TROPICAL CYCLONE REPORTS
Tropical Storm Allison
Tropical Depression Two
Tropical Storm Barry
Tropical Storm Chantal
Tropical Storm Dean
Tropical Depression Nine
Tropical Storm Jerry
Tropical Storm Lorenzo
Tropical Cyclone Report
21 - 27 September 2001
James L. Franklin
National Hurricane Center
30 October 2001
Humberto was a category 2 hurricane (on the Saffir-Simpson
Hurricane Scale) that passed about 120 n mi to the west of
a. Synoptic History
Humberto had its origin in a trough of low pressure that
extended southwestward from the circulation of Hurricane Gabrielle.
On 18 September, a westward-moving upper-level low passed over the
surface trough about 600 n mi south-southeast of Bermuda and
enhanced deep convection there. A weak low formed in this area the
following day and began to drift westward. The system gradually
became better organized, and Dvorak classifications were initiated
early on 20 September. The system became Tropical Depression Ten
near 1200 UTC 21 September, when deep convection developed near the
center of a broad cyclonic circulation defined by low-cloud vectors
in the satellite imagery, about 425 n mi south of Bermuda.
The "best track" chart of the tropical cyclone's path is given
in Figure 1, with the wind and pressure histories
shown in Figure 2
and Figure 3, respectively. The best track positions and
listed in Table 1Table 1.
Moving to the northwest at about 8 kt, the
depression's surface circulation remained weak and disorganized on
21 September despite a rapidly improving and impressive appearance
in satellite imagery. Surface development appeared to catch up to
the satellite signature the next day, however, and the depression
is estimated to have reached tropical storm strength by 1200 UTC 22
September, about 275 n mi south-southwest of Bermuda. Moving around
the periphery of the subtropical ridge, Humberto turned to the
north-northwest and gradually strengthened, becoming a hurricane at
1200 UTC 23 September, about 150 n mi west-southwest of Bermuda.
Humberto then turned to the north-northeast and reached its first
peak in intensity, 85 kt and 983 mb, shortly thereafter at 0000 UTC
Strong upper-level westerlies began to shear the system, and
within 12 h the estimated maximum surface winds in Humberto had
fallen to 70 kt. However, once Humberto passed north of the
subtropical ridge, it turned to the northeast and accelerated
within a more vertically uniform steering current and the weakening
trend slowed. With a large mid- to upper-level cutoff low over the
Midwestern states, downstream ridging caused Humberto to slow on 25
September. An eye was then apparent intermittently on satellite
images. Humberto moved underneath the downstream ridge and turned
briefly northward early the next day. At this point the eye became
much better defined and Humberto quickly strengthened over water
temperatures no higher than 25-26C, reaching its peak intensity of
90 kt and 970 mb at 1200 UTC 26 September, about 175 n mi
south-southeast of Sable Island.
The ridging that had been sheltering Humberto quickly collapsed
and upper-level westerly flow again impinged on the cyclone.
Humberto turned northeastward again and began to accelerate over
colder waters, weakening rapidly. Moving eastward at 28 kt,
Humberto weakened to a tropical storm at 1200 UTC 27 September
about 350 n mi southeast of Cape Race Newfoundland. As Humberto was
beginning to lose its tropical characteristics, its cloud pattern
becoming distorted and separated from the low-level wind center,
the cyclone dissipated when its weakening circulation degenerated
into an open trough shortly after 1800 UTC 27 September.
b. Meteorological Statistics
Observations in Humberto (Figure 2 and Figure 3)
Dvorak technique intensity estimates from the Tropical Analysis and
Forecast Branch (TAFB), the Satellite Analysis Branch (SAB) and the
U. S. Air Force Weather Agency (AFWA), as well as flight-level and
dropwindsonde observations from flights of the 53rd
Weather Reconnaissance Squadron of the U. S. Air Force Reserve
Command, and NOAA/Hurricane Research Division flights of the WP-3D
aircraft flown by the NOAA Aircraft Operations Center.
