WTPZ44 KNHC 260856 RRA

TCDEP4



HURRICANE HILARY DISCUSSION NUMBER  20

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP092017

300 AM MDT WED JUL 26 2017



THE STRUCTURE OF HILARY HAS CHANGED LITTLE DURING THE PAST SEVERAL

HOURS.  CONVENTIONAL SATELLITE IMAGERY SHOWS A CENTRAL DENSE

OVERCAST THAT IS SLIGHTLY ELONGATED NORTH-TO-SOUTH, MOST LIKELY DUE

TO THE IMPACT OF SOME NORTHERLY SHEAR.  AN EYE IS OCCASIONALLY

APPEARING IN INFRARED IMAGERY, AND MICROWAVE DATA CONTINUES TO

INDICATE AN EYE PRESENT UNDER THE OVERCAST.  HOWEVER, THE EYE HAS

NOT YET BECOME WELL-DEVELOPED ENOUGH TO JUSTIFY AN INCREASE IN THE

INITIAL INTENSITY, WHICH REMAINS 90 KT IN AGREEMENT WITH A BLEND OF

THE VARIOUS SATELLITE ESTIMATES.



THE INITIAL MOTION IS 285/11.  THE TRACK FORECAST IS ESSENTIALLY

UNCHANGED FOR THE NEXT 72 H, WHILE HILARY IS EXPECTED TO BE STEERED

BY THE MID-LEVEL SUBTROPICAL RIDGE TO THE NORTH.  AFTER THAT TIME,

THE GUIDANCE HAS BECOME INCREASINGLY DIVERGENT DUE TO DISAGREEMENTS

ON HOW MUCH A MID- TO UPPER-LEVEL TROUGH JUST WEST OF CALIFORNIA

AFFECTS THE RIDGE.  THE GFS USES THE TROUGH TO BREAK THE RIDGE AND

ALLOW HILARY TO MOVE ALMOST NORTHWARD BY THE END OF THE PERIOD,

WHICH IS A MAJOR SHIFT TO THE RIGHT SINCE THE PREVIOUS ADVISORY.

THE UKMET AND CANADIAN MODELS KEEP THE STRONGEST RIDGE AND FORECAST

A MORE WESTWARD MOTION.  THE ECMWF IS BETWEEN THESE EXTREMES, BUT

HAS SHIFTED NORTHWARD SINCE THE PREVIOUS ADVISORY.  AN ADDITIONAL

COMPLICATION IS THE POSSIBILITY OF INTERACTION WITH HURRICANE

IRWIN.  THE NEW GFS FORECAST CALLS FOR LITTLE INTERACTION, WHILE THE

UKMET AND CANADIAN MERGE THE TWO CYCLONES IN 3-4 DAYS AND THE ECMWF