WTPZ42 KNHC 152035 RRA

TCDEP2



TROPICAL STORM NORMA DISCUSSION NUMBER   6

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172017

300 PM MDT FRI SEP 15 2017



NORMA'S PRIMARY CONVECTIVE BAND CONTINUES TO ELONGATE AND CURLS

ABOUT ONE AND A HALF TIMES AROUND THE CIRCULATION.  OBJECTIVE AND

SUBJECTIVE DVORAK ESTIMATES ARE WIDE RANGING, GOING FROM 50 KT UP

TO 77 KT, BUT SINCE THE DEEP CONVECTIVE CLOUD TOPS ARE NOT VERY

COLD, THE INITIAL INTENSITY IS RAISED CONSERVATIVELY TO 60 KT.

THIS LOWER INTENSITY IS SUPPORTED BY RECENT ASCAT DATA, WHICH ONLY

SHOWED MAXIMUM WINDS OF 50 KT.



THE ENVIRONMENT APPEARS CONDUCIVE FOR CONTINUED GRADUAL

STRENGTHENING.  NORMA'S LARGE SIZE ARGUES AGAINST RAPID

INTENSIFICATION, BUT THE CYCLONE SHOULD BE ABLE TO AT LEAST

STEADILY STRENGTHEN DURING THE NEXT 36 HOURS OR SO.  VERTICAL

SHEAR BEGINS TO INCREASE AFTER THAT TIME, HOWEVER, AND NORMA IS

LIKELY TO WEAKEN BEGINNING IN ABOUT 48 HOURS.  THE HCCA GUIDANCE AND

THE ICON INTENSITY CONSENSUS WERE NOTABLY LOWER ON THIS CYCLE, AND

THE UPDATED NHC FORECAST HAS BEGUN TO FOLLOW THAT TREND, ESPECIALLY

ON DAYS 3 THROUGH 5.



THE CENTER HAS WOBBLED NORTHWESTWARD DURING THE DAY, AND THE

INITIAL MOTION ESTIMATE IS 325/2 KT.  NORMA SHOULD TURN SLOWLY

NORTHWARD DURING THE NEXT DAY OR SO, STEERED ALONG THE WESTERN

PERIPHERY OF THE SUBTROPICAL RIDGE.  THE TRACK MODELS CONTINUE TO

PLAY THE SAME TUNE, WITH THE ECMWF AND UKMET MODELS ON THE FAR

WESTERN SIDE OF THE GUIDANCE ENVELOPE AND THE HWRF AND GFS ON THE

EASTERN SIDE.  SINCE THE OVERALL TREND IN THE GUIDANCE HAS BEEN

WESTWARD, THE NEW NHC TRACK FORECAST LEANS LEFT OF THE CONSENSUS

AIDS AND IS A LITTLE LEFT OF THE PREVIOUS FORECAST.