Statement as of 5:00 am AST on August 20, 2009
during the last pass through the northeastern eyewall...an Air
Force Reserve hurricane hunter aircraft reported 123 kt winds at
700 mb...which is down almost 20 kt from earlier. An eyewall
dropsonde suggests the surface winds in that area were 105-110 kt.
Based on this and a slight weakening of the satellite cloud
pattern...the initial intensity is reduced to 110 kt. Analyses
from CIMSS at the University of Wisconsin show that Bill has been
experiencing 10-15 kt of westerly vertical wind shear. This may be
starting to diminish...as the cirrus outflow is increasing in the
western semicircle.
The initial motion is 305/16. Bill is moving toward a weakness in
the subtropical ridge caused by two mid/upper-level troughs...one
to the east-northeast of Bermuda and one between Bermuda and the
United States. The second trough is forecast to move southward and
westward during the next 24-48 hr...causing Bill to gradually turn
more northward as the systems interact. After 48 hr...all guidance
forecasts a large deep-layer trough to move into the eastern United
States...with a large mid/upper-level ridge building over the
central Atlantic. This combination should steer Bill northward and
eventually northeastward into the westerlies. The track guidance
is tightly clustered for the first 48-72 hr...then become somewhat
divergent in both direction and speed. The GFDN and NOGAPS are on
the left side of the guidance envelope...bringing the center of
Bill just southeast of New England and then over Nova Scotia and
Newfoundland. The corrected consensus models call for a much
sharper eastward turn...keeping the center well away from New
England and Canada. The track forecast is between these extremes
and down the middle of the guidance envelope in best overall
agreement with the GFDL. It should be noted that Bill will
probably not turn as sharply between 72-96 hr as implied by the
track graphic...and thus is likely to pass closer to New England
than the graphic would suggest.
The large-scale models suggest the current shear should diminish
during the next 24 hr...and this basis some re-intensification is
expected. The intensity forecast is most uncertain during the
24-72 hr time frame as Bill interacts with the current western
Atlantic trough. This trough could cause shear to weaken the
system. However...some of the large-scale models forecast
increased upper-level divergence over Bill...which could strengthen
the hurricane. For now...the intensity forecast calls for little
change in strength as a compromise. After 72 hr...Bill should
weaken rapidly as it moves over colder sea surface temperatures and
into strong vertical wind shear. Extratropical transition should
begin around 96 hr and be complete by 120 hr.
Large swells generated by this hurricane will affect the northern
Leeward Islands...the Bahamas...Bermuda and most of the eastern
U.S. Coast during the next few days. These swells will likely
cause extremely dangerous surf and life-threatening rip currents.
Please consult statements issued by your local weather office for
more details.
Forecast positions and Max winds
initial 20/0900z 21.6n 60.3w 110 kt
12hr VT 20/1800z 23.0n 62.3w 115 kt
24hr VT 21/0600z 25.2n 64.7w 120 kt
36hr VT 21/1800z 27.8n 66.5w 120 kt
48hr VT 22/0600z 30.8n 68.1w 120 kt
72hr VT 23/0600z 38.5n 68.0w 105 kt
96hr VT 24/0600z 46.6n 59.0w 70 kt
120hr VT 25/0600z 53.5n 36.0w 55 kt...extratropical
$$
forecaster Beven
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