11pm TPC discussion

[wow]

HURRICANE ISABEL DISCUSSION NUMBER 16
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 PM EDT TUE SEP 09 2003

THERE HAS BEEN SOME EROSION OF THE EYEWALL CONVECTION IN THE EASTERN
SEMICIRCLE AND THE EYE HAS BECOME LESS DISTINCT OVER THE PAST FEW
HOURS. WATER VAPOR IMAGERY ALSO INDICATES THAT THE OUTFLOW TO THE
NORTHEAST OF ISABEL IS BECOMING SLIGHTLY RESTRICTED. DVORAK T
NUMBERS ARE 5.5 FROM TAFB AND SAB...AND THE 3-HR AVERAGE RAW ODT IS
ALSO DOWN TO 5.4. THE INITIAL INTENSITY WILL BE HELD AT 115 KT FOR
THIS ADVISORY.

THE INITIAL MOTION ESTIMATE...BASED ON THE LAST 12 HOURS...IS
295/12...ALTHOUGH OVER THE PAST THREE HOURS THE MOTION HAS BEEN
TOWARD THE WEST. THIS IS QUITE POSSIBLY THE BEGINNING OF THE
EXPECTED WESTWARD TURN...BUT THREE HOURS OF MOTION IS NOT ENOUGH TO
BE SURE...ESPECIALLY GIVEN THE DEGRADATION OF THE EYE OBSERVED
DURING THIS PERIOD. REGARDLESS...ISABEL IS SOUTH OF A MID-LEVEL
RIDGE OF HIGH PRESSURE THAT SHOULD FORCE THE HURRICANE ON A
GENERALLY WESTWARD TRACK OVER THE NEXT THREE DAYS. AFTER THAT...
GLOBAL MODELS GENERALLY SHOW A BREAK IN THE RIDGE BETWEEN 75 AND
80W THAT WOULD ALLOW A TURN BACK TO THE WEST-NORTHWEST ON DAYS 4
AND 5 OF THE FORECAST.

FOR THE LAST COUPLE OF DAYS...THE GUIDANCE ENVELOPE HAS BEEN
CONSISTENTLY DEFINED BY THE UKMET ON THE NORTH AND THE GFS/NOGAPS
ON THE SOUTH. AMONG THE GLOBAL MODELS...THE UKMET HAS BEEN THE BEST
PERFORMER SO FAR WITH ISABEL AND THE GFS HAS BEEN THE WORST. THE
UKMET ALSO WAS THE BEST OF THE GLOBAL MODELS FOR FABIAN. THE
INITIAL VORTEX FOR ISABEL IN THE GFS HAS BEEN CONSISTENTLY MUCH
WEAKER THAN THE INITIALIZATION IN THE UKMET...ALTHOUGH I AM NOT
CERTAIN THAT THIS EXPLAINS THE DIFFERENCES BETWEEN THE TWO MODELS.
GIVEN THE RECENT MOTION...I HAVE NUDGED THE OFFICIAL FORECAST TRACK
A LITTLE SOUTH OF THE PREVIOUS ADVISORY. THE NEW TRACK IS IN BEST
AGREEMENT WITH THE GFDL...AND SLIGHTLY ON THE UKMET SIDE OF THE
GLOBAL MODEL CONSENSUS.

ENVIRONMENTAL CONDITIONS SHOULD BE GENERALLY FAVORABLE FOR THE NEXT
SEVERAL DAYS...SO LITTLE OVERALL CHANGE IN STRENGTH IS FORECAST.
HOWEVER...FLUCTUATIONS IN INTENSITY ARE COMMON IN MAJOR HURRICANES
AND THESE ARE GENERALLY VERY DIFFICULT TO FORECAST.

FORECASTER FRANKLIN

FORECAST POSITIONS AND MAX WINDS

INITIAL 10/0300Z 20.6N 48.8W 115 KT
12HR VT 10/1200Z 21.3N 50.4W 115 KT
24HR VT 11/0000Z 21.5N 52.3W 115 KT
36HR VT 11/1200Z 21.5N 53.9W 115 KT
48HR VT 12/0000Z 21.5N 55.5W 115 KT
72HR VT 13/0000Z 21.5N 58.5W 115 KT
96HR VT 14/0000Z 22.0N 61.5W 110 KT
120HR VT 15/0000Z 23.0N 65.0W 110 KT

[stormernie]

Forecast for next 3 days probably good, adjustments will be made in 4-5 outlook as the weekend approaches. 23N 65W seems to far north.

[wow]

any of you think she'll make it to cat 5 in her lifetime, or will she peak at 135mph?

[CocoaBill]

I say 135 is as far as she goes!

[JetMaxx]

A couple thoughts...

1) IMO the NHC forecast is too far north and too slow. Under a strengthening ridge, the tendency is normally an increase in forward speed...not a decrease. I expect a motion toward the west at least 10-12 mph or 4-5 degrees in longitude a day. I'm not sure Isabel will move due west yet, but believe the turn has begun -- and doubt the hurricane will be farther north than 21.0 N in 96 hours; and won't be surprised if it comes back below 21.0 (moving 260° for awhile between 55-65W longitude).

2) As for intensity....has Isabel peaked? In the short term I vote yes...but in the long term, I see this hurricane coming through the Turks & Caicos Islands and southern Bahamas toward Southern Florida. Those waters are very warm, and infamous for making major hurricanes into monsters...plus we have a 86° Gulf Stream between Andros Island and Miami. I'm no longer certain Isabel will reach cat-5 status, but still believe 150 mph or more isn't unreachable....especially if the track is toward the Florida Keys.