TROPICAL STORM BERTHA DISCUSSION NUMBER 13
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008
500 AM EDT SUN JUL 06 2008
NOT MANY CHANGES TO REPORT WITH BERTHA THIS MORNING. THE CYCLONE
CONTINUES TO MOVE QUICKLY WESTWARD ALONG A HEADING OF ABOUT 280/19
TO THE SOUTH OF THE SUBTROPICAL RIDGE. THE OBJECTIVE GUIDANCE IS IN
VERY GOOD AGREEMENT THROUGH ABOUT 48 HOURS ON A GENERAL
WEST-NORTHWESTWARD MOTION...AND A GRADUAL SLOWING OF THE FORWARD
SPEED AS HEIGHTS BUILD TO THE NORTHWEST OF BERTHA OVER THE
SUBTROPICAL WESTERN ATLANTIC. THE MODELS STILL DIVERGE
SIGNIFICANTLY AT DAYS 3 THROUGH 5...BUT THEY HAVE GENERALLY COME
INTO A LITTLE BETTER AGREEMENT ON A TURN TOWARD THE NORTHWEST BY
THEN...ALTHOUGH AT A FORWARD SPEED OF ABOUT HALF OF BERTHA'S
CURRENT MOTION. THE OUTLIER IS THE HWRF THAT FORECASTS A MUCH
SOONER TURN TOWARD THE NORTH ALONG ABOUT 58W. NONE OF THE
MODELS...HOWEVER...FORECAST BERTHA TO REACH 30 DEGREES NORTH
LATITUDE WITHIN FIVE DAYS. WHILE THE RIDGE OVER THE WESTERN
ATLANTIC IS FORECAST TO BE MUCH WEAKER BY THE END OF THE FIVE-DAY
FORECAST PERIOD...THE MODELS DO NOT DEPICT A HIGH-AMPLITUDE TROUGH
THAT WOULD ACCELERATE BERTHA INTO THE MIDDLE LATITUDES. SO IT IS
FAR TOO EARLY TO DETERMINE IF OR WHEN BERTHA WILL RECURVE. THE NEW
OFFICIAL TRACK FORECAST IS ADJUSTED SLIGHTLY TO THE RIGHT...TOWARD
BUT NOT AS FAR EAST AS THE CONSENSUS.
THE WATERS BENEATH BERTHA ARE CURRENTLY ABOUT 26 CELSIUS AND
CONTINUE TO GET WARMER AS THE CYCLONE PROCEEDS WESTWARD...AND BY 48
HOURS FROM NOW THEY SHOULD BE APPROACHING 28 CELSIUS. VERTICAL
WIND SHEAR SHOULD NOT STRENGTHEN DURING THIS PERIOD...SO GRADUAL
STRENGTHENING IS FORECAST IN LINE WITH NEARLY ALL OF THE OBJECTIVE
INTENSITY GUIDANCE...AND BERTHA COULD BE A HURRICANE IN A COUPLE OF
DAYS. BEYOND THAT TIME...THE WATERS WILL BE PLENTY WARM...SO THE
DETERMINING FACTOR ON THE INTENSITY OF BERTHA WILL PROBABLY BE WIND
SHEAR. THE UPPER-LEVEL PATTERN FORECAST BY THE MODELS AT DAYS 3
THROUGH 5 IS COMPLEX...AND IT IS HARD TO DETERMINE JUST HOW MUCH
SHEAR WILL BE AFFECTING BERTHA DURING THAT PERIOD. DESPITE TAKING
INTO ACCOUNT A MODEST INCREASE IN SHEAR BASED ON THE GFS MODEL WIND
FIELDS...THE SHIPS AND LGEM FORECAST 70-75 KT AT 72-120 HOURS...SO
THE NEW OFFICIAL INTENSITY FORECAST PEAKS AT 70 KT...A LITTLE
HIGHER THAN THE PREVIOUS ADVISORY. GIVEN THE UNCERTAINTIES IN THE
ATMOSPHERIC PATTERN AT THOSE LONG RANGES...HOWEVER...THIS IS NOT A
HIGH CONFIDENCE INTENSITY FORECAST.
FORECAST POSITIONS AND MAX WINDS
INITIAL 06/0900Z 17.3N 43.2W 45 KT
12HR VT 06/1800Z 17.9N 46.1W 50 KT
24HR VT 07/0600Z 18.7N 49.5W 55 KT
36HR VT 07/1800Z 19.6N 52.5W 60 KT
48HR VT 08/0600Z 20.4N 55.1W 65 KT
72HR VT 09/0600Z 22.5N 60.0W 70 KT
96HR VT 10/0600Z 24.5N 64.0W 70 KT
120HR VT 11/0600Z 27.0N 67.5W 70 KT
