Tropical Cyclone Report

Hurricane Helene

(AL082006)

12-24 September 2006

Daniel P. Brown

National Hurricane Center

15 November 2006

Helene was a long-lived Cape Verde hurricane that remained at sea and
attained category 3 intensity on the Saffir-Simpson Hurricane Scale.

a.  Synoptic History

    Helene developed from a vigorous tropical wave and broad area of low
    pressure that emerged from the coast of Africa on 11 September.
    Surface observations over western Africa indicate that this wave was
    accompanied by 24 h pressure falls of 3 to 4 mb. Shower and
    thunderstorm activity associated with the broad area of low pressure
    quickly increased and by 1200 UTC 12 September the system showed
    enough organization to be classified as a tropical depression, the
    eighth of the 2006 Atlantic hurricane season, while centered
    approximately 200 n mi south-southeast of the Cape Verde Islands.

    The "best track" chart of the tropical cyclone's path is given in
    Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3,
    respectively. The best track positions and intensities are listed in
    Table 1. As the center of the depression passed about 165 n mi south
    of the Cape Verde Islands the circulation remained rather large and
    broad, which along with some easterly shear likely delayed
    strengthening during the early stage of the system. Convective
    banding, however, began to increase over the northwestern semicircle
    of the circulation on 13 September and the depression slowly
    strengthened, becoming a tropical storm at 0000 UTC 14 September
    while centered about 425 n mi west-southwest of the Cape Verde
    Islands.

    Moving west-northwestward over the tropical Atlantic Ocean, Helene
    steadily intensified and became a hurricane at 1200 UTC 16
    September, while located about 1000 n mi east of the northern
    Leeward Islands. The next day Helene turned northwestward and slowed
    down, due in part to a weakness in the subtropical ridge created by
    Hurricane Gordon, which was centered a little over 1000 n mi
    northwest of Helene. Helene continued to strengthen, attaining
    category 3 status at 0000 UTC 18 September, and six hours later it
    reached its peak intensity of 105 kt. At this time, Helene and
    Gordon were at nearly the same longitude, with Gordon centered about
    875 n mi north of Helene.

    Once Gordon moved east of Helene's longitude late on 18 September, a
    narrow mid-to upper-level ridge built to the north of Helene, which
    caused the hurricane to turn westward on 19 September. During this
    time microwave satellite data indicated that Helene's inner eyewall
    gradually deteriorated (Fig. 4), and Helene weakened to a category 2
    hurricane. On 20 September, Helene turned northward ahead of a large
    deep-layer trough that was exiting the east coast of the United
    States. Moderate southwesterly shear ahead of this trough induced
    additional weakening and Helene was reduced to a category one
    hurricane while passing about 475 n mi east of Bermuda early on 21
    September. Thereafter, Helene turned east-northeastward over the
    open waters of the central Atlantic and retained hurricane strength
    until becoming extratropical by 1800 UTC 24 September about 275 n mi
    northwest of the Azores.

    On 25 and 26 September, the extratropical cyclone moved
    northeastward and weakened to a gale center before passing very near
    the west coast of Ireland on 27 September. A few hours later, the
    low merged with a larger extratropical low over the north Atlantic
    around 0000 UTC 28 September, just before reaching northwestern
    Scotland.

b.  Meteorological Statistics

    Observations in Helene (Figs. 2 and 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 several NOAA P-3 research
    missions into Helene. These missions were part of the Saharan Air
    Layer Experiment (SALEX) and the NOAA Ocean Winds Experiment.
    Stepped-Frequency Microwave Radiometer (SFMR) data (10 s averages)
    were also available on four consecutive days. Data from the NOAA
    aircraft missions were very useful in ascertaining the intensity of
    Helene during 17-20 September. Microwave satellite imagery from NOAA
    polar-orbiting satellites, the NASA Tropical Rainfall Measuring
    Mission (TRMM), the NASA QuikSCAT, and Defense Meteorological
    Satellite Program (DMSP) satellites were also useful in tracking
    Helene and assessing its internal structure.

