WEST 
AND THERE MAY BE A LAG IN THE WIND FIELD. ALSO...DROPSONDE DATA 
INDICATED ABOUT 105 KT SURFACE WINDS...BUT WINDS JUST A FEW HUNDRED 
ABOVE THE SURFACE HAVE BEEN AS HIGH AS 130 KT. THE THREE SATELLITE 
AGENCIES REPORTED A DVORAK SATELLITE INTENSITY ESTIMATE OF T6.0...OR 
115 KT...WHILE THE 3-HOUR OBJECTIVE DVORAK T-NUMBER WAS ALSO T6.0. 
BASED ON THIS INFORMATION...THE INITIAL INTENSITY WAS INCREASED TO 
110 KT.
 
THE INITIAL MOTION ESTIMATE IS 270/3.  RECON FIX POSITONS SINCE
ABOUT 12Z INDICATE ISIDORE HAS ACTUALLY REMAINED NEARLY STATIONARY 
THE PAST 6 HOURS...BUT RIGHT OVER SOME OF THE HOTTEST WATER IN THE 
ATLANTIC BASIN. STEERING CURRENTS REMAIN WEAK AND ARE FORECAST BY 
ALL OF THE GLOBAL TO REMAIN WEAK OR EVEN GET WEAKER. ISIDORE REMAINS 
CAUGHT BETWEEN A MID- TO UPPER-LEVEL LOW EAST OF FLORIDA AND ONE TO 
THE WEST OVER THE BAY OF CAMPECHE. A WEAK AND NARROW LOW- TO 
MID-LEVEL RIDGE EXTENDS ACROSS THE NORTHERN GULF OF MEXICO COAST 
FROM FLORIDA WESTWARD TO TEXAS. GIVEN THE RELATIVE WEAKNESS OF THIS 
RIDGE...ONLY 5880 METERS AT 500 MB...IT WOULD NOT TAKE MUCH OF A 
SHORTWAVE TROUGH TO ERODE IT AND ALLOW ISIDORE TO DRIFT SLOWLY 
POLEWARD. ALL OF THE NHC MODEL GUIDANCE...EXCEPT LBAR...KEEPS 
ISIDORE MOVING ON A SLOW WEST OR WEST-SOUTHWEST TRACK THROUGH THE 
FORECAST PERIOD. THE AVN AND GFDL ARE VERY SIMILAR IN HOOKING 
ISIDORE TO THE SOUTHWEST AROUND THE WESTERN YUCATAN PENINSULA IN 36 
TO 48 HOURS. THIS MAY BE SOME ARTIFACT OF TERRAIN INTERACTION WHICH 
I HAVE IGNORED FOR THIS ADVISORY PACKAGE SINCE ALL OF THE GLOBAL 
MODELS SHOW NO SIGNIFICANT RIDGING NORTH OF ISIODRE TO PUSH THE 
SYSTEM TO SOUTHWEST. THE FORECAST CALLS FOR A SLOW...LESS THAN 6 
KT...MOTION THROUGHOUT THE FORECAST PERIOD WITH A SLIGHT 
WEST-NORTHWESTWARD MOTION AFTER 48 HOURS AS A STRONG SHORTWAVE 
TROUGH DROPS DOWN THE WEST SIDE OF THE BROAD LONGWAVE TROUGH...WHICH 
IS EXPECTED TO ERODE THE WEAK RIDGE OVER TEXAS AND THE NORTHWEST 
GULF OF MEXICO AND ALLOW ISIDORE TO GAIN SOME LATITUDE. HOWEVER... 
THE SLOWER ISIDORE MOVES DURING THE NEXT 36 TO 48 HOURS WILL 
DETERMINE JUST HOW FAR NORTH AND THE CYCLONE WILL MOVE IN THE LONGER 
TIME PERIODS BEYOND 72 HOURS. THE GOOD NEWS IS THAT IT APPEARS THAT 
ISIDORE IS NOT GOING TO GO ANYWHERE FAST. THE BAD NEWS IS THAT IT 
WILL REMAIN OVER VERY HOT WATER.
 
THE CENTRAL PRESSURE HAS DROPPED 21 MB IN THE PAST 13 HOURS. A 
TYPICAL RAPID INTENSIFICATION PERIOD. THIS TREND WOULD NORMALLY 
CONTINUE FOR ANOTHER 12 HOURS OR SO...BUT RADAR AND RECON DATA 
INDICATE THAT THE EYE IS OPEN TO THE WEST AND THAT AN EYEWALL 
REPLACEMENT CYCLE MAY BE STARTING. AS SUCH...THE OFFICIAL INTENSITY 
FORECAST IS HELD BELOW THE SHIPS INTENSITY MODEL WHICH BRINGS 
ISIDORE TO 135 KT IN 36 HOURS AND 140 KT IN 60 HOURS. THIS TYPE OF 
INTENSIFICATION IS CERTAINLY POSSIBLE GIVEN THE LOW SHEAR...LESS 
THAN 6 KT...AND HIGH SSTS...ABOUT 30C/86F. HOWEVER...PREDICTING 
INTERNAL CONVECTIVE CHANGES IS NEALRY IMPOSSIBLE SO THERE COULD BE 
FLUCUATIONS BY AS MUCH 10 KT EITHER WAY FROM THE OFFICAL INTENSITY 
FORECAST. SOME COASTAL UPWELLING MAY WEAKEN THE HURRICANE SLIGHTLY 
AS IT MOVES WEST OF 88W LONGITUDE...BUT THEN SOME RESTRENGTNENING IS 
FORECAST AFTER 48 HOURS WHEN ISIDORE IS FORECAST TO MOVE BACK OVER 
WARMER WATER.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     21/2100Z 21.9N  86.2W   110 KTS
12HR VT     22/0600Z 21.9N  87.0W   120 KTS
24HR VT     22/1800Z 21.9N  88.0W   125 KTS
36HR VT     23/0600Z 21.9N  89.2W   125 KTS
48HR VT     23/1800Z 22.0N  90.3W   125 KTS
72HR VT     24/1800Z 22.5N  92.5W   130 KTS
 
 
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