Number range CHAPTERTropical Cyclone Report

Hurricane Irene

4 - 18 August 2005

Stacy R. Stewart

National Hurricane Center

20 January 2006

Hurricane Irene was a long-lived Cape Verde tropical cyclone that
remained over the open Atlantic Ocean throughout its lifetime and became
a Category 2 hurricane (on the Saffir-Simpson Hurricane Scale) when it
was located well to the north of Bermuda.

a.  Synoptic History

    A vigorous tropical wave, accompanied by a broad surface low
    pressure system and curved convective bands, moved off the African
    coast on 1 August. However, since the wave came off the coast at a
    relatively high latitude, convection weakened significantly as the
    system moved over relatively cool water located southeast of the
    Cape Verde Islands. The wave continued moving westward over the next
    couple of days and, as it passed over higher sea-surface
    temperatures, sufficient convection redeveloped for satellite
    classifications to be initiated. A QuikSCAT overpass at 1940 UTC 3
    August indicated the surface circulation had become better defined
    and that winds had increased to 20-25 kt. During the next 24 h,
    thunderstorms became better organized near the low pressure center
    and it is estimated that a tropical depression formed at 1800 UTC 4
    August about 600 n mi southwest 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.

    Early on 5 August, Tropical Depression Nine abruptly turned
    west-northwestward to northwestward, causing the cyclone to move
    beneath stronger upper-level northwesterly winds. This motion also
    caused the depression to move across cooler water and into more
    stable trade wind flow. The combination of these unfavorable
    environmental conditions resulted in a pronounced decrease in both
    the organization and amount of thunderstorm activity. However,
    despite the lack of organized convection, the cyclone continued to
    slowly strengthen. QuikSCAT overpasses at 0759 UTC and 2029 UTC 5
    August indicated winds has increased to and remained near 30 kt in
    the northeast quadrant. The depression might have derived energy
    from what available barotropic instability existed within the trade
    wind flow in which it was embedded. By 1200 UTC 7 August, deep
    convection had increased enough near the low-level center for the
    cyclone to be upgraded to Tropical Storm Irene about 1080 n mi east
    of the northern Leeward Islands. This intensity was supported by a
    QuikSCAT overpass near 0849 UTC that indicated winds were near 40
    kt. Irene maintained tropical storm status for the next 24 h as it
    continued west-northwestward before belligerent upper-level
    northwesterly shear and a dry, stable environment eroded the
    convection. This caused Irene to weaken back to a depression by 1200
    UTC 8 August.

    Irene made a turn toward the west on 9 August while the vertical
    depth of the weakened cyclone decreased. As a result, the low-level
    easterly winds on the south side of the large subtropical ridge to
    the north became the dominant steering flow. Despite moving over
    warmer sea-surface temperatures, Irene persisted as a tropical
    depression due to the unfavorable northwesterly wind shear and a
    continued dry and stable environment. By 0000 UTC 11 August, the
    vertical shear decreased enough for more convection to develop and
    persist near the low-level center, which allowed Irene to once again
    become a tropical storm about 300 n mi northeast of the Leeward
    Islands.

    Irene made a turn toward the northwest early on 11 August, the
    result of the now stronger vertical circulation being influenced by
    deeper southeasterly steering flow. While moving around the
    southwestern periphery of the subtropical ridge situated
    east-to-west across Bermuda, the vertical shear slowly decreased,
    allowing Irene to gradually strengthen as it moved into a more
    unstable environment. As the tropical storm moved into a weakness in
    the subtropical ridge about midway between Bermuda and the North
    Carolina Outer Banks, the cyclone reached hurricane strength at 0000
    UTC 15 August. After achieving hurricane status, Irene made a sharp
    turn toward the east-northeast and east as it moved north of the
    ridge axis and continued to strengthen, reaching its peak intensity
    of 90 kt at 1800 UTC 16 August about 300 n mi northeast or Bermuda.

