WTPZ41 KNHC 150832 RRA

TCDEP1



TROPICAL STORM ORLENE DISCUSSION NUMBER  18

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP162016

200 AM PDT THU SEP 15 2016



ORLENE HAS A QUITE SMALL, SYMMETRIC CENTRAL DENSE OVERCAST TONIGHT,

WITH A TINY EYE OCCASIONALLY MAKING AN APPEARANCE. OVERALL, THE

CONVECTIVE STRUCTURE HAS NOT CHANGED MUCH OVER THE LAST FEW HOURS.

A BLEND OF THE TAFB/SAB SUBJECTIVE DVORAK AND CIMSS ADVANCED DVORAK

SUGGESTS THAT ORLENE REMAINS A 60-KT TROPICAL STORM.



ORLENE IS SITUATED DIRECTLY UNDER AN UPPER-LEVEL SUBTROPICAL RIDGE

AND THUS IS EXPERIENCING VERY LOW VERTICAL SHEAR.  HOWEVER, THE

TROPICAL STORM IS EXPECTED TO RIDE ALONG THE 26C SST ISOTHERM WHILE

THE NOT-VERY-MOIST ENVIRONMENT BECOMES QUITE DRY DURING THE NEXT

THREE DAYS.  IN ADDITION, BY DAYS FOUR AND FIVE, THE SHEAR SHOULD GO

UP SUBSTANTIALLY AS ORLENE APPROACHES A MID- TO UPPER-LEVEL LOW. THE

BOTTOM LINE IS THAT THE TROPICAL STORM SHOULD GRADUALLY WEAKEN. THE

NHC INTENSITY FORECAST IS BASED UPON A NOW MORE TIGHTLY CLUSTERED

SET OF STATISTICAL AND DYNAMICAL MODEL GUIDANCE. PERHAPS THE BIGGEST

UNCERTAINTY IS WHEN ORLENE'S DEEP CONVECTION WILL CEASE.  THE

FORECAST IS FOR THAT TO OCCUR AROUND DAY FIVE, BUT IT WOULD NOT BE

SURPRISING IF THIS OCCURRED SIGNIFICANTLY EARLIER.



ORLENE IS MOVING TOWARD THE WEST AT ABOUT 6 KT.  THE SYSTEM IS

EXPECTED TO MOVE TOWARD THE WEST OR WEST-SOUTHWEST WITH A FASTER

FORWARD SPEED DURING THE NEXT THREE TO FOUR DAYS AS MID-LEVEL

RIDGING BUILDS TO ITS NORTH.  AROUND DAY FOUR OR FIVE, ORLENE SHOULD

BEGIN RESPONDING TO THE UPPER-LEVEL LOW THAT IT APPROACHES BY

TURNING TOWARD THE WEST-NORTHWEST.  THE NHC TRACK FORECAST IS NEARLY=