WTPZ42 KNHC 170244 RRA

TCDEP2



TROPICAL STORM NORMA DISCUSSION NUMBER  11

NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172017

900 PM MDT SAT SEP 16 2017



NORMA'S STRUCTURE HASN'T CHANGED OVER THE PAST 6 HOURS.  A SPRAWLING

BAND OF DEEP CONVECTION EXTENDS WELL OUT FROM THE CENTER, BUT THE

TROPICAL STORM STILL APPEARS TO LACK AN INNER CORE.  THE INTENSITY

HAS BEEN HELD AT 55 KT BASED ON THE DVORAK CURRENT INTENSITY

ESTIMATE FROM TAFB, BUT THAT COULD BE GENEROUS GIVEN THAT THE

EARLIER RECONNAISSANCE FLIGHT STRUGGLED TO FIND WINDS THAT HIGH, AND

THE THE FINAL-T NUMBER FROM TAFB WAS A LITTLE LOWER.



NORMA HAS CONTINUED MOVING SLOWLY NORTH, BUT A TURN TOWARD THE

NORTH-NORTHWEST IS EXPECTED OVERNIGHT.  THE GLOBAL MODELS AGREE FOR

THE NEXT 2 DAYS THAT A DEEP-LAYER RIDGE EXTENDING WESTWARD OVER

CENTRAL MEXICO WILL KEEP NORMA ON A NORTH-NORTHWEST OR NORTHWEST

HEADING.  BEYOND THAT TIME, THERE IS CONSIDERABLE SPREAD IN THE

GUIDANCE.  THE ECMWF FORECASTS THAT THE RIDGE WILL WEAKEN

SUFFICIENTLY TO CAUSE NORMA TO STALL OFF THE COAST OF THE BAJA

CALIFORNIA PENINSULA.  ON THE OTHER HAND, THE GFS MAINTAINS THE

RIDGE LONG ENOUGH TO FORCE NORMA FARTHER NORTH, AND INTO A FAIRLY

DRY AND STABLE ENVIRONMENT. THE OFFICIAL FORECAST SPLITS THESE

SCENARIOS, AND IS BASED MOSTLY ON THE TVCX CONSENSUS AID.



RECENT TOTAL PRECIPITABLE WATER IMAGERY SUGGESTS THAT SOME DRY AIR

MAY BE WRAPPING INTO THE CIRCULATION, AND THIS WILL LIKELY

CONTRIBUTE TO FURTHER WEAKENING.  THE RELIABLE INTENSITY MODELS ALL

INDICATE THAT STEADY WEAKENING WILL OCCUR THROUGHOUT THE FORECAST

PERIOD.  BY DAY 5, SSTS BELOW 26 C AND VERY DRY AIR SHOULD CAUSE THE