12 September 2003

Isabel becomes a CAT5 hurricane...

At 21Z yesterday, Isabel became the first Category 5 hurricane in the
Atlantic since Mitch 1998.  A few hours after reaching this level, she
underwent an eyewall replacement cycle and now has a larger eye, still
clear, and showing some wonderful features (mesovortices) as described
in http://einstein.atmos.colostate.edu/~mcnoldy/papers/KMS2002_MWR.pdf 
(coincidentally, Hurricane Erin '01 also had some beautiful mesovortices
in her eye on 9/11)

At 15Z today, Isabel was located at 21.6N 57.8W and tracking slowly W at
8kts.  Intensity is 140kts and 924mb.  NHC's forecast does not keep her
at CAT5 status, but rather just a bump down to 135kts (CAT4).  However,
keep in mind that a CAT5 is an extreme, and few forecasters are willing
to forecast extremes.  From this point on, aircraft will be flying the
storm to get in-situ intensity measurements; up until now, it's all been
satellite estimates.  I would not be surprised if the aircraft currently
in there finds a much lower pressure, perhaps 912mb or so.

Her track has shifted to due west now, and will be crossing over the
"remnants" of Fabian's cool wake Saturday afternoon, which could
temporarily disrupt the intensity.  However, on the west side of the
wake, SSTs are warmer than they are now, so rapid recovery is likely.

Given the ideal conditions, it's not impossible that Isabel could
accumulate a large number of hours as a CAT5.  Since 1947, there have
been only 20 hurricanes to reach CAT5 strength, and only 6 have lasted
longer than 24 hours as a CAT5.  It's very difficult for a storm to
maintain this intensity for very long because it pushes the envelope of
what our atmosphere can provide.  Any flaw in conditions such as
decreased SST, increased vertical shear, change in direction, etc
usually causes a drop in intensity.

Again, for some stunning 1-minute imagery of Isabel, visit
http://www.cira.colostate.edu/ramm/rmsdsol/RSOMAIN.HTML and hit the GOES
East Visible Floater Loop link.  This marks the 4th consecutive day of
GOES-12 SRSO on Isabel.  (GOES-12 is the satellite, and SRSO is Super
Rapid Scan Operations, meaning 1-minute imagery over a small predefined
area, such as a hurricane)

Please visit my tropical Atlantic headquarters.

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