WTPZ45 KNHC 230853 RRA

TCDEP5



TROPICAL STORM IRWIN DISCUSSION NUMBER   4

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP102017

200 AM PDT SUN JUL 23 2017



AN ASCAT-B OVERPASS AT 0516 UTC SHOWED SEVERAL VECTORS OF 32-33 KT

WINDS AND A MAX VALUE OF 34 KT IN THE NORTHEAST QUADRANT OF THE

TROPICAL CYCLONE.  BASED ON THAT DATA, THE SYSTEM HAS BEEN NAMED

TROPICAL STORM IRWIN.  IRWIN IS THE FIFTH SYSTEM TO BE NAMED IN THE

EASTERN NORTH PACIFIC SO FAR IN JULY.



ALTHOUGH THE SYSTEM HAS BEEN UPGRADED BASED ON THE ASCAT DATA, THE

STRUCTURE HAS NOT IMPROVED.  SHORTWAVE-IR IMAGERY SHOWS THAT THE

LOW-LEVEL CENTER IS CLEARLY EXPOSED TO THE NORTHWEST OF THE DEEP

CONVECTION, AND DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB HAVE

NOT INCREASED.  THE GLOBAL MODELS SUGGEST THAT THE SHEAR SHOULD

DECREASE QUITE A BIT AFTER ABOUT 12 TO 24 HOURS, WHICH SHOULD ALLOW

FOR SOME INTENSIFICATION.  THE REGIONAL DYNAMICAL MODELS HWRF AND

COAMPS-TC DEPICT IRWIN QUICKLY RECOVERING FROM ITS CURRENT SHEARED

STATE AND INTENSIFYING TO A CATEGORY 2 OR 3 HURRICANE.  ON THE OTHER

HAND, THE STATISTICAL MODELS STILL ONLY SHOW MODEST STRENGTHENING,

AND KEEP IRWIN BELOW HURRICANE STRENGTH.  AS A COURSE OF LEAST

REGRET, THE INTENSITY FORECAST SPLITS THESE TWO SCENARIOS, BUT

SLIGHTLY FAVORS THE LOWER STATISTICAL GUIDANCE SINCE THE DYNAMICAL

MODELS MAY SHOW THE STRUCTURE OF IRWIN RECOVERING FROM THE CURRENT

SHEAR TOO QUICKLY.  NEAR THE END OF THE FORECAST PERIOD, IRWIN IS

EXPECTED TO ENCOUNTER A HIGHER SHEAR ENVIRONMENT, DUE IN PART TO

OUTFLOW FROM HILARY TO THE EAST, SO THE FORECAST DEPICTS GRADUAL

WEAKENING.



THE INITIAL MOTION IS 280/6 KT.  IRWIN CONTINUES TO BE STEERED BY A