HURRICANE BERTHA DISCUSSION NUMBER 17

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WTNT42 KNHC 070900
TCDAT2
HURRICANE BERTHA DISCUSSION NUMBER 17
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008
500 AM EDT MON JUL 07 2008

IN THE HOURS LEADING UP TO 00Z...SEVERAL CONSECUTIVE MICROWAVE
OVERPASSES OCCURRED ON ROUGHLY AN HOURLY BASIS...AND ALMOST MADE IT
SEEM LIKE WE WERE RECEIVING RADAR FIXES. THOSE IMAGES REVEALED A
WELL-DEFINED AND VERTICALLY-ALIGNED EYE FEATURE...AND MORE RECENTLY
AN IMAGE FROM AMSU TAKEN AT 0517Z ALSO DEPICTED AN EYE. THE
MICROWAVE EYE DIAMETER APPEARS TO BE ABOUT 15-20 N MI. GOES
INFRARED IMAGERY HAS INTERMITTENTLY EXHIBITED HINTS OF AN
EYE...ESPECIALLY AROUND 06Z WHEN SUBJECTIVE DVORAK INTENSITY
ESTIMATES FROM TAFB AND SAB WERE BOTH 65 KT. OBJECTIVE DVORAK
ESTIMATES REMAIN SLIGHTLY BELOW THE HURRICANE THRESHOLD...BUT GIVEN
THE MICROWAVE SIGNATURES THE SYSTEM IS UPGRADED TO A HURRICANE...
THE FIRST OF THE 2008 ATLANTIC SEASON...WITH AN INTENSITY OF 65 KT.
WHILE A HURRICANE DOES NOT FORM EVERY YEAR IN JULY IN THE ATLANTIC
BASIN...IT HAS CERTAINLY HAPPENED BEFORE...INCLUDING IN 1996 WHEN
ANOTHER HURRICANE BERTHA FORMED...COINCIDENTALLY ALSO ON JULY 7.

EXTRAPOLATION OF THE EARLIER SERIES OF MICROWAVE IMAGES MATCHES UP
QUITE WELL WITH MORE RECENT GEOSTATIONARY FIXES TO YIELD AN INITIAL
MOTION ESTIMATE OF 285/17. THE SUBTROPICAL RIDGE TO THE NORTH OF
BERTHA IS PROVIDING THE STEERING...BUT THAT FEATURE IS FORECAST BY
THE DYNAMICAL MODELS TO GRADUALLY WEAKEN OVER THE WESTERN ATLANTIC
DURING THE NEXT FEW DAYS...ALLOWING BERTHA'S TRACK TO STEADILY BEND
TO THE RIGHT...BUT ALSO SLOW DOWN SUBSTANTIALLY. ALL OF THE MODELS
GENERALLY AGREE ON THIS SCENARIO...BUT THERE REMAINS A HEALTHY
AMOUNT OF SPREAD REGARDING AT WHAT LONGITUDE BERTHA WILL TURN. THE
NEW OFFICIAL TRACK FORECAST IS EDGED SLIGHTLY TO THE RIGHT TOWARD
THE CONSENSUS AND IS A TAD SLOWER AT THE END. GIVEN THE WEAK
STEERING CURRENTS AND LACK OF A HIGH-AMPLITUDE TROUGH OVER THE
WESTERN ATLANTIC DURING THE NEXT SEVERAL DAYS...IT IS STILL NOT
GUARANTEED THAT BERTHA WILL RECURVE.

THE UPPER-LEVEL WINDS NEAR BERTHA ARE NOT FORECAST TO CHANGE
APPRECIABLY DURING THE NEXT COUPLE OF DAYS...WHILE UNDERLYING
SEA-SURFACE TEMPERATURES WARM A LITTLE MORE...SO SOME ADDITIONAL
STRENGTHENING IS FORECAST ALONG THE LINES OF THE SHIPS AND LGEM
GUIDANCE. AT 3 TO 5 DAYS...THE GLOBAL MODELS DISAGREE ON THE
EVOLUTION OF AN UPPER-LEVEL TROUGH IN THE VICINITY OF THE TROPICAL
CYCLONE...AND SO IT REMAINS UNCLEAR HOW MUCH WIND SHEAR WILL AFFECT
BERTHA THEN. THE NEW OFFICIAL INTENSITY FORECAST PEAKS AT 80 KT
WITHIN THE NEXT COUPLE OF DAYS...BUT THEN SHOWS GRADUAL WEAKENING
DUE TO THE POTENTIAL FOR INCREASING SHEAR...AND DUE TO COOLER
WATERS AS BERTHA PROGRESSES NORTHWARD ALONG THE FORECAST TRACK.

FORECAST POSITIONS AND MAX WINDS

INITIAL 07/0900Z 19.3N 50.2W 65 KT
12HR VT 07/1800Z 19.9N 52.5W 70 KT
24HR VT 08/0600Z 20.8N 55.0W 75 KT
36HR VT 08/1800Z 21.7N 57.1W 80 KT
48HR VT 09/0600Z 22.7N 59.0W 80 KT
72HR VT 10/0600Z 25.0N 62.0W 75 KT
96HR VT 11/0600Z 28.0N 64.0W 70 KT
120HR VT 12/0600Z 30.5N 64.5W 70 KT


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