    The strongest winds measured by a NOAA reconnaissance aircraft were
    111 kt at a flight level of 850 mb in the eastern eyewall at 1831
    UTC 17 September. Using a standard eyewall adjustment factor (ratio
    of surface to flight-level winds of 0.8) yields a surface wind of 89
    kt. This wind estimate is in good agreement with both a dropsonde
    that recorded a 90 kt surface wind and an SFMR maximum wind of 88 kt
    on this flight. The 105 kt peak intensity of Helene, which occurred
    about 12 h later at 0600 UTC 18 September, is based on a blend of
    subjective Dvorak intensity estimates.

    Since the NOAA research mission on 18 September was flown at a
    flight level of 500 mb, obtaining surface winds from this
    higher-than-normal flight level is problematic, as it is not
    possible to confidently adjust winds from this altitude to the
    surface. The SFMR instrument on this flight recorded a peak surface
    wind of 86 kt and a minimum pressure of 963 mb was reported from the
    aircraft center fix. Since this flight only made three passes
    through the center, it may not have adequately sampled the
    hurricane's strongest winds. Therefore, the 95 kt maximum wind speed
    estimate at 1800 UTC 18 September is from a blend of satellite
    estimates and the aircraft data.

    On 19 September, the NOAA aircraft observed maximum flight-level
    (between 850 and 700 mb) winds of 98 kt at 1726 UTC, and at nearly
    the same time the SFMR measured maximum surface winds of 83 kt. The
    NOAA SALEX flight on 20 September provided two center "fixes", but
    it may not have adequately sampled the inner core to determine the
    maximum winds.

    QuikSCAT data on 22-23 September support the analysis of Helene as a
    hurricane until its extratropical transition on 24 September. A
    QuikSCAT overpass at 0916 UTC 23 September indicated a large area of
    hurricane-force winds over the southwestern semicircle of the
    circulation with maximum winds around 80 kt. Since there was no deep
    convection over this portion of the circulation, these winds did not
    suffer from rain contamination and are likely representative of the
    maximum winds at that time.

    Ship and buoy reports of winds of tropical storm force associated
    with Helene are given in Table 2. A few hours before Helene became
    extratropical it passed within about 20 n mi of drifting buoy 44613
    (40.3°N 39.6°W), which reported a minimum pressure of 971.8 mb at
    0900 UTC 24 September. Later that day, drifting buoy 44774 (43.5°N
    35.8°W) reported a minimum pressure of 967.0 mb at 2300 UTC as the
    center of extratropical Helene passed nearly directly over that
    buoy.

    After losing tropical characteristics, the system produced
    gale-force wind gusts across much of Ireland and northwestern
    Scotland on 27-28 September. The highest reported wind gust in
    Ireland was 49 kt at the Valentia Observatory, located on the
    extreme southwestern coast. A wind gust to 64 kt was reported on
    South Uist Island in the Outer Hebrides of western Scotland.

c.  Casualty and Damage Statistics

    There were no reports of damage or casualties associated with
    Helene.

d.  Forecast and Warning Critique

The genesis of Helene was fairly well anticipated. Mention of the
tropical wave from which Helene developed was introduced into the
Atlantic Tropical Weather Outlook as it emerged from the coast of Africa
about 24 h prior to tropical cyclone formation. Customarily, NHC waits
until a tropical wave has fully emerged from the coast of Africa before
mentioning the possibility of development. The outlooks prior to the
development of Helene discussed the potential for genesis, but they did
not specifically mention tropical depression formation until a few hours
prior to formation. It should be noted that many of the global forecast
models accurately predicted the genesis of Helene.