The first reconnaissance mission in the depression near 1800 UTC
21 September was not able to formally close off the center;
however, the data did suggest that a small circulation was present.
Subsequent reconnaissance missions indicated that Humberto's
circulation remained small. When the reconnaissance aircraft
entered then-Tropical Depression Ten near 1800 UTC 22 September,
they found a peak flight-level (1500 ft) wind of 57 kt (estimated
surface-equivalent of 46 kt), and visually estimated the surface
winds to be 55-60 kt (Figure 2). It is therefore assumed that
Humberto had reached tropical storm strength by 1200 UTC that day.
The first peak intensity of 85 kt at 0000 UTC 24 September is based
in part on a dropwindsonde spot surface report of 87 kt. While
surface winds derived from dropsonde layer averages are lower,
suggesting that this surface value may not have been
representative, the surface report is consistent with data from the
Stepped Frequency Microwave Radiometer (SFMR) on board the NOAA
WP-3D research aircraft (Figure 4).
Humberto's peak intensity of 90
kt on 26 September is based on Dvorak satellite classifications
from TAFB and SAB.
Ship reports of winds of tropical storm force associated with
Humberto are given in Table 2. Bermuda experienced a maximum
sustained wind of 24 kt, with gusts to 37 kt, when Humberto passed
120 n mi to the west of the island. Bermuda also reported a storm
total rainfall of 1.69 inches.
c. Casualty and Damage Statistics
There were no reports of damage or casualties associated with
d. Forecast and Warning Critique
Average official track errors (with the number of cases in
parentheses) for Humberto were 41 (20), 76 (18), 121 (16), 125
(14), and 176 (10) n mi for the 12, 24, 36, 48, and 72 h forecasts,
respectively. These errors are comparable to the previous 10-year
averages for 12-36 h, and less than the 10-year averages at 48 and
72 h (Table 4). A number of models outperformed the official
forecast out to 36 hours, including the barotropic models and the
Aviation (AVNI). However, none of the guidance models outperformed
the official forecast for 72 h.
As was the case for track, the longer-range intensity forecasts
were better than the shorter-range forecasts, relative to the
long-term means. Average official intensity errors were 9, 14, 15,
14, and 13 kt for the 12, 24, 36, 48, and 72 h forecasts,
respectively. For comparison, the average official intensity errors
over the 10-yr period 1991-2000 are 7, 11, 14, 16, and 20 kt,
respectively. For the most part the intensity of Humberto was
under-forecast, with biases of 10-15 kt from 24-72 h. The SHIPS
guidance had similar mean errors but smaller biases than the
official forecast, while the GFDI and AVNI also had significant
under-forecast biases. Figure 5 shows selected official intensity
forecasts for Humberto. The first intensification period was
modestly under-forecast, but the second intensification was missed
entirely, even after it was underway.
There were no watches or warnings associated with Humberto.
Table 1: Best track for Hurricane Humberto, 21 - 27 September 2001.