    Shortly after Irene peaked in intensity, upper-level west to
    southwesterly winds began to increase ahead of a large mid-latitude
    trough and frontal system that was quickly approaching from the
    west. This produced unfavorable vertical shear across the cyclone,
    inducing a slow but steady weakening trend, and Irene weakened to a
    tropical storm by 0000 UTC 18 August over the far North Atlantic
    Ocean about 450 n mi south of Cape Race Newfoundland. Later that
    day, Irene turned toward the northeast and began to move over cooler
    water. Deep convection near the center started to wane. During the
    morning hours, the extratropical low and frontal system located to
    the west continued to accelerate northeastward at a forward speed
    faster than that of Irene. After 1200 UTC, no convection existed
    within 200 n mi of the center, and Irene may have briefly become an
    extratropical low before it was finally absorbed by the larger
    extratropical low pressure system at around 1800 UTC 18 August about
    250 n mi east-southeast of Cape Race, Newfoundland.

b.  Meteorological Statistics

    Observations in Irene (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 dropsonde observations from flights of the 53^rd^ Weather
    Reconnaissance Squadron (WRS) of the U. S. Air Force Reserve
    Command. 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 Irene.

    The 53^rd^ WRS made 23 aircraft fixes on Irene as the cyclone moved
    northwestward in the general direction of Bermuda and the U.S. east
    coast during the period 12-15 August. The maximum 850 mb
    flight-level winds observed were 88 kt at 2340 UTC 14 August in the
    southeast quadrant. Those flight-level winds correspond to
    approximately 70-kt surface winds, which was the basis for upgrading
    Irene to a hurricane. The lowest central pressure measured by a
    dropwindsonde was 989 mb at 0001 UTC 15 August, but the last
    reconnaissance flight occurred before Irene's peak intensity of 90
    kt at 1800 UTC 16 August. The peak intensity is based on Dvorak
    satellite intensity estimates of T5.0 or 90 kt from the TAFB and SAB
    near that time. A University of Wisconsin-Cooperative Institute for
    Meteorological Satellite Studies (UW-CIMSS) AMSU intensity estimate
    of 85 kt at 1911 UTC 16 August also supports the 90-kt intensity.
    The peak intensity is also coincident with Irene's most impressive
    appearance in conventional and microwave (Fig. 4) satellite imagery.

    Despite Irene's long track, there were no reliable buoy or ship
    reports of tropical storm-force winds.

c.  Casualty and Damage Statistics

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

d.  Forecast and Warning Critique

    Average official track errors (with the number of cases in
    parentheses) for Irene were 40 (54), 68 (52), 103 (50), 142 (48),
    201 (44), 268 (40), and 345 (36) n mi for the 12, 24, 36, 48, 72,
    96, and 120 h forecasts, respectively. These errors are smaller than
    the average official track errors through 72 hours and then larger
    than average at 96 and 120 h for the 10-yr period 1995-2004[^1]
    \[(42, 75, 107, 138, 202, 236, and 310 n mi, respectively), (Table
    2)\]. The first few forecasts had a right bias after 72 hours as a
    result of the models prematurely eroding the western portion of the
    subtropical ridge. However, subsequent official forecast tracks
    beginning on 9 August correctly predicted that Irene would pass
    about midway between Bermuda and the U.S. east coast, with only a
    slight left bias. After Irene recurved through the mean ridge axis
    and began moving in an easterly direction, both the official
    forecast tracks and the majority of the NHC model guidance had a
    pronounced left bias. This was the result of the models
    overdeveloping a mid- to upper-level trough over the northeastern
    United States that was expected to lift the cyclone northeastward
    immediately after recurvature into the westerlies had occurred.

    Average official intensity errors were 4, 7, 8, 11, 11, 11, and 12
    kt for the 12, 24, 36, 48, 72, 96, and 120 h forecasts,
    respectively. These errors were much lower than the average official
    intensity errors over the 10-yr period 1995-2004 are 6, 10, 12, 15,
    18, 20, and 22 kt, respectively. While early intensity forecasts
    that called for Irene to become a hurricane over the central
    tropical Atlantic had a high bias, later forecast calling for Irene
    to become a hurricane at a higher latitude were fairly accurate.

