Hurricane Florence was a category one hurricane (on the
Saffir-Simpson Hurricane Scale) of subtropical
origins that passed within about 65 n mi
of Bermuda.
a. Synoptic History
During the first week of September a cold front edged off the North American
continent and became stationary over the western subtropical Atlantic. A
weak wave along the front amplified slowly on the 8th and
9th of September, and then began to deepen more rapidly on the
10th underneath an upper-level cold low. During the day the flow
around the developing low became less wavelike and more circular, and it is
estimated that the low had lost its surface frontal structure and become a
subtropical depression by 1800 UTC on the 10th, when it was about
325 miles west-southwest of Bermuda. At this time convection was present
west of and close to the circulation center, but very limited in areal
coverage. It is presumed that the upper circulation was still cold-core,
hence the subtropical designation. After 0000 UTC on the 11th,
the convection began to increase in depth and coverage as it rotated to the
south side of the low-level circulation center. An Advanced Microwave
Sounder Unit (AMSU) temperature cross-section analysis from the Cooperative
Institute for Research in the Atmosphere (CIRA) suggests a weak warm core
had developed by 1243 UTC on the 11th, and it is estimated that
the subtropical depression had become tropical (or at least sufficiently so
to warrant the designation) by 0600 UTC. The "best track" chart of the
tropical cyclone's path, including the subtropical stage, is given in
Figure 1, with the wind and pressure histories shown in
Figure 2 and Figure 3, respectively.
The best track is listed in Table 1.
The convective burst early on the 11th continued to rotate around
to the east side of the low-level circulation center where it was associated
with a rapid increase in wind speed. The depression reached tropical storm
strength by 1200 UTC and hurricane strength by 1800 UTC that day, the latter
event occurring when Florence was about 425 n mi west-southwest of Bermuda.
However, the strong winds were confined to a very small area near the edge
of the convection, and after 0000 UTC on the 12th, when the
cyclone entrained some drier mid-tropospheric air and the convection
collapsed, Florence's winds temporarily weakened below hurricane strength. A
reconnaissance aircraft later on the 12th reported pressure falls,
the development of an elliptical eyewall, and a return of hurricane force
winds.
On the 11th, Florence moved very slowly westward under the
influence of a middle-level ridge along the mid-Atlantic coast; however,
this flow was soon balanced by ridging to the southeast of the cyclone and
Florence moved little from the 12th to the 14th. By
1200 UTC on the 13th central pressures began to rise and Florence
again weakened to a tropical storm, due in part perhaps to local reduction
of the sea-surface temperatures under the hurricane. (Sea-surface
temperature analyses from Johns Hopkins University Applied Physics
Laboratory show decreases of up to 6C near the cyclone center during this
time.) North-northwesterly shear began to increase as well on the
14th, and the maximum winds dropped to 40 kt.
An approaching short-wave trough in the westerlies broke the steering
stalemate and began to accelerate Florence to the east-northeast on the
15th. With most of the convection and strong winds in its
southeast quadrant, Florence strengthened late on the 15th, turned
rapidly to the northeast, and attained hurricane strength for the third time
at 0000 UTC on the 16th, when it was about 175 n mi west-southwest
of Bermuda. Florence made its closest approach to Bermuda, passing about
65 n mi to the northwest of the island, around 0800 UTC on the
16th. Later on the 16th an eye was briefly apparent as
Florence reached its peak intensity of 70 kt. Weakening ensued over cooler
waters, and Florence became a tropical storm for the fourth time at 0000 UTC
on the 17th, when it was about 425 n mi northeast of Bermuda.
After 1800 UTC on the 17th, when it was about 125 n mi south of
St. Johns Newfoundland, the circulation of Florence was absorbed by the
extratropical surface low associated with the short wave trough.
b. Meteorological Statistics
Observations in Florence (Figure 2 and
Figure 3) include satellite-based 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. Although not included in
Figure 2, a
QuikSCAT analysis from 1014 UTC 11 September suggests that the maximum winds
at that time were at least 40 kt, and is the basis for the designation of
the cyclone as a tropical storm. Florence was upgraded to a hurricane based
on an aircraft reconnaissance report of 79 kt at a flight altitude of 1500
ft, a speed which corresponds to 67 kt at the surface using the standard
reduction for that altitude (85%). Although not reflected in the best
track, the flight meteorologist made a visual (sea-state) surface wind
estimate of 80 kt shortly thereafter. Florence's second upgrade to
hurricane strength was based on a visual estimate of 65-70 kt over a
substantial area. The timing of the peak intensity (70 kt) was based on
satellite imagery showing the brief appearance of an eye.
