Alex was the first tropical cyclone of the 1998 season
and remained over the open waters of the tropical Atlantic throughout its lifetime.
a. Synoptic History
A well-organized tropical wave emerged from the west coast of Africa on 26 July
and moved westward at 15 to 20 knots. Early on 27th, ship reports and satellite scatterometer winds supported the presence
of a surface circulation in association with the wave. On this basis, it is estimated that the system attained
tropical depression status around 1200 UTC 27 July while located about 300 n mi south-southwest
of the Cape Verde Islands (Figure 1) (21K GIF)
The depression changed little in organization on 27 July and most of the 28th, with minimal deep
convection near the center, while moving on a general west-northwestward track at 15 to 20 knots. During this period, satellite imagery
characterized the depression as a large and elongated circulation which was still involved with the Intertropical
Convergence Zone. By the evening of the 28th, deep convection increased near the center
and meteorologists from both the Tropical Analysis and Forecast Branch (TAFB) at the Tropical Prediction
Center and the Satellite Analysis Branch (SAB)
from the National Environmental Satellite, Data, and Information Service assigned Dvorak
T-numbers of 2.5, i.e., 35 knots. Consequently, the system was upgraded to
Tropical Storm Alex at 0000 UTC 29 July.
Alex continued to move on a general west to west-northwest course at 10 to 15 knots in response to a deep-layer ridge
over the tropical eastern Atlantic. During the next several days, Alex's development was hampered by a mid- to upper-level
trough, and attendant cyclonic circulation, located to its north and west. By 30 July, satellite imagery indicated that Alex
was experiencing southerly vertical wind shear. During the evening of the 30th, satellite imagery showed a burst of deep
convection just east of the center. It is estimated that Alex reached a peak intensity of 45 knots from
1800 UTC 30 July to 0600 UTC 31 July, and a minimum central pressure of 1002 mb near 0000 UTC 31 July. Shortly thereafter, increased
southerly vertical wind shear induced by the mid- to upper-level tropospheric trough to the west of Alex curtailed further
strengthening.
Over the next few days the vertical wind shear took its toll with the low-level center of Alex becoming fully exposed
south of the remaining deep convection on 1 August. Alex turned toward the northwest later that day and continued to gradually
weaken. Alex was downgraded to a depression by midday on the 2nd. Later that afternoon, data from an
Air Force Reserve Hurricane Hunter
reconnaissance aircraft showed that the system no
longer had a closed low-level circulation, and the system dissipated.
b. Meteorological Statistics
The "best-track" intensities in Table 1 were obtained from the
data in Figures 2 (21K GIF) and 3 (24K GIF) which depict
the curves of minimum central sea-level pressure (mb) and maximum sustained one-minute average "surface" (10 meters
above ground level) wind speed, respectively, as a function of time. These figures also contain data upon which the curves
are based; United States Air Force Reserve (USAFR) aircraft reconnaissance data, satellite-based
Dvorak-technique intensity estimates from TAFB,
SAB, and
Air Force Global Weather Agency (AFGWA in figures).
There were no surface observations of tropical storm force winds in association with Alex.
c. Casualty and Damage Statistics
Alex is not known to have caused any casualties or damages.
d. Forecast and Warning Critique
The official track forecast errors, listed in Table 2, were below the most recent 10-year average.
The average 12-, 24-, 36-, 48-, and 72-hr official forecast errors and associated number of cases (in parenthesis) were 44 (17),
75 (15), 90 (13), 96 (11), and 136 (7) n mi respectively. The 36 to 72 hr errors are 31% to 45% lower than the most recent 10-year
average track errors.
The average 12-, 24-, 36-, 48-, and 72-hr official intensity forecasts ranged from about 5 to 10 knots between 12
and 36 hours, near 20 knots at 48 hours, and 25 knots at 72 hours. The 12 to 36 hr errors are 20% to 30% lower than the most
recent 10-year average intensity errors while the 72 hr errors were 10-15 % higher. SHIFOR had the best performance
scores with average intensity errors generally less than 10 knots. The 12 to 48 hr SHIPS errors were generally below the
official forecast while the GFDL and GFDI had lower errors only at 48 hr.
With the exception of SHIFOR, all the 72 hr model errors were larger than official forecast errors.
No watches or warnings were issued for Alex.
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