    Irene's potential development into a tropical depression was first
    mentioned in the 0212 UTC 4 August Tropical Weather Outlook issued
    by the National Hurricane Center. This resulted in approximately a
    16-hour genesis lead time.

    No coastal tropical cyclone watches or warnings were required for
    Irene.

Number range CHAPTERTable 1. Best track for Hurricane Irene, 4-18 August
2005.

+---------+--------+--------+--------+-----------+--------------------+
| Da      | La     | Lon    | Pr     | Wind      | Stage              |
| te/Time | titude | gitude | essure | Speed     |                    |
|         |        |        |        |           |                    |
| (UTC)   | (N)   | (W)   | (mb)   | (kt)      |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 04 /    | 12.9   | 33.5   | 1009   | 25        | tropical           |
| 1800    |        |        |        |           | depression         |
+---------+--------+--------+--------+-----------+--------------------+
| 05 /    | 13.6   | 34.5   | 1009   | 25        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 05 /    | 14.6   | 35.5   | 1009   | 25        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 05 /    | 15.6   | 36.8   | 1008   | 30        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 05 /    | 16.6   | 38.4   | 1008   | 30        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 06 /    | 17.2   | 39.8   | 1008   | 30        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 06 /    | 17.7   | 40.5   | 1008   | 30        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 06 /    | 18.2   | 41.6   | 1008   | 30        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 06 /    | 18.8   | 42.8   | 1008   | 30        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 07 /    | 19.3   | 43.5   | 1008   | 30        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 07 /    | 19.7   | 44.2   | 1007   | 30        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 07 /    | 20.2   | 45.0   | 1005   | 35        | tropical storm     |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 07 /    | 20.8   | 46.0   | 1005   | 35        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 08 /    | 21.3   | 47.2   | 1005   | 35        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 08 /    | 21.8   | 48.3   | 1006   | 35        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 08 /    | 22.2   | 49.9   | 1009   | 30        | tropical           |
| 1200    |        |        |        |           | depression         |
+---------+--------+--------+--------+-----------+--------------------+
| 08 /    | 22.4   | 50.9   | 1009   | 30        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 09 /    | 22.5   | 52.0   | 1009   | 30        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 09 /    | 22.4   | 52.9   | 1009   | 30        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 09 /    | 22.4   | 53.7   | 1009   | 30        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 09 /    | 22.4   | 54.8   | 1009   | 30        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 10 /    | 22.4   | 55.7   | 1009   | 30        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 10 /    | 22.3   | 56.5   | 1009   | 30        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 10 /    | 22.4   | 57.2   | 1009   | 30        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 10 /    | 22.8   | 58.1   | 1008   | 30        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 11 /    | 23.3   | 59.3   | 1005   | 35        | tropical storm     |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 11 /    | 23.9   | 60.4   | 1003   | 40        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 11 /    | 24.7   | 61.7   | 1001   | 45        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 11 /    | 25.4   | 62.9   | 1001   | 45        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 12 /    | 26.3   | 63.8   | 1001   | 45        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 12 /    | 27.2   | 65.1   | 998    | 55        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 12 /    | 27.9   | 66.3   | 996    | 55        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 12 /    | 28.4   | 67.1   | 995    | 55        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 13 /    | 28.9   | 67.7   | 993    | 60        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 13 /    | 29.6   | 68.2   | 996    | 60        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 13 /    | 30.0   | 69.0   | 997    | 60        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 13 /    | 30.8   | 69.4   | 1000   | 60        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 14 /    | 31.6   | 69.9   | 997    | 60        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 14 /    | 32.1   | 70.2   | 999    | 55        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 14 /    | 33.1   | 70.1   | 999    | 55        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 14 /    | 34.0   | 70.0   | 994    | 60        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 15 /    | 34.9   | 69.5   | 989    | 70        | hurricane          |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 15 /    | 35.7   | 68.7   | 985    | 75        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 15 /    | 36.3   | 67.7   | 985    | 75        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 15 /    | 36.5   | 66.5   | 980    | 80        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 16 /    | 36.6   | 65.1   | 980    | 80        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 16 /    | 36.6   | 64.0   | 978    | 80        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 16 /    | 36.6   | 63.0   | 975    | 85        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 16 /    | 36.5   | 62.0   | 970    | 90        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 17 /    | 36.5   | 61.3   | 978    | 80        | \"                 |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 17 /    | 36.5   | 60.6   | 983    | 75        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 17 /    | 36.9   | 59.1   | 987    | 65        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 17 /    | 37.6   | 57.3   | 987    | 65        | \"                 |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 18 /    | 38.6   | 55.0   | 990    | 60        | tropical storm     |
| 0000    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 18 /    | 40.3   | 52.3   | 990    | 60        | \"                 |
| 0600    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 18 /    | 43.3   | 48.9   | 992    | 50        | \"                 |
| 1200    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+
| 18 /    |        |        |        |           | Absorbed by larger |
| 1800    |        |        |        |           | extratropical low  |
+---------+--------+--------+--------+-----------+--------------------+
| 16 /    | 36.5   | 62.0   | 970    | 90        | minimum pressure   |
| 1800    |        |        |        |           |                    |
+---------+--------+--------+--------+-----------+--------------------+