Ship reports of winds of tropical storm force associated with Florence are
given in Table 2. The report from ELVO3 has been
questioned and is not reflected in the best track. Bermuda reported a
maximum sustained wind of
36 kt with a peak gust of 50 kt at 0400 UTC on the 16th. The
minimum pressure observed there was 1007.5 mb, and the storm total
precipitation was 0.47 inches.
c. Casualty and Damage Statistics
Rip currents associated with Florence were blamed for three surf deaths in
North Carolina on the 12th. There were no reports of damage
associated with Florence.
d. Forecast and Warning Critique
Average official track errors (with the number of cases in parentheses) for
Florence were 42 (24), 78 (22), 118 (20), 142 (18), and 168 (14) n mi for
the 12, 24, 36, 48, and 72 h forecasts, respectively. These errors are
comparable to, or smaller than the average official track errors for the
10-yr period 1990-1999 (46, 85, 122, 158, and 235 n mi, respectively).
However, Figure 4 shows that the earlier forecasts
tended to begin the eastward acceleration too soon, and did not anticipate
the southward component of motion associated with the northwesterly shearing
flow. The official forecasts were better than the interpolated GFDL and
UKMET guidance. The forecasts from the Aviation model (AVNI) were very
good, and on average, better than the official forecast at all times
except 72 h (Table 3).
Average official intensity errors were 8, 13, 16, 16, and 12 kt for the 12,
24, 36, 48, and 72 h forecasts, respectively. For comparison, the average
official intensity errors over the 10-yr period 1990-1999 are 7, 11, 13, 16,
and 19 kt, respectively. Florence's intensity was consistently
over-forecast during the first half of the cyclone's existence when it was
moving more slowly than forecast. The intensification to hurricane strength
as Florence accelerated northeastward was under-forecast.
Table 4 lists the watches and warnings associated with
Florence. The tropical storm watch and warnings for Bermuda were issued
approximately 38 and 20 hours in advance, respectively, of the observed
sustained tropical storm force winds. Although a hurricane warning was
posted for Bermuda, only tropical storm conditions occurred there.
Table 1.
Best track for Hurricane Florence, 10-17 September 2000.
Date/Time (UTC) | Position |
Pressure (mb) |
Wind Speed (kt) | Stage |
Lat. (°N) | Lon. (°W) |
10 / 1800 | 30.9 | 70.9 | 1007 | 30 | subtropical depression |
11 / 0000 | 30.8 | 71.3 | 1007 | 30 | " |
11 / 0600 | 30.7 | 71.8 | 1006 | 30 | tropical depression |
11 / 1200 | 30.4 | 72.2 | 1002 | 45 | tropical storm |
11 / 1800 | 30.1 | 72.6 | 998 | 65 | hurricane |
12 / 0000 | 30.1 | 72.7 | 992 | 65 | " |
12 / 0600 | 30.2 | 72.8 | 993 | 60 | tropical storm |
12 / 1200 | 30.3 | 73.1 | 991 | 60 | " |
12 / 1800 | 30.6 | 73.3 | 987 | 65 | hurricane |
13 / 0000 | 30.8 | 73.7 | 986 | 65 | " |
13 / 0600 | 30.7 | 74.0 | 986 | 65 | " |
13 / 1200 | 30.7 | 73.8 | 987 | 60 | tropical storm |
13 / 1800 | 30.5 | 73.7 | 989 | 55 | " |
14 / 0000 | 30.2 | 73.6 | 991 | 50 | " |
14 / 0600 | 29.6 | 73.6 | 993 | 45 | " |
14 / 1200 | 29.5 | 73.4 | 994 | 45 | " |
14 / 1800 | 29.3 | 73.1 | 995 | 45 | " |
15 / 0000 | 29.2 | 72.8 | 995 | 45 | " |
15 / 0600 | 29.1 | 72.4 | 996 | 45 | " |
15 / 1200 | 29.8 | 71.2 | 997 | 50 | " |
15 / 1800 | 30.1 | 69.7 | 997 | 60 | " |
16 / 0000 | 30.8 | 67.5 | 994 | 65 | hurricane |
16 / 0600 | 32.6 | 66.1 | 988 | 65 | " |
16 / 1200 | 34.3 | 64.2 | 987 | 65 | " |
16 / 1800 | 36.1 | 61.8 | 985 | 70 | " |
17 / 0000 | 37.9 | 59.5 | 990 | 60 | tropical storm |
17 / 0600 | 40.1 | 57.4 | 995 | 55 | " |
17 / 1200 | 42.5 | 55.0 | 1000 | 50 | " |
17 / 1800 | 45.5 | 53.0 | 1002 | 50 | " |
18 / 0000 | | absorbed by extratropical low |
|
16 / 1800 | 36.1 | 61.8 | 985 | 70 | minimum pressure |
Table 2.
