855 
 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
  
 $$
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