Number range CHAPTERNumber range CHAPTERNumber range CHAPTERTable 2.
Preliminary forecast evaluation (heterogeneous sample) for Hurricane
Irene, 4-18 August 2005. 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.

+-----------+-------+-------+-------+-------+-------+-------+-------+
| Number    | For   |       |       |       |       |       |       |
| range     | ecast |       |       |       |       |       |       |
| CHAPTE    | P     |       |       |       |       |       |       |
| RForecast | eriod |       |       |       |       |       |       |
| Technique | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CLP5      | 55    | 97    | 134   | 166   | 224   | 310   | 402   |
|           | (54)  | (52)  | (50)  | (48)  | (44)  | (40)  | (36)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFNI      | *     | 69    | 105   | 144   | 222   | 343   | 610   |
|           | *38** | (43)  | (41)  | (37)  | (29)  | (23)  | (19)  |
|           | (49)  |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDI      | *     | *     | *     | *     | **    | **    | 360   |
|           | *37** | *58** | *72** | *95** | 170** | 263** | (21)  |
|           | (53)  | (47)  | (45)  | (39)  | (31)  | (26)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDL\*    | 41    | *     | *     | *     | **    | **    | **    |
|           | (53)  | *56** | *68** | *87** | 140** | 239** | 337** |
|           |       | (49)  | (44)  | (36)  | (31)  | (26)  | (20)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDN\*    | 42    | *     | *     | **    | 211   | 301   | 528   |
|           | (46)  | *65** | *95** | 136** | (29)  | (24)  | (20)  |
|           |       | (44)  | (39)  | (35)  |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFSI      | 50    | 84    | 133   | 182   | 254   | 391   | 430 ( |
|           | (46)  | (41)  | (39)  | (36)  | (21)  | (11)  | 5)    |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFSO\*    | 56    | 86    | 123   | 172   | 255   | 410 ( | 394 ( |
|           | (41)  | (37)  | (33)  | (31)  | (21)  | 8)    | 3)    |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| AEMI      | 44    | 77    | 116   | 161   | 243   | 331   | 501   |
|           | (52)  | (50)  | (48)  | (45)  | (35)  | (26)  | (11)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NGPI      | 43    | 80    | 121   | 160   | 237   | 333   | 427   |
|           | (52)  | (50)  | (48)  | (46)  | (42)  | (38)  | (33)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NGPS\*    | 48    | 74    | 110   | 144   | 214   | 305   | 425   |
|           | (51)  | (49)  | (47)  | (45)  | (41)  | (37)  | (33)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| UKMI      | 42    | 77    | 146   | 193   | 213   | **    | **    |
|           | (52)  | (50)  | (48)  | (44)  | (34)  | 252** | 275** |
|           |       |       |       |       |       | (28)  | (26)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| UKM\*     | 53    | 70    | **    | **    | **    | **    | **    |
|           | (27)  | (26)  | 102** | 133** | 181** | 220** | 244** |
|           |       |       | (25)  | (23)  | (18)  | (15)  | (13)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| A98E      | 52    | 81    | 116   | 158   | 242   | 300   | 400   |
|           | (54)  | (52)  | (50)  | (48)  | (44)  | (40)  | (36)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| A9UK      | 56    | 84    | 119   | 162   | 249   |       |       |
