Tropical Storm BERTHA

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TROPICAL STORM BERTHA DISCUSSION NUMBER   5
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL022008
500 AM EDT FRI JUL 04 2008
 
A STRONG BURST OF DEEP CONVECTION BEGAN AT ABOUT 05Z...RESULTING IN
SUBJECTIVE DVORAK INTENSITY ESTIMATES AT 06Z OF 45-55 KT.  THESE
ESTIMATES...HOWEVER...WERE CONTINGENT UPON THE LOW-LEVEL
CIRCULATION CENTER BEING LOCATED MUCH FARTHER NORTH THAN PREVIOUSLY
THOUGHT...AND RIGHT IN THE MIDDLE OF THE CONVECTION.  PASSIVE
MICROWAVE IMAGES FROM TRMM AT 0146Z AND AMSR-E AT 0326Z SUGGEST THE
LOW-LEVEL CENTER HAS BEEN MOVING NORTH OF THE PREVIOUS MOTION
ESTIMATE...BUT BENEATH THE SOUTHERN PORTION OF THE COLD CLOUD TOP
CANOPY.  SINCE 06Z...THE CLOUD TOPS HAVE WARMED A BIT...SO I DO
NOT THINK THE DVORAK ESTIMATES WOULD BE AS HIGH NOW.  BEST I CAN
TELL FROM FIRST LIGHT VISIBLE IMAGES...THE LOW-LEVEL CENTER SEEMS TO
BE MOVING ALONG THE HEADING SUGGESTED BY THE MICROWAVE IMAGERY...OR
290/12... BENEATH THE SOUTHERN PORTION OF THE DEEP CONVECTION.  THE
INITIAL INTENSITY WILL BE HELD AT 40 KT FOR NOW...TO SEE IF THE
DEEP CONVECTION PERSISTS AS BERTHA HEADS TOWARD SLIGHTLY COOLER
WATERS TODAY.

THE FORECAST REASONING HAS NOT CHANGED TOO MUCH.  THE FIRST TWO DAYS
OF THE TRACK FORECAST HAVE BEEN ADJUSTED SLIGHTLY NORTHWARD...
MAINLY TO ACCOUNT FOR THE CHANGE IN THE INITIAL MOTION. 
THEREAFTER...ONCE BERTHA FINDS ITSELF DUE SOUTH OF THE MID-LEVEL
RIDGE TO ITS NORTH...THE TRACK HAS BEEN SPED UP AND EDGED A LITTLE
TO THE LEFT OR WEST...IN ACCORDANCE WITH A WESTWARD SHIFT IN MOST
OF THE MODELS.  ALL OF THE MODELS STILL FORECAST A WEAKNESS IN THE
RIDGE TO ALLOW BERTHA TO TURN TO THE RIGHT LATE IN THE FORECAST
PERIOD...BUT RECENT RUNS HAVE BEEN TRENDING TOWARD DELAYING THIS
TURN.  THE NEW OFFICIAL TRACK FORECAST IS KEPT CLOSE TO THE MODEL
CONSENSUS...BUT THE MODEL SPREAD IS SUBSTANTIAL AT DAYS 4-5.  THE
MODELS THAT FORECAST A STRONGER BERTHA...SUCH AS THE
GFDL...FORECAST MORE OF A NORTHWESTWARD MOTION BY DAY 5...WHILE
MODELS SUCH AS THE ECMWF AND UKMET INSIST ON A WEAKER CYCLONE
CONTINUING WEST-NORTHWESTWARD.  THE OFFICIAL INTENSITY
FORECAST...LIKE THE TRACK FORECAST...IS CLOSE TO THE
CONSENSUS...TAKING A STRONG TROPICAL STORM DOWN THE MIDDLE OF THE
GUIDANCE ENVELOPE...IN CLOSEST AGREEMENT WITH THE LGEM GUIDANCE. 
STRENGTHENING SHOULD BE LIMITED BY COOL WATERS DURING THE NEXT TWO
DAYS...AND BY INCREASING SHEAR ON DAYS 4 AND 5.  IT IS NOTABLE THAT
THE GFDL AND HWRF INTENSITY FORECASTS ARE VERY MUCH IN
DISAGREEMENT...INDICATIVE OF GREAT UNCERTAINTY IN THE INTENSITY
FORECAST AS WELL.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      04/0900Z 14.2N  28.3W    40 KT
 12HR VT     04/1800Z 15.0N  30.6W    45 KT
 24HR VT     05/0600Z 15.9N  34.0W    45 KT
 36HR VT     05/1800Z 16.6N  37.7W    50 KT
 48HR VT     06/0600Z 17.3N  41.4W    55 KT
 72HR VT     07/0600Z 19.0N  48.0W    60 KT
 96HR VT     08/0600Z 21.5N  53.0W    60 KT
120HR VT     09/0600Z 25.0N  56.5W    60 KT
 
$$
FORECASTER KNABB
 
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