first, in response to someone asking if the trough goes further down would it turn the storm more:
if the trough digs more, it would imply more amplified ridging upstream and a further west track. the trough would have to slip by to the north and erode the northwest side of the ridge to do that.. with a strong hurricane like katrina on the ridge periphery pumping the ridge and trying to force itself northward, i doubt you'll see the track shift much to the right.
the symmetry on katrina is excellent, but when you look at the inner core it's still kind of uneven. there's finally an eye trying to redefine itself on satellite, but the inner core bands are still uneven.. there isn't that circular eye and solid convective ring that is indicative of a strong four/five. i'm not certain that katrina will develop that traditional structure, since thus far it's kept a sloppy inner core even with lessening, weak shear and a barometric pressure more often associated with a category 4.
invest 90L is moving faster and on a lower trajectory than i'd anticipated. it will be to the islands in three days if it keeps a similar pace and track... the upper air environment currently involves some weak easterly shear.. but that should lessen as it gets further west. that's gonna be lee. 97L seems to have given up on developing (shear has pretty much decoupled the low and the vorticity perturbation aloft driving that convective burst coincident with the system's track).
irwin is looking rather pathetic in the eastpac... and may be the last in the burst they've had over there for a while. i'm not sure but the atlantic may start revving down for a while... will look into this when a major hurricane isn't threatening NO/biloxi anymore.
HF 0426z28august