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)
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