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TROPICAL DEPRESSION OLGA DISCUSSION NUMBER 39
NATIONAL WEATHER SERVICE MIAMI FL
4 AM EST TUE DEC 04 2001
OLGA REMAINS A VERY PERSISTENT AND WELL DEVELOPED CIRCULATION.
SATELLITE IMAGERY INDICATES THAT SIGNIFICANT CONVECTION...TOPS TO
-60C...HAS ONE AGAIN DEVELOPED JUST EAST OF THE LOW-LEVEL CENTER.
DVORAK SATELLITE INTENSITY ESTIMATES FROM ALL THREE AGENCIES IS
30 KT...T2.0...WITH SHEAR PATTERN DATA T-NUMBERS NEAR T2.5...OR
35 KT. OUTFLOW IS RESTRICTED TO THE EASTERN SEMICIRCLE OWING TO THE
MODERATE UPPER-LEVEL SOUTHWESTERLY SHEAR IMPINGING ON THE CYCLONE.
THE INITIAL MOTION ESTIMATE IS 090/06. HOWEVER...A DECREASE IN
FORWARD SPEED AND A TURN TO THE SOUTH OR SOUTHWEST IS EXPECTED TO
DEVELOP LATER TODAY OR TONIGHT. WATER VAPOR IMAGERY INDICATES THE
DEEP-LAYER TROUGH TO THE NORTH IS MOVING RAPIDLY AWAY FROM OLGA
WHILE A BROAD UPPER-LEVEL TROUGH IS DIGGING SOUTHWARD ALONG THE WEST
SIDE OF THE CYCLONE. IT ALSO APPEARS THAT A SMALL VORTICITY CENTER
MAY BE TRACKING INWARD TOWARD THE CENTER OF OLGA AND THIS COULD ADD
A LITTLE MORE ENERGY TO THE SYSTEM AND MAKE THE VERTICAL CIRCULATION
A LITTLE STRONGER AND DEEPER...AND MORE SHEAR RESISTANT. THE NHC
MODEL GUIDANCE IS LESS DIVERGENT THAN THIS TIME YESTERDAY AND THE
FORECAST TRACKS ARE CONFINED TO THE SOUTHERN SEMICIRCLE IN RESPONSE
TO THE DEEP-LAYER RIDGE BUILDING BACK IN TO THE NORTH OF OLGA. ALL
OF THE GLOBAL MODELS AND THE 3 BAM MODELS TAKE OLGA SOUTH AND THEN
SOUTHWESTWARD DURING THE NEXT 24 HOURS...WITH A GRADUAL TURN TO THE
WEST-SOUTHWEST OR WEST IN 36 TO 48 HOURS. ALL OF THE GLOBAL MODELS
ALSO AGREE ON THE REMNANT CIRCULATION OR INVERTED TROUGH PASSING
ACROSS THE CENTRAL AND NORTHERN BAHAMAS AND SOUTH FLORIDA IN 72 TO
96 HOURS. GIVEN THE TENACITY OF THIS SYSTEM...AND THE FACT THAT IT
IS FORECAST TO TRACK OVER WARMER WATER...I HAVE DECIDED TO ADD A
72-HOUR POSITION FOR CONTINUITY PURPOSES IN THE EVENT OLGA DECIDES
TO RE-STRENGTHEN BACK TO A SIGNIFICANT TROPICAL CYCLONE.
GIVEN THE SOMEWHAT HYBRID NATURE OF THIS SYSTEM...IT IS DIFFICULT TO
ASSESS WHAT EFFECT VERTICAL SHEAR WILL HAVE ON OLGA DURING THE NEXT
24 HOURS. BASED ON THE LATEST SATELLITE IMAGES...THE UPPER-LEVEL
SHEAR APPEARS TO BE HAVING LITTLE EFFECT OTHER THAN TO DISPLACE THE
CONVECTION TO THE EAST OF THE CENTER. HOWEVER...ALL OF THE GLOBAL
MODELS AGREE THAT THE SHEAR WILL LESSEN AND BECOME MORE NORTHERLY
DURING THE NEXT 24 HOURS...WHICH WILL BE ALONG THE FORECAST TRACK.
BY 36 HOURS...A 200 MB ANTICYCLONE IS FORECAST TO DEVELOP JUST NORTH
OF OLGA...PLACING THE CYCLONE UNDER MORE FAVORABLE LIGHT UPPER-LEVEL
EASTERLY FLOW. GIVEN THAT OLGA WILL BE TRACKING SOUTHWARD OVER
WARMER WATER...SSTS NEAR 26C... SIGNIFICANT RE-DEVELOPMENT IS A
DISTINCT POSSIBILITY...ESPECIALLY IF A LOW-LEVEL CIRCULATION REMAINS
INTACT AFTER 36 TO 48 HOURS.
FORECASTER STEWART
FORECAST POSITIONS AND MAX WINDS
INITIAL 04/0900Z 27.8N 67.9W 30 KTS
12HR VT 04/1800Z 27.4N 67.6W 30 KTS
24HR VT 05/0600Z 26.3N 68.4W 25 KTS...DISSIPATING
36HR VT 05/1800Z 25.2N 70.0W 25 KTS...DISSIPATING
48HR VT 06/0600Z 24.5N 72.2W 20 KTS...REMNANT LOW
72HR VT 07/0600Z 24.5N 78.0W 20 KTS...REMNANT LOW
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