WTPZ45 KNHC 231456 RRA

TCDEP5



TROPICAL STORM IRWIN DISCUSSION NUMBER   5

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP102017

800 AM PDT SUN JUL 23 2017



IRWIN'S CENTER IS EXPOSED TO THE NORTH AND WEST OF ITS ASSOCIATED

DEEP CONVECTION DUE TO ABOUT 15 KT OF NORTHERLY SHEAR.  SUBJECTIVE

AND OBJECTIVE DVORAK INTENSITY ESTIMATES ARE A CONSENSUS T2.5, SO

35 KT WILL REMAIN THE INITIAL INTENSITY FOR THIS ADVISORY.



THE INITIAL MOTION REMAINS WESTWARD AT 280/6 KT, WITH IRWIN LOCATED

SOUTH OF A WEAK LOW-LEVEL RIDGE AND ALMOST MIDWAY BETWEEN TROPICAL

STORMS GREG AND HILARY.  THIS ORIENTATION SHOULD CAUSE IRWIN TO

MAINTAIN A WESTWARD, ALBEIT MUCH SLOWER, MOTION DURING THE NEXT

SEVERAL DAYS.  HOWEVER, ONCE GREG DEGENERATES INTO A REMNANT LOW IN

ABOUT 4 DAYS, A BINARY INTERACTION BETWEEN A STRONGER IRWIN AND

HILARY IS MORE LIKELY TO OCCUR, WHICH COULD FORCE IRWIN TO TURN

WEST-SOUTHWESTWARD AT THE END OF THE FORECAST PERIOD.  THERE IS

SOME NOTABLE SPREAD IN THE GUIDANCE, HIGHLIGHTED BY THE GFS AND ITS

ENSEMBLE MEAN ON THE SOUTHERN END OF THE GUIDANCE ENVELOPE, AND THE

ECMWF AND ITS ENSEMBLE MEAN ON THE NORTHERN EDGE.  THE HWRF IS ALSO

NORTH OF MOST MODELS BY THE END OF THE FORECAST PERIOD, PRESUMABLY

BECAUSE IT IS NOT CAPTURING ANY INTERACTION BETWEEN IRWIN AND

HILARY.  THE UPDATED NHC FORECAST IS NUDGED A LITTLE SOUTH ON THIS

ADVISORY, AND IT IS SOUTH OF THE MODEL CONSENSUS AFTER DAY 3 TO

ACCOUNT FOR AN IRWIN-HILARY TRACK INTERACTION.



THE SHEAR OVER IRWIN IS EXPECTED TO DECREASE WITHIN THE NEXT 12-24

HOURS, WHICH SHOULD ALLOW THE CYCLONE TO BEGIN A SUSTAINED PERIOD

OF STRENGTHENING.  SEA SURFACE TEMPERATURES WILL BE BETWEEN 27-28