A verification of official and guidance model track forecasts is given
in Table 3. Average official track errors for Helene were 27, 42, 61,
86, 124, 161, and 192 n mi for the 12, 24, 36, 48, 72, 96, and 120 h
forecasts, respectively. The number of forecasts ranged from 47 at 12 h
to 29 at 120 h. These errors are considerably lower than the average
long-term official track errors for the 5-year period 2001-2005 (Table
3). The track errors were exceptionally small as forecasts accurately
predicted most of Helene's path over the deep tropical Atlantic.
Dynamical track models, however, did not correctly forecast the
reduction in Helene's forward speed and its turn to the northwest on 17
and 18 September. It appears that the global models did not accurately
forecast weakening of the subtropical ridge that was located between
Hurricanes Gordon and Helene. During this time the global models also
did not properly predict the recurvature of Helene, and the official 4
and 5-day track forecasts had track errors that were much larger than
the 5-year mean. The track models had a westward or left bias during
this period. Helene's recurvature along 57°W longitude was not captured
well by any of the global models at the extended forecast ranges. The
GFS and UKMET models indicated that Helene would continue westward to
west-northwestward for a longer period of time, suggesting that the
hurricane could become a possible threat to Bermuda. Meanwhile, the GFDL
model was more accurate in forecasting Helene to turn northward between
57°W and 60°W longitude, and the official track forecast leaned closer
to this solution. Overall, the official forecasts had smaller errors at
120 h than all of the available guidance, and at 96 h only the Florida
State Superensemble (FSSE) and the CONU and GUNA consensus models beat
the NHC track forecasts.

Average official intensity errors were 7, 10, 12, 12, 11, 11, and 10 kt
for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively (Table
4). These forecasts are comparable to the average long-term official
intensity errors (Table 4) through 48 hours, but they are much lower
than the average long-term errors at 72, 96, and 120 h. During the
depression stage, the official forecasts predicted the rate of Helene's
strengthening fairly accurately. Intensity forecasts on 17 and 18
September, when Helene was near peak intensity, predicted additional
strengthening to a category 4 hurricane, which did not materialize.

Table 1. Best track for Hurricane Helene, 12-24 September 2006.

