Reports from Air Force reserve and NOAA hurricane hunter aircraft confirm that Isabel is a category 5 hurricane. Maximum observed flight-level winds so far are 156 kt at 700 mb...supporting surface winds near 140 kt. The eye is 30 nm wide with a closed Wall and a minimum pressure of 920 mb. The initial intensity thus remains 140 kt...which agrees with satellite intensity estimates. It should be noted that Isabel looks a little less organized in satellite imagery than it did 24 hr ago. Thus it might have been a little stronger yesterday. The eye has recently nudged to the right...but the longer term motion remains 270/8. There is little change in the synoptic pattern or reasoning from the previous forecast. Isabel is currently south of a deep layer ridge...with an anticyclone seen in water vapor imagery near 29n55w...and upper-level troughs along 68w and 42w. In the short term...this should produce a westward to possibly west-northwestward motion. Upstream...a deep layer cyclonic circulation is over the southeastern United States with a mid/upper-level ridge building over southeastern Canada. Large-scale guidance forecasts the southeastern system to move northward...leaving a weakness on the southwest side of the Canadian ridge as it builds eastward into the Atlantic. NHC track guidance agrees on a westward to west-northwestward motion through 72 hr... and to that time the forecast track is an update of the previous package. Considerable divergence occurs after 72 hr. The UKMET calls for a gradual turn toward the northwest by 120 hr in response to the weakness. The GFS...NOGAPS...GFDL...and Canadian models all call for a 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 continues to look a little dubious. Thus...the 3-5 day official forecast will lean toward the UKMET. It should be noted that the UKMET is calling for more northward motion after 96 hr than forecast earlier...and the official forecast is thus shifted to the right of the previous forecast after 72 hr. Given the divergence of the models at 120 hr...it is still too early to speculate which parts...if any...of eastern coast of the United States may get affected by Isabel. 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 eyewall replacement cycles. The intensity forecast will thus call for a slow weakening through that time based on continuity from previous forecasts and climatology. After 72 hr...the GFS and ships models suggest Isabel may be affected by southwesterly vertical shear. Based on this...the intensity forecast will call for a somewhat faster weakening after 72 hr. Wind radii have been revised on the basis of aircraft data. It should be noted that large-scale models all forecast Isabel to grow in size beyond 72 hr even if the inner core weakens. Forecaster Beven |