Bill became the first
hurricane of the 1997 season.
It spent its brief life over waters the north Atlantic Ocean.
a. Synoptic History
Bill developed from a large upper-level low that separated
from the mid-oceanic trough northeast of Puerto Rico. On 7 July,
satellite images indicate that cloudiness and showers associated
with the upper- level low began to increase and although surface
pressures were quite high north of Puerto Rico there was a small
perturbation of the wind field and a trough at the surface. A
low pressure center formed from the trough just east of the
Bahamas and moved toward the west-northwest. The upper-level low
moved southwestward into the Caribbean Sea resulting in a
decrease in the wind shear over the surface low. The first
indications that a tropical depression
might be forming was a 24-hour pressure drop of near 3 mb in the eastern
Bahamas as the area of low pressure approached. Convection then gradually
became organized and it is estimated that a tropical depression
formed near 0600 UTC 11 July. By then, the
was already moving northeastward ahead of a cold front located over
the eastern United States. The system reached
status by 1200 UTC on the same day.
was dispatched to the area early on 11 July and measured
at 700 feet to the southeast of the center. The minimum surface pressure was
1013 mb, which is not very low for a tropical cyclone but environmental
pressures were also high. The prevailing pressure gradient would support
tropical storm force winds.
Bill continued moving toward the northeast about 20 to 25
knots and reached cool waters. An eye was depicted
on high resolution visible images at 1300 UTC 12 July, suggesting that
Bill reached hurricane strength in spite of the cool waters. A
special Dvorak classification
from the Tropical Analysis and Forecast Branch
(TAFB) indicated that Bill reached its peak intensity of
65 knots at 1500 UTC 12 July. The minimum
pressure estimated at that time was 986 mb. Thereafter, Bill became
absorbed by a frontal system and was no longer identifiable by
0600 UTC 13 July.
Bill's track is shown in Fig. 1. (18K GIF)
Table 1 is a listing, at
six-hourly intervals, of the best-track position,
estimated minimum central pressure and maximum 1-minute surface wind speed.
b. Meteorological Statistics
The best track pressure and wind curves as a function of time
are shown in Fig. 2 (12K GIF) and 3
(10K GIF) and are based on reconnaissance and surface observations, satellite intensity
estimates from TAFB, the
Satellite Analysis Branch
(SAB) and the Air Force Global
Weather Center (AFGWC). The latter reported an intermittent eye
feature as early as 0415 UTC 12 July.
c. Casualty and Damage Statistics
There are no reports of casualties or damage associated with
d. Forecast and Warning Critique
Bill was never forecast to become a hurricane mainly because
it was expected to move over cool waters. Neither SHIPS97 nor
SHIFOR indicated that Bill would reach hurricane strength.
Bill was a short-lived storm and there were only a few
official track forecasts. The official forecast errors (5
forecasts) at 12 hours were 66 n mi and reached 303 n mi at 36
hours (1 forecast). These numbers are high in comparison to the
long-term mean but in general the largest errors occur when
systems are embedded in the westerlies and accelerating like
Bill. The CLIPER error at 36 hours was 451 n mi.
Since most of the tropical storm force winds were to the east
of the center of Bill and the storm was forecast to pass not too
far from Bermuda, a tropical storm warning was issued for
Bermuda at 1600 UTC 11 July. It was discontinued at 0300 UTC 12 July
after the storm passed by the island. Bermuda did not report
tropical storm force winds.