+-----------+---------+---------+------------+--------+--------------+
| Date/Time | L       | Lo      | Pressure   | Wind   | Stage        |
|           | atitude | ngitude |            | Speed  |              |
| (UTC)     |         |         | (mb)       |        |              |
|           | \(N\)   | \(W\)   |            | (kt)   |              |
+-----------+---------+---------+------------+--------+--------------+
| 12 / 1200 | 11.9    | 22.0    | 1007       | 25     | tropical     |
|           |         |         |            |        | depression   |
+-----------+---------+---------+------------+--------+--------------+
| 12 / 1800 | 11.9    | 23.2    | 1007       | 30     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 13 / 0000 | 11.9    | 24.6    | 1007       | 30     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 13 / 0600 | 12.0    | 26.1    | 1007       | 30     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 13 / 1200 | 12.2    | 28.0    | 1007       | 30     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 13 / 1800 | 12.5    | 30.0    | 1006       | 30     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 14 / 0000 | 12.9    | 31.9    | 1005       | 35     | tropical     |
|           |         |         |            |        | storm        |
+-----------+---------+---------+------------+--------+--------------+
| 14 / 0600 | 13.2    | 33.8    | 1005       | 35     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 14 / 1200 | 13.6    | 35.6    | 1003       | 40     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 14 / 1800 | 14.0    | 37.0    | 1003       | 40     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 15 / 0000 | 14.4    | 38.3    | 1002       | 40     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 15 / 0600 | 14.8    | 39.6    | 1000       | 45     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 15 / 1200 | 15.5    | 40.8    | 997        | 50     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 15 / 1800 | 16.2    | 42.1    | 995        | 55     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 16 / 0000 | 17.0    | 43.3    | 992        | 60     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 16 / 0600 | 17.7    | 44.3    | 990        | 60     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 16 / 1200 | 18.4    | 45.3    | 987        | 65     | hurricane    |
+-----------+---------+---------+------------+--------+--------------+
| 16 / 1800 | 18.9    | 46.2    | 986        | 65     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 17 / 0000 | 19.4    | 47.1    | 983        | 70     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 17 / 0600 | 19.9    | 47.9    | 979        | 75     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 17 / 1200 | 20.3    | 48.4    | 976        | 80     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 17 / 1800 | 20.8    | 48.8    | 970        | 90     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 18 / 0000 | 21.4    | 49.1    | 962        | 100    | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 18 / 0600 | 22.0    | 49.4    | 955        | 105    | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 18 / 1200 | 22.7    | 49.7    | 958        | 105    | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 18 / 1800 | 23.6    | 50.4    | 962        | 95     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 19 / 0000 | 24.0    | 51.0    | 962        | 95     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 19 / 0600 | 24.3    | 51.7    | 960        | 95     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 19 / 1200 | 24.4    | 52.5    | 958        | 95     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 19 / 1800 | 24.4    | 53.5    | 956        | 90     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 20 / 0000 | 24.5    | 54.4    | 958        | 90     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 20 / 0600 | 24.9    | 55.2    | 958        | 90     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 20 / 1200 | 25.4    | 56.0    | 959        | 90     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 20 / 1800 | 25.9    | 56.7    | 960        | 80     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 21 / 0000 | 26.8    | 57.0    | 962        | 80     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 21 / 0600 | 27.8    | 57.0    | 964        | 75     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 21 / 1200 | 29.0    | 57.0    | 968        | 75     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 21 / 1800 | 30.2    | 57.0    | 970        | 75     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 22 / 0000 | 31.4    | 56.6    | 970        | 75     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 22 / 0600 | 32.5    | 55.9    | 970        | 70     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 22 / 1200 | 33.6    | 55.1    | 970        | 70     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 22 / 1800 | 34.9    | 53.7    | 968        | 70     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 23 / 0000 | 35.7    | 51.7    | 964        | 75     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 23 / 0600 | 36.4    | 49.6    | 962        | 80     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 23 / 1200 | 37.1    | 47.5    | 962        | 80     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 23 / 1800 | 37.7    | 45.4    | 963        | 75     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 24 / 0000 | 38.6    | 43.2    | 964        | 70     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 24 / 0600 | 39.5    | 40.9    | 964        | 65     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 24 / 1200 | 40.6    | 38.2    | 964        | 65     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 24 / 1800 | 42.0    | 35.9    | 964        | 65     | e            |
|           |         |         |            |        | xtratropical |
+-----------+---------+---------+------------+--------+--------------+
| 25 / 0000 | 43.2    | 34.7    | 964        | 65     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 25 / 0600 | 44.1    | 33.3    | 966        | 65     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 25 / 1200 | 44.7    | 31.7    | 968        | 60     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 25 / 1800 | 44.9    | 29.8    | 972        | 55     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 26 / 0000 | 45.0    | 27.6    | 977        | 50     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 26 / 0600 | 45.5    | 25.2    | 980        | 50     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 26 / 1200 | 46.0    | 22.7    | 984        | 50     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 26 / 1800 | 47.2    | 20.1    | 986        | 45     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 27 / 0000 | 49.0    | 17.0    | 988        | 45     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 27 / 0600 | 51.3    | 13.3    | 988        | 45     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 27 / 1200 | 53.9    | 10.1    | 988        | 40     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 27 / 1800 | 56.3    | 7.6     | 988        | 40     | \"           |
+-----------+---------+---------+------------+--------+--------------+
| 28 / 0000 |         |         |            |        | merged with  |
|           |         |         |            |        | e            |
|           |         |         |            |        | xtratropical |
|           |         |         |            |        | low          |
+-----------+---------+---------+------------+--------+--------------+
| 18 / 0600 | 22.0    | 49.4    | 955        | 105    | minimum      |
|           |         |         |            |        | pressure     |
+-----------+---------+---------+------------+--------+--------------+

Table 2. Selected ship and buoy observations with winds of at least 34
kt for Hurricane Helene, 12-24 September 2006.

+-----------+-----------+-----------+-----------+-----------+-----------+
| Date/Time | Ship call | Latitude  | Longitude | Wind      | Pressure  |
| (UTC)     | sign      |           |           |           |           |
|           |           | \(N\)     | \(W\)     | dir/speed | (mb)      |
|           |           |           |           | (kt)      |           |
+-----------+-----------+-----------+-----------+-----------+-----------+
| 14/1700   | buoy      | 15.0      | 38.0      | 34        |           |
|           | 13008     |           |           |           |           |
+-----------+-----------+-----------+-----------+-----------+-----------+
| 19 / 0600 | S6TV      | 23.5      | 45.2      | 100 / 39  | 1012.0    |
+-----------+-----------+-----------+-----------+-----------+-----------+
| 22 / 1000 | buoy      | 34.5      | 55.9      | 100 / 47  | 995.4     |
|           | 41643     |           |           |           |           |
+-----------+-----------+-----------+-----------+-----------+-----------+
| 22 / 1300 | buoy      | 34.5      | 55.9      | 030 / 41  | 991.3     |
|           | 41643     |           |           |           |           |
+-----------+-----------+-----------+-----------+-----------+-----------+
| 23 / 0600 | DEDM      | 33.5      | 47.4      | 230 / 49  | 1007.0    |
+-----------+-----------+-----------+-----------+-----------+-----------+
| 23 / 1200 | PBHU      | 33.1      | 44.8      | 180 / 37  | 1010.9    |
+-----------+-----------+-----------+-----------+-----------+-----------+

