[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 300857
TCDAT3
TROPICAL STORM HANNA DISCUSSION NUMBER   9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
500 AM EDT SAT AUG 30 2008

THERE HAS BEEN NO SIGNIFICANT CHANGE IN THE CLOUD PATTERN OF HANNA.
THE CENTER CONTINUES TO RUN AWAY FROM THE CONVECTION AND NEW
THUNDERSTORM ACTIVITY REDEVELOPS OVER IT. THIS MEANS THAT THE SHEAR
HAS NOT RELAXED YET. A NOAA BUOY...41043...HAS BEEN EXTREMELY USEFUL
IN TRACKING THE CYCLONE.  IT HAS DEPICTED THE WELL-DEFINED SURFACE
CIRCULATION ASSOCIATED WITH HANNA AND HAS RECENTLY
REPORTED A MINIMUM PRESSURE OF NEAR 1002 MB. AS LONG AS THE
MODERATE SHEAR PREVAILS...CAUSED BY THE RESILIENT UPPER-LOW...HANNA
MOST LIKELY WILL NOT STRENGTHEN. THE OFFICIAL FORECAST CALLS FOR NO
CHANGE
IN INTENSITY DURING THE NEXT 12 HOURS OR SO. GLOBAL MODELS INSIST
ON WEAKENING THE UPPER-LOW...CREATING A MORE FAVORABLE ENVIRONMENT
FOR HANNA TO STRENGTHEN IN THE PERIOD BETWEEN 12 AND 48 HOURS.
THEREAFTER...THE SHEAR IS FORECAST TO INCREASE AND THIS IS
REFLECTED IN THE OFFICIAL FORECAST BY SHOWING SOME SLIGHT
WEAKENING. MOST OF THE MODELS CONTINUE TO INTENSIFY HANNA...SOME OF
THEM QUITE SIGNIFICANTLY...DESPITE THE SHEAR. I DO NOT HAVE AN
EXPLANATION FOR IT.

THE CENTER IS VERY DIFFICULT TO LOCATE ON IR IMAGES BUT SATELLITE
FIXES AND DATA FROM THE NOAA BUOY SUGGEST THAT HANNA HAS BEEN
MOVING WESTWARD OR 280 DEGREES AT 10 KNOTS. FOR ABOUT 12 TO 24
HOURS...A WEST-NORTHWEST MOTION AROUND THE UPPER-LOW IS
ANTICIPATED. THEREAFTER...A STRONG SUBTROPICAL HIGH WHICH IS
FORECAST TO BUILD TO THE NORTH OF HANNA WILL THEN PROVIDE THE
DOMINANT STEERING FLOW. THIS SHOULD FORCE THE CYCLONE ON A
WESTWARD AND THEN AN EVEN SLOWER WEST-SOUTHWESTWARD TRACK. I TRIED
VERY HARD NOT TO FORECAST THE WEST-SOUTHWEST TURN SINCE THIS IS NOT
A VERY COMMON TROPICAL CYCLONE TRACK BUT RELIABLE DYNAMICAL
GUIDANCE HAS FORCED ME TO DO SO. THE OFFICIAL FORECAST IS
PRACTICALLY ON TOP OF THE MODEL CONSENSUS.  I ALWAYS WONDERED HOW
TODAY'S SOPHISTICATED DYNAMICAL MODELS WOULD HAVE FORECAST
HURRICANE BETSY BACK IN 1965. THIS HURRICANE MOVED SOUTHWESTWARD
OVER THE BAHAMAS.


FORECAST POSITIONS AND MAX WINDS

INITIAL      30/0900Z 21.9N  66.3W    45 KT
 12HR VT     30/1800Z 22.6N  68.0W    45 KT
 24HR VT     31/0600Z 23.3N  70.0W    50 KT
 36HR VT     31/1800Z 23.5N  71.5W    60 KT
 48HR VT     01/0600Z 23.5N  72.5W    65 KT
 72HR VT     02/0600Z 23.5N  74.0W    65 KT
 96HR VT     03/0600Z 23.0N  75.5W    60 KT
120HR VT     04/0600Z 22.5N  77.0W    55 KT

$$
FORECASTER AVILA

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