000 WTPZ45 KNHC 131438 TCDEP5 TROPICAL STORM GUILLERMO DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP102009 800 AM PDT THU AUG 13 2009 CONVENTIONAL SATELLITE IMAGERY INDICATES THE FORMATION OF A TRANSIENT EYE-LIKE FEATURE WITHIN A DEVELOPING CENTRAL DENSE OVERCAST...WITH A WELL-DEFINED BAND IN THE SOUTHERN SEMICIRCLE. THESE INCREASES IN ORGANIZATION SUGGEST GUILLERMO IS LIKELY APPROACHING HURRICANE STRENGTH. DVORAK T-NUMBERS FROM TAFB AND SAB ARE 3.5 AND 4.0...RESPECTIVELY...AND SUPPORT AN INCREASE IN THE INITIAL INTENSITY TO 55 KT FOR THIS ADVISORY. THE INITIAL MOTION IS ESTIMATED AT 285/12. NO MAJOR CHANGES HAVE BEEN MADE TO THE PREVIOUS FORECAST TRACK...AS GUILLERMO CONTINUES TO MOVE AROUND THE SOUTHWEST PERIPHERY OF A MID-LEVEL RIDGE EXTENDING WESTWARD FROM THE SOUTH CENTRAL UNITED STATES TO AROUND 140W. MODEL SPREAD IS FAIRLY SMALL DURING THE NEXT 72 HOURS AND THE OFFICIAL FORECAST TRACK IS IN THE MIDDLE OF THE GUIDANCE ENVELOPE...CLOSE TO THE MODEL CONSENSUS. THE MODEL SOLUTIONS DIVERGE AFTER 72 HOURS...WITH THE GFS/UKMET/ECMWF FARTHER SOUTH THAN THE HWRF AND GFDL. THE LIKELY CAUSE FOR THIS IS AN INTERACTION BETWEEN GUILLERMO AND TROPICAL DEPRESSION NINE-E. WHILE THIS CANNOT BE RULED OUT...IT APPEARS AS A MORE REMOTE POSSIBILITY AT THIS TIME. GIVEN RECENT STRUCTURAL CHANGES...IT MIGHT NOT BE LONG TILL GUILLERMO BECOMES A HURRICANE. THE CYCLONE IS IN A LOW SHEAR ENVIRONMENT AND IS CURRENTLY MOVING OVER MARGINAL SEA SURFACE TEMPERATURES...WHICH ARE FORECAST TO DROP BELOW 26.5C IN ABOUT 24 HOURS. SEA SURFACE TEMPERATURES CONTINUE TO GRADUALLY DECREASE ALONG THE FORECAST TRACK THROUGH ABOUT 72 HOURS...AND SOUTHWESTERLY FLOW ALOFT BEGINS TO INCREASE AROUND THIS TIME. BASED UPON THESE FACTORS...IT APPEARS THAT GUILLERMO WILL HAVE A NARROW WINDOW OF OPPORTUNITY TO INTENSIFY. THE OFFICIAL FORECAST CALLS FOR GUILLERMO TO REACH HURRICANE STRENGTH IN 12 HOURS AND BEGIN TO SLOWLY WEAKEN AFTER 36 HOURS. THE INCREASING SOUTHWESTERLY SHEAR LATER IN THE PERIOD SHOULD ACCELERATE THE WEAKENING TREND...AND IT IS POSSIBLE THAT GUILLERMO COULD WEAKEN MORE THAN FORECAST BEYOND 72 HOURS. FORECAST POSITIONS AND MAX WINDS INITIAL 13/1500Z 17.1N 121.6W 55 KT 12HR VT 14/0000Z 17.5N 123.7W 65 KT 24HR VT 14/1200Z 17.9N 126.4W 65 KT 36HR VT 15/0000Z 18.3N 129.0W 65 KT 48HR VT 15/1200Z 18.8N 131.8W 60 KT 72HR VT 16/1200Z 19.5N 137.0W 50 KT 96HR VT 17/1200Z 20.0N 143.0W 45 KT 120HR VT 18/1200Z 20.0N 148.5W 35 KT $$ FORECASTER KIMBERLAIN