Table 3 . Preliminary track forecast evaluation (heterogeneous sample)
for Hurricane Helene, 12-24 September 2006. Forecast errors (n mi) are
followed by the number of forecasts in parentheses. Errors smaller than
the NHC official forecast are shown in bold-face type. Verification
includes the depression stage, but does not include the extratropical
stage.

+-----------+-------+-------+-------+-------+-------+-------+-------+
| Forecast  | For   |       |       |       |       |       |       |
| Technique | ecast |       |       |       |       |       |       |
|           | P     |       |       |       |       |       |       |
|           | eriod |       |       |       |       |       |       |
|           | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CLP5      | 41    | 85    | 131   | 177   | 261   | 299   | 219   |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (29)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFNI      | 34    | 58    | 83    | 110   | 180   | 223   | 307   |
|           | (42)  | (40)  | (38)  | (36)  | (32)  | (28)  | (26)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDI      | 29    | 49    | 68    | 95    | 144   | 171   | 265   |
|           | (46)  | (44)  | (42)  | (40)  | (36)  | (32)  | (28)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFSI      | **24  | **41  | 62    | 91    | 162   | 223   | 304   |
|           | (     | (     | (42)  | (40)  | (36)  | (32)  | (28)  |
|           | 46)** | 44)** |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| AEMI      | **24  | **39  | **57  | **82  | 133   | 165   | 242   |
|           | (     | (     | (     | (     | (36)  | (32)  | (28)  |
|           | 46)** | 44)** | 42)** | 40)** |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NGPI      | 29    | 45    | 61    | **81  | 127   | 199   | 322   |
|           | (43)  | (41)  | (39)  | (     | (33)  | (29)  | (26)  |
|           |       |       |       | 37)** |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| UKMI      | 34    | 55    | 74    | 94    | 159   | 190   | 250   |
|           | (46)  | (44)  | (42)  | (40)  | (36)  | (32)  | (28)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMD      | 35    | 61    | 87    | 118   | 203   | 279   | 410   |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (29)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMM      | 35    | 65    | 98    | 124   | 170   | 230   | 319   |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (29)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMS      | 44    | 79    | 113   | 140   | 188   | 242   | 327   |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (29)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CONU      | **25  | **38  | **53  | **73  | **123 | **157 | 199   |
|           | (     | (     | (     | (     | (     | (     | (28)  |
|           | 46)** | 44)** | 42)** | 40)** | 36)** | 32)** |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GUNA      | **25  | **37  | **50  | **70  | **120 | **144 | 201   |
|           | (     | (     | (     | (     | (     | (     | (26)  |
|           | 43)** | 41)** | 39)** | 37)** | 33)** | 29)** |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | **24  | **38  | **59  | **83  | 132   | **153 | 210   |
|           | (     | (     | (     | (     | (31)  | (     | (23)  |
|           | 41)** | 39)** | 37)** | 35)** |       | 27)** |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 27    | 42    | 61    | 86    | 124   | 161   | 192   |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (29)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 37    | 65    | 91    | 118   | 171   | 231   | 303   |
| Official  |       |       |       |       |       |       |       |
|           | \(1   | \(1   | \(1   | \(1   | \(1   | \(    | \(    |
| (         | 930\) | 743\) | 569\) | 410\) | 138\) | 913\) | 742\) |
| 2001-2005 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

Table 4. Preliminary intensity forecast evaluation (heterogeneous
sample) for Hurricane Helene, 12-24 September 2006. Forecast errors (kt)
are followed by the number of forecasts in parentheses. Errors smaller
than the NHC official forecast are shown in bold-face type. Verification
includes the depression stage, but does not include the extratropical
stage.

