WTPZ45 KNHC 300238 RRA

TCDEP5



HURRICANE ROSA DISCUSSION NUMBER  20

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP202018

800 PM PDT SAT SEP 29 2018



RECENT MICROWAVE IMAGES INDICATE THAT ROSA'S STRUCTURE HAS BEGUN TO

DETERIORATE AS A RESULT OF INCREASING SOUTHERLY SHEAR.  DESPITE THE

COLD CLOUD TOPS NOTED IN INFRARED IMAGERY, THE HURRICANE'S EYEWALL

IS OPEN ON THE SOUTH SIDE, AND THERE IS VERY LITTLE CONVECTIVE

BANDING WITHIN THE SOUTHERN HALF OF THE CIRCULATION.  THE VARIOUS

SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES HAVE EITHER HELD

STEADY OR FALLEN A BIT FROM SIX HOURS AGO, AND GIVEN THE MICROWAVE

SIGNATURE, THE INITIAL INTENSITY IS LOWERED TO 85 KT.



ROSA IS MOVING NORTHWARD, OR 360 DEGREES AT 10 KT, ALONG THE

WESTERN EDGE OF A SUBTROPICAL RIDGE WHICH EXTENDS ACROSS NORTHERN

MEXICO.  THE CYCLONE IS FORECAST TO TURN NORTH-NORTHEASTWARD AND

ACCELERATE SLIGHTLY DURING THE NEXT 48-72 HOURS AS IT IS STEERED

BETWEEN THE RIDGE AND A LARGE MID- TO UPPER-LEVEL TROUGH LOCATED

OFF THE WEST COAST OF THE UNITED STATES.  THE TRACK GUIDANCE IS

CLUSTERED FAIRLY TIGHTLY, ALTHOUGH THE ECMWF IS NOTABLY SLOWER THAN

THE REST OF THE MODELS, TAKING A LITTLE LONGER TO BRING ROSA'S

CENTER TO THE BAJA CALIFORNIA PENINSULA.  THE UPDATED NHC TRACK

FORECAST WAS NUDGED SOUTHEASTWARD BEYOND 36 HOURS TO ACCOUNT FOR

THE LATEST MODEL GUIDANCE, AND IT SHOWS ROSA REACHING THE COAST IN

ABOUT 48 HOURS.  A 72-HOUR FORECAST IS PROVIDED FOR CONTINUITY, BUT

ROSA'S SURFACE CIRCULATION IS LIKELY TO DISSIPATE BEFORE THAT TIME

OVER NORTHWESTERN MEXICO OR SOUTHERN ARIZONA, WITH THE MID-LEVEL

REMNANTS CONTINUING NORTHWARD ACROSS THE DESERT SOUTHWEST AND