11:00AM NHC Discussion

[TampaFl]

Hurricane Isabel Discussion Number 26


Statement as of 11:00 am EDT on September 12, 2003


Morning satellite imagery and microwave data from last night show
that Isabel has completed a concentric eyewall cycle. It now has a
well-defined 35 nm wide eye well embedded in the central dense
overcast. The cirrus outflow is still good in the southeast and
northwest quadrants...but is only fair to poor elsewhere and on the
whole looks less good than yesterday. Satellite intensity estimates
are all 140 kt...so that is the initial intensity. However...since
the time of the estimates there has been some warming of the
eyewall convective tops. NOAA and Air Force reserve hurricane
hunter aircraft will investigate the hurricane this afternoon.

The initial motion is 270/8. Isabel is currently south of a
low/mid-level ridge...with an anticyclone center in water vapor
imagery near 29n55w...and between large upper-level troughs along
68w and 41w. In the short term...this should produce a westward to
possibly west-northwestward motion. Further upstream...a deep
layer cyclonic circulation is over the southeastern United States
with a mid/upper-level ridge building over southeastern Canada.
The evolution of these features...along with the westerlies over
the central United States...will determine the long-range course of
Isabel. NHC track guidance generally agrees on a westward to
west-northwestward motion through 72 hr...and the official forecast
track follows along the south edge of the guidance. Considerable
divergence occurs after 72 hr. The UKMET calls for a gradual turn
toward the northwest by 120 hr toward a weakness in ridge left by
the deep-layer low as it lifts northward. On the other hand...the
GFS...NOGAPS...GFDL... Canadian...and ecmwf models all call for an
earlier and more northward motion. Since all models keep some deep
layer ridging over the northwest Atlantic north and northeast of
Isabel...so sharp of a turn looks a little dubious. Thus...the 3-5
day official forecast will lean toward the UKMET at a slower
forward speed.

Isabel is forecast to remain in a warm-water low-shear environment
for roughly the next 72 hr. During that time...the intensity
should be controlled mainly by hard-to-forecast eyewall replacement
cycles. The intensity forecast will thus call for a slow weakening
through that time based on continuity from the previous forecasts
and climatology. After 72 hr...the GFS and NOGAPS suggest that
Isabel may move north of a 200 mb anticyclone and get exposed to
some shear. The SHIPS model responds to this by forecasting 30 kt
of southwesterly shear by 120 hr. Based on this...the intensity
forecast will call for a somewhat faster weakening. It should be
noted that the large-scale models all forecast Isabel to grow in
size beyond 72 hr...which may be reflected in later wind radii
forecasts.
Given the divergence of the models at 120 hr...it is still to early
to even speculate which parts...if any...of eastern coast of the
United States may get affected by Isabel.

Forecaster Beven

forecast positions and Max winds

initial 12/1500z 21.6n 57.8w 140 kt
12hr VT 13/0000z 21.8n 59.1w 135 kt
24hr VT 13/1200z 22.2n 61.2w 135 kt
36hr VT 14/0000z 22.8n 63.4w 135 kt
48hr VT 14/1200z 23.4n 65.5w 130 kt
72hr VT 15/1200z 24.5n 69.5w 125 kt
96hr VT 16/1200z 25.5n 72.0w 110 kt
120hr VT 17/1200z 27.5n 75.0w 105 kt

[opera ghost]

Given the divergence of the models at 120 hr...it is still to early
to even speculate which parts...if any...of eastern coast of the
United States may get affected by Isabel.

I think this is the key for this afternoon- they clarified thier earlier statement and smacked in that "if any" this time.