WTPZ42 KNHC 162101 RRA

TCDEP2



TROPICAL STORM NORMA DISCUSSION NUMBER  10

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172017

300 PM MDT SAT SEP 16 2017



THE CONVECTIVE STUCTURE HAS CONTINUED TO DEGRADE DURING THE DAY

WITH SYMMETRIC, BUT ILL-DEFINED BANDS IN PLACE WITHIN 180 NM OF THE

CENTER.  AN AIR FORCE RECONNAISSANCE MISSION THIS AFTERNOON HAS

MEASURED PEAK FLIGHT-LEVEL WINDS OF ONLY 58 KT, WHICH SUPPORTS AN

INITIAL INTENSITY OF AT MOST 55 KT. THUS NORMA IS DOWNGRADED TO A

TROPICAL STORM.



WHILE THE VERTICAL SHEAR IS CURRENTLY LOW AND SHOULD REMAIN THAT WAY

FOR THE NEXT COUPLE OF DAYS, THE LACK OF MOTION OVER THE LAST DAYS

WOULD LIKELY INDUCE OCEAN UPWELLING AND COOLER WATERS UNDERNEATH THE

CORE OF NORMA.  ADDITIONALLY, A PROMINENT TONGUE OF DRY AIR SEEN IN

THE TOTAL PRECIPITABLE WATER IMAGERY HAS WRAPPED AROUND THE SOUTHERN

SEMICIRCLE ABOUT 200 NM FROM THE CENTER. MODEL GUIDANCE SUGGESTS

THAT THE MID-LEVEL HUMIDITIES SHOULD DRY SUBSTANTIALLY WHILE NORMA

ENCOUNTERS INCREASINGLY COOLER SSTS DURING THE NEXT FEW DAYS.

ADDITIONALLY THE VERTICAL SHEAR IS NOW ANTICIPATED TO SUBSTANTIALLY

INCREASE AROUND DAYS 4 OR 5 (IF THE GFS MODEL IS CORRECT) OR A DAY

OR TWO EARLIER (IF THE ECMWF/UKMET MODELS ARE CORRECT).  THE

OFFICIAL INTENSITY FORECAST THUS NO LONGER INDICATES ANY

INTENSIFICATION, ESPECIALLY GIVEN THE REDUCED INITIAL MAXIMUM WINDS.

CORRESPONDINGLY, A FASTER DECAY IS SHOWN COMPARED WITH THE PREVIOUS

ADVISORY AND NORMA IS EXPECTED TO BECOME A REMNANT LOW IN FIVE DAYS,

IF NOT SOONER.  THIS FORECAST IS CLOSE TO THE MEAN OF THE TIGHTLY

PACKED STATISTICAL AND DYNAMICAL INTENSITY GUIDANCE.



NORMA IS MOVING TOWARD THE NORTH AT ABOUT 3 KT, AS IT IS BEING