+-----------+-------+-------+-------+-------+-------+-------+-------+
| Forecast  | For   |       |       |       |       |       |       |
| Technique | ecast |       |       |       |       |       |       |
|           | P     |       |       |       |       |       |       |
|           | eriod |       |       |       |       |       |       |
|           | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| SHF5      | **7.3 | **9.2 | *     | *     | *     | 11.7  | 13.8  |
|           | (     | (     | *10.0 | *10.6 | *10.4 | (33)  | (29)  |
|           | 47)** | 45)** | (     | (     | (     |       |       |
|           |       |       | 43)** | 41)** | 37)** |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDI      | 7.4   | **9.4 | **8.5 | **9.8 | 11.3  | 12.5  | 10.6  |
|           | (46)  | (     | (     | (     | (36)  | (32)  | (28)  |
|           |       | 44)** | 42)** | 40)** |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| SHIP      | 8.0   | 11.5  | 13.8  | 15.1  | 16.5  | 11.9  | **6.4 |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (     |
|           |       |       |       |       |       |       | 29)** |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| DSHP      | 8.0   | 11.5  | 13.8  | 15.1  | 16.5  | 11.9  | **6.4 |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (     |
|           |       |       |       |       |       |       | 29)** |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | 9.0   | 11.2  | 12.2  | 12.9  | 12.6  | **7.4 | **6.0 |
|           | (41)  | (39)  | (37)  | (35)  | (31)  | (     | (     |
|           |       |       |       |       |       | 27)** | 23)** |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| ICON      | 7.4   | **9.0 | **9.2 | **9.4 | *     | **9.6 | **6.4 |
|           | (43)  | (     | (     | (     | *10.7 | (     | (     |
|           |       | 41)** | 39)** | 37)** | (     | 29)** | 27)** |
|           |       |       |       |       | 33)** |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 7.4   | 10.4  | 11.6  | 11.8  | 11.2  | 10.6  | 9.8   |
|           | (47)  | (45)  | (43)  | (41)  | (37)  | (33)  | (29)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 6.3   | 9.8   | 12.1  | 14.3  | 18.4  | 19.8  | 21.8  |
| Official  | (     | (     | (     | (     | (     | (913) | (742) |
|           | 1930) | 1743) | 1569) | 1410) | 1138) |       |       |
| (         |       |       |       |       |       |       |       |
| 2001-2005 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

![](media/image1.wmf){width="7.8694444444444445in"
height="6.102083333333334in"}

Figure 1. Best track positions for Hurricane Helene, 12-24 September
2006. Track during the extratropical stage is partially based on
analyses from the NOAA Ocean Prediction Center.

![](media/image2.wmf){width="7.877777777777778in"
height="5.3493055555555555in"}

Figure 2. Selected wind observations and best track maximum sustained
surface wind speed curve for Hurricane Helene, 12-24 September 2006.
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). Estimates during the extratropical stage are partially
based on analyses from the NOAA Ocean Prediction Center.

![](media/image3.wmf){width="8.998611111111112in"
height="6.110416666666667in"}

Figure 3. Selected pressure observations and best track minimum central
pressure curve for Hurricane Helene, 12-24 September 2006. Estimates
during the extratropical stage are based on analyses from the NOAA Ocean
Prediction Center.

**A) 1205 UTC 18 September B) 2322 UTC 18 September C) 1152 UTC 19
September**

![](media/image4.jpeg){width="2.5in"
height="2.5in"}![](media/image5.jpeg){width="2.5in"
height="2.5in"}![](media/image6.jpeg){width="2.5in" height="2.5in"}

**D) 2309 UTC 19 September E) 0532 UTC 20 September F) 1449 UTC 20
September**

![](media/image7.jpeg){width="2.5in"
height="2.5in"}![](media/image8.jpeg){width="2.5in"
height="2.5in"}![](media/image9.jpeg){width="2.5in" height="2.5in"}

Figure 4. Series f composite 85-91 GHz passive microwave images of
Hurricane Helene during 18-20 September 2006. Images courtesy of the
Fleet Numerical Meteorology and Oceanography Center (FNMOC).