|Lat. (°N)||Lon. (°W)
|21 / 1200||25.1||64.2||1012||25||tropical depression
|21 / 1800||25.8||65.0||1012||25||"
|22 / 0000||26.5||65.5||1010||25||"
|22 / 0600||27.2||65.9||1010||30||"
|22 / 1200||27.9||66.3||1007||40||tropical storm
|22 / 1800||28.6||66.7||998||50||"
|23 / 0000||29.3||67.0||995||55||"
|23 / 0600||30.1||67.3||994||55||"
|23 / 1200||31.0||67.5||990||65||hurricane
|23 / 1800||31.9||67.4||986||80||"
|24 / 0000||32.8||67.1||983||85||"
|24 / 0600||33.8||66.8||989||75||"
|24 / 1200||35.0||66.3||990||70||"
|24 / 1800||35.9||65.4||991||70||"
|25 / 0000||36.8||64.3||992||65||"
|25 / 0600||37.7||63.4||992||65||"
|25 / 1200||38.5||62.4||992||65||"
|25 / 1800||39.2||61.6||990||65||"
|26 / 0000||39.6||60.7||990||65||"
|26 / 0600||40.4||60.3||980||75||"
|26 / 1200||41.0||59.2||970||90||"
|26 / 1800||41.8||57.5||977||80||"
|27 / 0000||42.2||54.6||987||70||"
|27 / 0600||42.4||51.3||991||65||"
|27 / 1200||42.2||47.5||994||60||tropical storm
|27 / 1800||41.5||44.5||996||45||"
|28 / 0000||dissipated
|26 / 1200||41.0||59.2||970||90||minimum pressure
Table 2: Selected ship observations of tropical storm or greater
winds associated with Hurricane Humberto, 21 - 27 September 2001.
|Ship Name or Call Sign||Date/Time (UTC)||Lat. (°N)||Lon. (°W)||Wind dir/speed (deg/kt)||Pressure (mb)
Table 3: Table 3. Preliminary track forecast evaluation (heterogeneous
sample) for Hurricane Humberto, 21-27 Sept. 2001. Forecast errors
for tropical storm and hurricane stages (n mi) are followed by the
number of forecasts in parentheses. Errors smaller than the NHC
official forecast are shown in bold-face type.
|Forecast Technique||Period (hours)
|CLIP||51 (19)||111 (17)||131 (15)||143 (14)||193 (10)
|GFDI||48 (20)||88 (18)||112 (16)||135 (14)||203 (10)
|LBAR||26 (19)||65 (17)||99 (15)||154 (14)||179 (10)
|AVNI||42 (20)||73 (18)||105 (16)||133 (14)||220 (10)
|BAMD||26 (19)||45 (17)||75 (15)||105 (14)||227 (10)
|BAMM||34 (19)||66 (17)||104 (15)||132 (14)||192 (10)
|BAMS||52 (19)||101 (17)||144 (15)||187 (14)||255 (10)
|NGPI||49 (19)||102 (17)||162 (15)||262 (13)||489 (9)
|UKMI||53 (19)||96 (17)||91 (15)||103 (13)||267 (5)
|GUNS||46 (19)||83 (17)||93 (15)||126 (13)||201 (5)
|NHC Official||41 (20)||76 (18)||121 (16)||125 (14)||176 (10)
|NHC Official (1991-2000 mean)||44 (2049)||82 (1835)||118 (1646)||151 (1475)||226 (1187)
|*Output from these models was unavailable at time of forecast issuance.
Best track positions for Hurricane Humberto, 21-27 Sept. 2001.
Best track maximum sustained surface wind speed curve for Hurricane
Humberto, 21-27 Sept. 2001, and the observations on which the best track
curve is based. Aircraft observations have been adjusted for elevation using
90%, 80%, and 80% reduction factors for observations from 700 mb, 850 mb,
and 1500 ft, respectively. Dropwindsonde observations include actual 10 m
winds (sfc), as well as surface estimates derived from the mean wind over
the lowest 150 m of the wind sounding (LLM), and from the sounding boundary
layer mean (MBL).
Best track minimum central pressure curve for Hurricane Humberto, 21-27
Sept. 2001, and the observations on which the best track curve is based.
Horizontal wind profiles of flight-level (grey line) and SFMR-derived
surface wind (heavy black line) during a northeast to southwest traverse
across the core of Hurricane Humberto from 2304-2358 UTC 23 September 2001.
Flight level of the NOAA WP-3D aircraft was 850 mb. Diagram courtesy of Dr.
Peter Black, NOAA/Hurricane Research Division.
Selected official intensity forecasts (dashed lines) for Hurricane Humberto,
21-27 Sept. 2001. The best track intensity is given by the thick solid line.