|           | (27)  | (26)  | (25)  | (24)  | (22)  |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMD      | 42    | 72    | **    | **    | **    | **    | **    |
|           | (53)  | (51)  | 100** | 128** | 167** | 233** | 315** |
|           |       |       | (49)  | (48)  | (44)  | (40)  | (36)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMM      | 41    | 69    | *     | **    | **    | **    | 379   |
|           | (53)  | (51)  | *98** | 128** | 182** | 239** | (36)  |
|           |       |       | (49)  | (48)  | (44)  | (40)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMS      | 43    | 70    | *     | **    | **    | **    | 370   |
|           | (53)  | (51)  | *91** | 112** | 160** | 232** | (36)  |
|           |       |       | (49)  | (48)  | (44)  | (40)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CONU      | *     | *     | **    | **    | **    | **    | 365   |
|           | *36** | *63** | 100** | 130** | 175** | 257** | (33)  |
|           | (53)  | (51)  | (49)  | (47)  | (42)  | (38)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GUNA      | *     | *     | *     | 144   | 244   | **    | **    |
|           | *34** | *57** | *98** | (28)  | (13)  | 229** | 245** |
|           | (44)  | (38)  | (36)  |       |       | ( 7)  | ( 4)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | *     | *     | **    | 150   | 239   | 336   | 405   |
|           | *30** | *62** | 101** | (34)  | (29)  | (21)  | (17)  |
|           | (41)  | (39)  | (36)  |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 40    | 68    | 103   | 142   | 201   | 268   | 345   |
|           | (54)  | (52)  | (50)  | (48)  | (44)  | (40)  | (36)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 44    | 78    | 112   | 146   | 217   | 248   | 319   |
| Official  |       |       |       |       |       |       |       |
|           | \(3   | \(2   | \(2   | \(2   | \(1   | \(    | \(    |
| (         | 172\) | 894\) | 636\) | 368\) | 929\) | 421\) | 341\) |
| 1994-2003 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

\*Output from these models was unavailable at forecast time.

Errors given for the 96 and 120 h periods are averages over the
four-year period 2001-4

Number range CHAPTER![](media/image1.wmf){width="7.623611111111111in"
height="5.873611111111111in"}

Number range CHAPTERFigure 1. Best track positions for Hurricane Irene,
4-18 August 2005.

![](media/image2.wmf){width="8.247222222222222in"
height="5.7347222222222225in"}

Number range CHAPTERFigure 2. Selected wind observations and best track
maximum sustained surface wind speed curve for

Hurricane Irene, 4-18 August 2005. Aircraft observations have been
adjusted for elevation using an 80%

adjustment factor for observations from 850 mb.

![](media/image3.wmf){width="8.371527777777779in"
height="5.826388888888889in"}

Number range CHAPTERFigure 3. Selected pressure observations and best
track minimum central pressure curve for Hurricane Irene,

4-18 August 2005.

![](media/image4.jpeg){width="7.250694444444444in"
height="5.872916666666667in"}

Number range CHAPTERFigure 4. An SSM/I microwave image (85 GHz) at 1329
UTC 16 August 2005 showing a well-defined

eye just a few hours prior to Irene reaching its peak intensity of 90 kt
at a relatively high latitude (image

courtesy of the U.S. Navy Fleet Numerical Meteorology and Oceanography
Center, Monterey, CA).

[^1]: Errors given for the 96 and 120 h periods are averages over the
    four-year period 2001-4.