WTPA41 PHFO 010903
TCDCP1

TROPICAL STORM LANA DISCUSSION NUMBER   8
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   EP062009
1100 PM HST FRI JUL 31 2009

A BURST OF CONVECTION ALONG THE NORTHERN QUADRANT OF LANA DURING THE
EVENING...ALLOWED THE SYSTEM TO MAINTAIN ITS INTENSITY DESPITE THE
PRESENCE OF VERTICAL WIND SHEAR.  THIS LATEST ROUND OF
CONVECTION...LED TO SOME DIFFICULTY IN LOCATING THE LOW LEVEL
CENTER AND CAUSED A VARIETY IN THE DVORAK CURRENT INTENSITY
ESTIMATES. THE 37 GHZ MICROWAVE IMAGERY FROM SSMIS AND TRMM NEAR 
0300Z AND SSMIS AT 0530Z POINTED TO THE LOW LEVEL CENTER FOLLOWING 
CLOSE TO THE OFFICIAL TRACK...WHILE THE 0312Z AMSUB 89 GHZ SUGGESTED 
THAT THE UPPER PORTION OF THE SYSTEM IS TILTED NORTHWARD WITH HEIGHT 
DUE TO VERTICAL WIND SHEAR. PHFO AND SAB DVORAK CURRENT INTENSITY
NUMBERS CAME IN AT 3.5...AND JTWC REPORTED A 4.0...WHILE CIMSS ADT
CALCULATED AN ADJUSTED T NUMBER OF 3.1. THE INITIAL INTENSITY
REMAINS UNCHANGED AT 45 KT...FOLLOWING CLOSELY THE INTENSITY
SUGGESTED BY ADT AND SAB WHICH PLACED THE LOW LEVEL CENTER CLOSER
TO THE 37 GHZ IMAGERY. THE WIND RADII WERE ADJUSTED SLIGHTLY BASED 
ON NEARBY QUIKSCAT PASS.

THERE IS LITTLE CHANGE TO THE FORECAST FROM THE PREVIOUS PACKAGE
REGARDING TRACK. LANA CONTINUES TO MOVE WESTWARD TO THE SOUTH OF A
MID-LEVEL SUBTROPICAL RIDGE...WHILE SOUTHWESTERLY WINDS ALOFT
CONTINUE TO PROMOTE CONVECTION AND OUTFLOW ACROSS THE NORTHERN
QUADRANT OF THE SYSTEM. THE MID LEVEL RIDGE IS FORECAST TO BUILD
WESTWARD OVER THE NEXT SEVERAL DAYS...KEEPING THE TROPICAL CYCLONE
MOVING GENERALLY TOWARD THE WEST TO WEST-NORTHWEST THROUGH THE
FORECAST PERIOD. THE FORECAST TRACK IS CLOSE TO THE CONSENSUS
THROUGH THE NEXT FIVE DAYS.

INCREASING VERTICAL WIND SHEAR WILL BE THE MAIN FACTOR IN THE
INTENSITY FORECAST...SINCE SST VALUES WILL REMAIN AROUND 27C FOR
THE NEXT FEW DAYS. AN UPPER LEVEL TROUGH CENTERED JUST WEST OF THE
MAIN HAWAIIAN ISLANDS IS PRODUCING THE WIND SHEAR...WHICH INCREASES
RAPIDLY TO THE WEST OF THE TROPICAL CYCLONE ACCORDING TO THE LATEST
CIMSS ANALYSIS. LANA WILL WEAKEN AS IT MOVES UNDER THE STRONGER
WINDS ALOFT...BUT THAT WEAKENING TREND HAS BEEN DELAYED TO BE IN
BETTER AGREEMENT WITH THE GFDL AND SHIPS GUIDANCE. STEADY WEAKENING
IS FORECAST FROM 24 HOURS AND BEYOND DUE TO THE EFFECTS OF
INCREASING VERTICAL SHEAR ALONG LANA/S PROJECTED TRACK. THE
FORECAST IS FOR LANA TO BE A REMNANT LOW BY 96 HOURS AND
POTENTIALLY SOONER.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/0900Z 14.4N 150.0W    45 KT
 12HR VT     01/1800Z 14.8N 152.4W    45 KT
 24HR VT     02/0600Z 15.3N 155.2W    40 KT
 36HR VT     02/1800Z 15.7N 158.0W    35 KT
 48HR VT     03/0600Z 16.1N 160.8W    30 KT
 72HR VT     04/0600Z 16.9N 166.3W    25 KT...DISSIPATING
 96HR VT     05/0600Z 17.5N 171.8W    20 KT...REMNANT LOW
120HR VT     06/0600Z 18.0N 177.2W    20 KT...REMNANT LOW
 
$$
FORECASTER WROE/HOUSTON