The Insiders' Guide to St. Barths: Bertha Thursday

WTNT32 KNHC 100833
BULLETIN
HURRICANE BERTHA ADVISORY NUMBER 29
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008
500 AM AST THU JUL 10 2008

...BERTHA MOVING A LITTLE SLOWER TO THE NORTHWEST...

IT IS STILL TOO EARLY TO DETERMINE THE EXTENT TO WHICH BERTHA COULD
IMPACT BERMUDA. INTERESTS ON THAT ISLAND SHOULD CLOSELY MONITOR
THE PROGRESS OF BERTHA DURING THE NEXT SEVERAL DAYS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...INCLUDING POSSIBLE
INLAND WATCHES AND WARNINGS...PLEASE MONITOR PRODUCTS ISSUED
BY YOUR LOCAL WEATHER OFFICE.

AT 500 AM AST...0900Z...THE CENTER OF HURRICANE BERTHA WAS LOCATED
NEAR LATITUDE 26.0 NORTH...LONGITUDE 59.4 WEST OR ABOUT 545 MILES...
875 KM...SOUTHEAST OF BERMUDA.

BERTHA IS MOVING TOWARD THE NORTHWEST NEAR 9 MPH...15 KM/HR. A
GRADUAL TURN TOWARD THE NORTH WITH A DECREASE IN FORWARD SPEED IS
EXPECTED DURING THE NEXT COUPLE OF DAYS.

MAXIMUM SUSTAINED WINDS ARE NEAR 105 MPH...165 KM/HR...WITH HIGHER
GUSTS. BERTHA IS A CATEGORY TWO HURRICANE ON THE SAFFIR-SIMPSON
SCALE. SOME STRENGTHENING IS POSSIBLE DURING THE NEXT 24 HOURS.

HURRICANE FORCE WINDS EXTEND OUTWARD UP TO 30 MILES...45 KM...FROM
THE CENTER...AND TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO 140
MILES...220 KM.

ESTIMATED MINIMUM CENTRAL PRESSURE IS 970 MB...28.64 INCHES.

LARGE SWELLS AND HIGH SURF ARE EXPECTED TO AFFECT PORTIONS OF
BERMUDA DURING THE NEXT FEW DAYS.

REPEATING THE 500 AM AST POSITION...26.0 N...59.4 W. MOVEMENT
TOWARD...NORTHWEST NEAR 9 MPH. MAXIMUM SUSTAINED WINDS...105 MPH.
MINIMUM CENTRAL PRESSURE...970 MB.

THE NEXT ADVISORY WILL BE ISSUED BY THE NATIONAL HURRICANE CENTER AT
1100 AM AST.

WWWW
WTNT42 KNHC 100834
HURRICANE BERTHA DISCUSSION NUMBER 29
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008
500 AM EDT THU JUL 10 2008

DVORAK T-NUMBERS REMAIN AT 5.0 CORRESPONDING TO 90 KT...WHICH IS
USED FOR THE ADVISORY INTENSITY. THERE HAS BEEN SOME EROSION OF
DEEP CONVECTION OVER THE SOUTHWEST QUADRANT OF THE
HURRICANE...PROBABLY DUE TO THE COMBINATION OF INCREASED
SOUTH-SOUTHWESTERLY SHEAR AND THE INTRUSION OF DRIER AIR. GIVEN
THIS EVOLUTION...I HAVE ELECTED TO BACK OFF SLIGHTLY ON THE
INTENSITY PREDICTION. THE CURRENT OFFICIAL WIND SPEED FORECAST IS
A BLEND OF THE STATISTICAL/DYNAMICAL GUIDANCE SHIPS AND LGEM ALONG
WITH THE PURELY DYNAMICAL MODELS GFDL AND HWRF. SEA SURFACE
TEMPERATURES SHOULD REMAIN SUFFICIENTLY WARM FOR STRENGTHENING
THROUGH AT LEAST 3-4 DAYS...BUT THE SHIPS MODEL DIAGNOSES 27-31 KT
OF VERTICAL SHEAR IN 3-5 DAYS. THEREFORE WEAKENING IS LIKELY TO
OCCUR IN THE LATTER PART OF THE FORECAST PERIOD.

BERTHA'S FORWARD MOTION HAS SLOWED A BIT AND IS NOW ABOUT 315/8. AN
ANTICYCLONE TO THE NORTH OF THE HURRICANE WILL CONTINUE TO SLIDE
EASTWARD OVER THE NEXT DAY OR TWO...SO THE TRACK OF THE HURRICANE
IS LIKELY TO BEND TOWARD THE NORTH THROUGH 48 HOURS. BEYOND THAT
TIME...THE TRACK FORECAST IS COMPLICATED BY THE FACT THAT THE
DYNAMICAL MODELS...IN PARTICULAR THE GFS...SHOW A MID-LEVEL
ANTICYCLONE DEVELOPING TO THE NORTH OF BERTHA IN AROUND 72 HOURS.
THIS DEVELOPMENT WOULD AT LEAST RETARD THE NORTHWARD PROGRESS OF
THE HURRICANE AND...IF THE ANTICYCLONE TURNS OUT TO BE STRONG
ENOUGH...COULD EVEN FORCE A TURN TO THE LEFT AS INDICATED BY THE
GFS. THE GFDL AND HWRF MODELS RESPOND TO THE INCREASED RIDGING
MAINLY BY SLOWING THE TROPICAL CYCLONE AS IT CROSSES 30N LATITUDE.
IT IS CERTAINLY POSSIBLE FOR BERTHA TO MOVE ERRATICALLY IN ABOUT 3
DAYS...BUT THE DETAILS OF THAT MOTION ARE OF COURSE UNKNOWN. BY
THE END OF THE FORECAST PERIOD...THE FLOW AHEAD OF A LARGE-
AMPLITUDE 500 MB TROUGH NEARING THE U.S EAST COAST IS EXPECTED TO
CAUSE BERTHA TO MOVE NORTH-NORTHEASTWARD WITH SOME INCREASE IN
FORWARD SPEED.

THE RADII OF 12-FT SEAS HAVE BEEN ADJUSTED BY THE TROPICAL ANALYSIS
AND FORECAST BRANCH BASED ON A JASON SATELLITE ALTIMETER PASS OVER
BERTHA...WHICH IS QUITE CONSISTENT WITH THE LATEST OUTPUT FROM THE
NORTH ATLANTIC HURRICANE WAVE MODEL.

FORECAST POSITIONS AND MAX WINDS

INITIAL 10/0900Z 26.0N 59.4W 90 KT
12HR VT 10/1800Z 27.0N 60.3W 95 KT
24HR VT 11/0600Z 28.0N 61.1W 95 KT
36HR VT 11/1800Z 29.1N 61.8W 95 KT
48HR VT 12/0600Z 29.9N 62.0W 90 KT
72HR VT 13/0600Z 31.0N 62.0W 80 KT
96HR VT 14/0600Z 32.5N 62.0W 70 KT
120HR VT 15/0600Z 36.0N 61.0W 60 KT

WWWW

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