WTPZ45 KNHC 232032 RRA

TCDEP5



TROPICAL STORM IRWIN DISCUSSION NUMBER   6

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP102017

200 PM PDT SUN JUL 23 2017



DEEP CONVECTION HAS COVERED UP IRWIN'S CENTER SINCE THE LAST

ADVISORY, A SIGN THAT THE SHEAR AFFECTING THE SYSTEM CONTINUES TO

DIMINISH.  DVORAK INTENSITY ESTIMATES HAVE INCREASED TO T3.0/45 KT

FROM TAFB AND SAB, BUT OBJECTIVE NUMBERS ARE STILL AT T2.5/35 KT.

BASED ON THESE ESTIMATES, AND A 1748 UTC ASCAT-B PASS, THE INITIAL

INTENSITY IS RAISED TO 40 KT.



IRWIN IS MOVING WESTWARD, OR 270/7 KT, BETWEEN TROPICAL STORMS GREG

AND HILARY AND TO THE SOUTH OF A WEAK LOW- TO MID-LEVEL RIDGE

EXTENDING WEST OF THE BAJA CALIFORNIA PENINSULA.  SINCE THE RIDGE

IS EXPECTED TO REMAIN WEAK, IRWIN IS FORECAST TO MOVE ONLY SLOWLY

WESTWARD THROUGH 48 HOURS.  AFTER THAT TIME, ITS MOTION IS LIKELY

TO BECOME INCREASINGLY INFLUENCED BY HILARY, WITH THE TWO BEGINNING

SOME DEGREE OF A BINARY INTERACTION.  THE TRACK GUIDANCE NOW

INDICATES THAT IRWIN WILL TURN WEST-SOUTHWESTWARD ON DAYS 3 AND 4,

AND THEN POSSIBLY TURN BACK TO THE NORTHEAST AROUND HILARY'S

SOUTHERN SIDE ON DAY 5.  WITH A SHIFT IN THE OVERALL GUIDANCE

ENVELOPE, THE NEW NHC TRACK FORECAST HAS BEEN SHIFTED SOUTHWARD

TOWARD HCCA AND THE TVCN MULTI-MODEL CONSENSUS, AND IT ALSO NOW

SHOWS IRWIN STALLING OR DRIFTING NORTHWARD ON DAY 5.



IRWIN IS LOCATED OVER WARM SEA SURFACE TEMPERATURES AND IN A

RELATIVELY MOIST ENVIRONMENT, AND THE NORTHERLY SHEAR AFFECTING THE

CYCLONE SHOULD CONTINUE TO DECREASE OVER THE NEXT DAY OR SO.

THEREFORE, ADDITIONAL GRADUAL STRENGTHENING IS ANTICIPATED, AND

IRWIN HAS THE POSSIBILITY OF REACHING HURRICANE STRENGTH IN ABOUT