WTPZ45 KNHC 242033 RRA

TCDEP5



TROPICAL STORM IRWIN DISCUSSION NUMBER  10

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP102017

200 PM PDT MON JUL 24 2017



IRWIN HAS BEEN DISPLAYING A DIMPLE IN VISIBLE SATELLITE IMAGERY,

WHICH APPEARS TO COINCIDE WITH A MID-LEVEL EYE THAT HAS BEEN

APPARENT IN RECENT MICROWAVE IMAGERY.  HOWEVER, THE MICROWAVE DATA

ALSO SUGGEST THAT IRWIN'S CIRCULATION IS TILTED FROM SOUTH TO NORTH

WITH HEIGHT, WITH THE LOW-LEVEL CENTER LOCATED SOUTH OF THE FEATURE

NOTED IN VISIBLE IMAGERY.  SUBJECTIVE DVORAK ESTIMATES HAVE RISEN

TO T3.5/55 KT FROM TAFB AND T4.0/65 KT FROM SAB, WHILE THE

OBJECTIVE ADT IS STILL DOWN AROUND T3.1/47 KT.  GIVEN THE CYCLONE'S

TILTED STRUCTURE, I'M COMFORTABLE GOING DOWN THE MIDDLE AND

INITIALIZING THE INTENSITY AT 55 KT.



VERTICAL SHEAR IS CURRENTLY LOW OVER IRWIN AND SHOULD REMAIN SO FOR

THE NEXT 24 HOURS OR SO.  THIS WILL LIKELY ALLOW IRWIN TO CONTINUE

STRENGTHENING, REACHING HURRICANE STATUS OVERNIGHT.  INCREASED

SHEAR, POSSIBLY ENHANCED BY HILARY'S OUTFLOW, COULD BEGIN TO

IMPINGE ON IRWIN AFTER 24 HOURS, WHICH WOULD LIKELY LIMIT THE

INTENSIFICATION TREND.  THE NEW INTENSITY GUIDANCE HAS COME DOWN A

BIT AFTER 24 HOURS, AND ONLY GRADUAL WEAKENING IS ANTICIPATED

THROUGH THE END OF THE FORECAST PERIOD AS IRWIN AND HILARY

INTERACT.  IRWIN WILL ALSO BE APPROACHING THE 26 DEG C SST ISOTHERM

IN ABOUT 5 DAYS, WHICH WILL ALSO FOSTER SOME WEAKENING.  THE

UPDATED NHC FORECAST IS CLOSE TO THE HCCA GUIDANCE, AND IT'S JUST

SLIGHTLY BELOW THE PREVIOUS FORECAST FROM 36 HOURS AND BEYOND.



IRWIN'S IS DRIFTING WESTWARD WITH AN INITIAL MOTION OF 280/3 KT.

AS IRWIN AND HILARY'S CIRCULATIONS GET CLOSER TOGETHER OVER THE