Selected ship reports with winds of at least 34 kt for Hurricane Florence,
10-17 September 2000.
Ship Call Sign
|
Date/ Time (UTC) |
Lat. (°N) |
Lon. (°W) |
Wind dir/speed (deg/kt) |
Pressure (mb) |
11 / 0000 | ELVO3 | 31.7 | 72.2 | 050/39 | 1009.0 |
16 / 1800 | WWXA | 39.8 | 58.0 | 170/35 | 1006.1 |
17 / 0000 | WWXA | 40.1 | 58.4 | 160/49 | 1005.2 |
17 / 1200 | C6JS | 41.8 | 56.8 | 000/34 | 1007.0 |
17 / 1800 | C6JS | 42.2 | 54.2 | 320/34 | 1011.0 |
Table 3.
Preliminary forecast evaluation (heterogeneous sample) for Hurricane
Florence, 10-17 September 2000. 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. NGPI and GUNS forecasts are not included due to their small
sample sizes.
Forecast Technique |
Period (hours) |
12 | 24 | 36 | 48 | 72 |
CLIP | 62 (24) | 143 (22) | 247 (20) | 360 (18) | 481 (14) |
GFDI | 42 (24) | 84 (22) | 131 (20) | 173 (18) | 242 (14) |
LBAR | 42 (24) | 81 (22) | 127 (20) | 156 (18) | 219 (14) |
AVNI | 34 (23) | 49 (21) | 76 (19) | 118 (17) | 203 (13) |
BAMD | 42 (24) | 74 (22) | 117 (20) | 169 (18) | 331 (14) |
BAMM | 40 (24) | 67 (22) | 93 (20) | 130 (18) | 293 (14) |
BAMS | 56 (24) | 91 (22) | 120 (20) | 147 (18) | 231 (14) |
UKMI | 62 (21) | 95 (20) | 113 (18) | 162 (18) | 169 (8) |
NHC Official | 42 (24) | 78 (22) | 118 (20) | 142 (18) | 168 (14) |
NHC Official (1990-1999 mean) | 46 (2057) | 85 (1842) | 122 (1650) | 158 (1471) | 235 (1164) |
Table 4.
Watch and warning summary for Hurricane Florence, 10-17 September 2000.
Date/Time (UTC) |
Action |
Location |
14 / 1500 | Tropical Storm Watch issued | Bermuda |
15 / 0900 | Tropical Storm Warning issued | Bermuda |
15 / 2130 | Hurricane Warning replaces Tropical Storm Warning | Bermuda |
16 / 0900 | Tropical Storm Warning replaces Hurricane Warning | Bermuda |
16 / 1500 | Tropical Storm Warning Discontinued | Bermuda |
Figure 1.
Best track positions for Hurricane Florence, 10-17 September 2000.
Figure 2.
Best track maximum sustained surface wind speed curve for Hurricane
Florence, 10-17 September 2000, and the observations on which the best track
curve is based. Aircraft observations have been adjusted for elevation
using 90%, 80%, and 85% 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).
Figure 3.
Best track minimum central pressure curve for Hurricane Florence, 10-17
September 2000, and the observations on which the best track curve is based.
Figure 4.
Official track forecasts from 0000 and 1200 UTC for the period 11-15
September (dashed lines, with 0, 12, 24, 36, 48, and 72 h positions
indicated) for Hurricane Florence. The best track is given by the thick
solid line with positions given at 6 h intervals.