NHC 11 P.M. COUNTING ON 2ND TROUGH TO TURN ISABEL........

[dixiebreeze]

GAVE UP ON THE FIRST TROUGH:

Statement as of 11:00 PM EDT on September 13, 2003

satellite images show that cloud tops have warmed over the past 6
hours and Isabel may have weakened a little. But since the previous
advisory value of 140 knots was based on aircraft reconnaissance
and the next reconnaissance mission is at 06z...the official wind
speed is kept the same as the previous advisory value of 140 knots.

The global models generally show little change in wind speed for 5
days but the ships statistical dynamical model gradually weakens
Isabel to 70 knots in 5 days...as a result of increasing vertical
shear and climatology among other factors. The official forecast is
for weakening to 100 knots in 5 days...a compromise between ships
and the global models.

The initial motion is 290/11 based on the past 12 hours of motion.
The global models are in good agreement about a gradual turn toward
the northwest to north-northwest over the next 5 days. The
hurricane is forecast to move around the periphery of a deep-layer
anticyclone centered south of Nova Scotia. An approaching
short-wave trough moving across the eastern United States is
forecast to bypass Isabel on about day 3...but the next short-wave
trough should pick up the hurricane by about day 5. The official
forecast track is adjusted slightly to the east of the previous
advisory but is still close to the western-most guidance which is
the GFS model. The official track forecast shows a decrease in
forward speed for 3 days followed by an increase in forward speed
on day 4 and 5. Note that the 5 day forecast position is rather
close to the mid-Atlantic U.S. Coast.

Forecaster Lawrence

[dixiebreeze]

See related post by John about Isabel missing the first "bus."