288
WTPZ42 KNHC 190252
TCDEP2
HURRICANE NORMA DISCUSSION NUMBER 6
NWS NATIONAL HURRICANE CENTER MIAMI FL EP172023
900 PM MDT WED OCT 18 2023
NORMA'S RAPID INTENSIFICATION CONTINUES. MICROWAVE IMAGERY FROM LATE
THIS AFTERNOON INDICATED THAT A VERY WELL-DEFINED LOW- TO MID-LEVEL
EYE HAS FORMED WITH A SURROUNDING RING OF SHALLOW TO MODERATE
CONVECTION ASSOCIATED WITH RAPID INTENSIFICATION. WHILE THAT EYE IS
NOT YET VISIBLE IN CONVENTIONAL SATELLITE IMAGERY, SUBJECTIVE
DVORAK ESTIMATES FROM TAFB AND SAB HAVE INCREASED TO 77 KT AND 90
KT, RESPECTIVELY, WHILE RECENT OBJECTIVE EVALUATIONS FROM UW-CIMSS
RANGE FROM ABOUT 75 KT TO 85 KT. BASED ON THE WHOLE OF THE DATA,
THE INTENSITY FOR NORMA HAS BEEN RAISED TO 80 KT.
FOR THE NEXT 24 TO 36 HOURS, THE FORECAST FOR NORMA APPEARS TO BE
RELATIVELY STRAIGHTFORWARD. CONTINUED RAPID STRENGTHENING APPEARS
LIKELY TONIGHT BASED ON NORMA'S CONVECTIVE STRUCTURE AND THE
CONDUCIVE SURROUNDING ENVIRONMENT. THE HURRICANE SHOULD CONTINUE
MOVING GENERALLY NORTHWARD DURING THAT TIME, STEERED BY A WEAK
MID-LEVEL RIDGE TO THE EAST OF NORMA AND AN UPPER-LEVEL TROUGH TO
ITS NORTHWEST. SHORT-TERM FLUCTUATIONS IN INTENSITY DUE TO EYEWALL
REPLACEMENT CYCLES WILL BECOME POSSIBLE STARTING TONIGHT, BUT
CONFIDENCE IN THE FORECAST IS REASONABLY HIGH THROUGH 36 H.
AFTER THAT TIME, THE UNCERTAINTY GROWS CONSIDERABLY. SOUTHWESTERLY
UPPER-LEVEL FLOW ASSOCIATED WITH THE AFOREMENTIONED UPPER-LEVEL
TROUGH WILL CONTINUE, BUT THE MID- TO LOWER-LEVEL STEERING WILL
LIKELY BECOME LIGHT OR EVEN SWITCH TO NORTHERLY. THE RESULTING
INCREASE IN SHEAR SHOULD CAUSE NORMA TO WEAKEN, BUT THE RATE OF THAT
WEAKENING REMAINS IN DOUBT. AS LONG AS THE HURRICANE RETAINS ITS
VERTICAL DEPTH, UPPER-LEVEL FLOW SHOULD KEEP IT MOVING NORTHWARD.
HOWEVER, ONCE NORMA WEAKENS TO THE POINT THAT ITS STEERING IS
DOMINATED BY LOW- TO MID-LEVEL FLOW, ITS FORWARD SPEED SHOULD SLOW
DOWN CONSIDERABLY. IT IS POSSIBLE, THEREFORE, THAT NORMA WILL STALL
JUST SOUTH OF BAJA CALIFORNIA PENINSULA, A SOLUTION FAVORED BY THE
DETERMINISTIC ECMWF AND A NUMBER OF ITS ENSEMBLE MEMBERS. HOWEVER,
THE HURRICANE MODELS (HAFS-A/B, HMON, AND HWRF), WHICH SHOULD
RESOLVE THE STRUCTURE OF NORMA BETTER THAN A GLOBAL MODEL, ALL
INDICATE THAT THE HURRICANE WILL RETAIN ITS DEPTH LONG ENOUGH TO
REACH THE BAJA CALIFORNIA PENINSULA. THE NHC TRACK AND INTENSITY
FORECASTS FAVOR THE REGIONAL MODELS, BLENDING THEIR LATEST CONSENSUS
WITH THE PREVIOUS FORECAST TO TRY TO MAINTAIN CONTINUITY. THE
UPDATED OFFICIAL TRACK FORECAST IS SLIGHTLY FASTER THAN THE PREVIOUS
ONE, BUT IS GENERALLY SIMILAR DESPITE THE HIGH UNCERTAINTY.
TROPICAL STORM OR HURRICANE WATCHES COULD BE REQUIRED FOR PORTIONS
OF THE BAJA CALIFORNIA PENINSULA LATER TONIGHT, AND INTERESTS THERE
SHOULD CONTINUE TO MONITOR UPDATES ON NORMA, ESPECIALLY SINCE
LARGER-THAN-NORMAL CHANGES TO THE FORECAST ARE POSSIBLE DUE TO THE
HIGH UNCERTAINTY IN THIS CASE.
KEY MESSAGES:
1. NORMA IS FORECAST TO CONTINUE RAPIDLY STRENGTHENING TONIGHT
WHILE IT PASSES WELL OFFSHORE OF SOUTHWESTERN MEXICO.
2. THERE IS A GREATER THAN NORMAL UNCERTAINTY IN THE TRACK AND
INTENSITY FORECASTS FOR NORMA LATER THIS WEEK AND THIS WEEKEND WHEN
IT IS EXPECTED TO APPROACH THE BAJA CALIFORNIA PENINSULA AND
WESTERN MEXICO. REGARDLESS OF NORMA'S EXACT TRACK OR INTENSITY,
THERE IS AN INCREASING THREAT OF WIND AND RAINFALL IMPACTS TO
SOUTHERN PORTIONS OF BAJA CALIFORNIA SUR BY SATURDAY, AND WATCHES
COULD BE REQUIRED THERE LATER TONIGHT OR ON THURSDAY.
FORECAST POSITIONS AND MAX WINDS
INIT 19/0300Z 16.4N 108.0W 80 KT 90 MPH
12H 19/1200Z 17.2N 108.1W 100 KT 115 MPH
24H 20/0000Z 18.3N 108.4W 100 KT 115 MPH
36H 20/1200Z 19.3N 108.9W 90 KT 105 MPH
48H 21/0000Z 20.6N 109.5W 80 KT 90 MPH
60H 21/1200Z 22.1N 109.8W 70 KT 80 MPH
72H 22/0000Z 23.0N 109.8W 65 KT 75 MPH
96H 23/0000Z 23.5N 109.5W 60 KT 70 MPH
120H 24/0000Z 24.0N 108.5W 45 KT 50 MPH
FORECASTER D. ZELINSKY
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