Tropical Storm Bertha Forecast Discussion #9

By: National Weather Service
By: National Weather Service

000
WTNT42 KNHC 050900
TCDAT2
TROPICAL STORM BERTHA DISCUSSION NUMBER 9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008
500 AM EDT SAT JUL 05 2008

BERTHA IS PASSING OVER SEA-SURFACE TEMPERATURES OF ROUGHLY 25
DEGREES CELSIUS...ABOUT THE COOLEST THE CYCLONE HAS ENCOUNTERED YET
DURING THE USUAL DIURNAL CONVECTIVE MAXIMUM...AND IT HAS BEEN
STRUGGLING TO PRODUCE VERY MUCH DEEP CONVECTION OVERNIGHT. DVORAK
INTENSITY ESTIMATES AT 06Z WERE 35-45 KT...AND WHILE IT IS POSSIBLE
THE WINDS HAVE WEAKENED...THE ADVISORY INTENSITY WILL REMAIN 45 KT
FOR NOW. DURING THE NEXT THREE DAYS...BERTHA HAS AN OPPORTUNITY TO
STRENGTHEN AS THE UNDERLYING WATER TEMPERATURES STEADILY WARM BY
ABOUT A DEGREE CELSIUS PER DAY ALONG THE FORECAST TRACK. ALL OF
THE OBJECTIVE INTENSITY MODELS FORECAST INTENSIFICATION THROUGH 72
HOURS...WITH THE SHIPS AND LGEM REACHING 65 KT BY THEN...WHILE HWRF
AND GFDL ARE MORE CONSERVATIVE. THE OFFICIAL INTENSITY FORECAST IS
A BLEND OF THESE SOLUTIONS AND IS THE SAME AS THE PREVIOUS ADVISORY
IN PEAKING AT 60 KT. ONE SHOULD NOT PAY TOO MUCH ATTENTION TO THE
EXACT INTENSITY FORECAST...HOWEVER. THE MAXIMUM WIND SPEED
PROBABILITY TABLE INCLUDED IN THIS FORECAST PACKAGE INDICATES
ROUGHLY EQUAL CHANCES OF BERTHA BEING A TROPICAL STORM OR A
HURRICANE AT 3 TO 5 DAYS. WHETHER OR NOT BERTHA BECOMES A
HURRICANE WOULD SEEM TO DEPEND ON HOW WELL IT SURVIVES ITS STAY
OVER COOLER WATERS...AND JUST HOW MUCH WIND SHEAR IMPACTS THE
CYCLONE ONCE IT REACHES THE WARMER WATERS...AND BOTH OF THOSE
FACTORS ARE SOMEWHAT UNCERTAIN RIGHT NOW.

A COUPLE OF RECENT MICROWAVE OVERPASSES FROM AMSU AND AMSR-E PROVIDE
THE BASIS FOR THE INITIAL MOTION ESTIMATE OF 285/18. OVERALL THE
DYNAMICAL MODEL GUIDANCE PROVIDES NO STRONG INDICATION THAT BERTHA
WILL DEVIATE MUCH FROM THE CURRENT WEST-NORTHWESTWARD HEADING
DURING THE NEXT SEVERAL DAYS. THE WEAKNESS IN THE SUBTROPICAL
RIDGE CURRENTLY ALONG ABOUT 55W IS FORECAST BY THE MODELS TO BECOME
LESS PRONOUNCED AND TO PROPAGATE EASTWARD DURING THE NEXT COUPLE OF
DAYS...AND MOST OF THE GUIDANCE TRACKS SHOW BERTHA BYPASSING THIS
FEATURE WITH JUST A SLIGHT BEND TO THE RIGHT ON DAYS 3 THROUGH 5.
THE HWRF IS THE ONLY DYNAMICAL MODEL CURRENTLY CALLING FOR BERTHA
TO TURN NORTHWARD BEFORE REACHING 60W. THE MODEL CONSENSUS HAS
HARDLY BUDGED ON THIS CYCLE...AND THE NEW OFFICIAL TRACK FORECAST
IS ADJUSTED ONLY SLIGHTLY FASTER AND TO THE LEFT AT DAYS 3 THROUGH
5. THIS NEW TRACK IS MOST SIMILAR TO THE GFDL...GFS...AND
ECMWF...BUT THE TRACK FORECAST UNCERTAINTY REMAINS SIGNIFICANT AS
REFLECTED BY THE CONTINUING LARGE SPREAD IN THE LONG-RANGE MODEL
GUIDANCE.

FORECAST POSITIONS AND MAX WINDS

INITIAL 05/0900Z 16.5N 35.3W 45 KT
12HR VT 05/1800Z 17.0N 38.0W 45 KT
24HR VT 06/0600Z 17.6N 41.8W 50 KT
36HR VT 06/1800Z 18.2N 45.6W 55 KT
48HR VT 07/0600Z 19.0N 49.3W 60 KT
72HR VT 08/0600Z 20.5N 56.0W 60 KT
96HR VT 09/0600Z 22.5N 61.0W 60 KT
120HR VT 10/0600Z 24.5N 66.0W 60 KT

$$
FORECASTER KNABB


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