Tropical Cyclone Report

Hurricane Karen

(AL122007)

25-29 September 2007

Richard J. Pasch

National Hurricane Center

27 November 2007

Karen was briefly a hurricane over the tropical Atlantic. It did not
threaten land, and it dissipated just to the east of the Leeward
Islands.

a.  Synoptic History

    A tropical wave emerged from western Africa on 21 September
    accompanied by only a few showers and thunderstorms, but with a
    large envelope of low pressure at low latitudes. As the system
    continued westward over the next couple of days, there was an
    increase in the associated deep convection and a broad low-level
    circulation gradually became better-defined. Curved banding features
    soon became evident, and Dvorak classifications were initiated early
    on 23 September while the disturbed weather area was located about
    550 n mi south of the Cape Verde Islands. The system moved
    west-northwestward with little change in organization until late on
    24 September, when convective banding features became well-enough
    defined to designate the formation of a tropical depression centered
    about 720 n mi west-southwest of the Cape Verde Islands at 0000 UTC
    25 September. Strengthening into a tropical storm occurred about 6 h
    thereafter. The "best track" chart of Karen's path is shown in Fig.
    1, with the wind and pressure histories displayed in Figs. 2 and 3,
    respectively. The best track positions and intensities are listed in
    Table 1.

    For about a day after its formation, there was only a slight
    increase in the cyclone's organization and intensity. Early on 26
    September, however, Karen's cloud signature became much better
    organized, and it strengthened significantly. The cyclone reached
    hurricane strength, and its peak intensity of about 65 kt, by 1200
    UTC 26 September. Later that day, a sharp upper-level trough to the
    west of Karen produced a substantial increase in southwesterly
    vertical shear over the hurricane. Karen quickly lost organization,
    and it weakened below hurricane status by 0000 UTC 27 September.
    Later that same day, as southwesterly shear continued to impact the
    tropical cyclone, the low-level circulation center became exposed to
    the west and southwest of the deep convection. Karen continued to
    gradually weaken in an unfavorable upper-level wind environment, and
    diminished to marginal tropical storm intensity on 28 September.
    Meanwhile, Karen's heading changed temporarily from
    west-northwestward to northward, perhaps due to changes in the
    convective involvement of the low-level center or to a temporary
    surge of low-level environmental southerlies. Although intermittent
    bursts of deep convection occurred near the center, the relentless
    southwesterly shear caused further weakening. By early on 29
    September, Karen weakened to a tropical depression and turned
    westward in response to the low-level easterly environmental flow.
    After 1200 UTC 29 September, Karen's circulation became so distorted
    that the system could no longer be tracked, indicating dissipation.
    A remnant area of showers and squalls lingered near and east of the
    Leeward Islands for a few more days.

b\. Meteorological Statistics

Observations in Karen (Figs. 2 and 3) include satellite-based Dvorak
technique intensity estimates from the Tropical Analysis and Forecast
Branch (TAFB) and the Satellite Analysis Branch (SAB), as well as
flight-level and Stepped-Frequency Microwave Radiometer (SFMR)
measurements from two NOAA Hurricane Hunter aircraft missions. Data and
imagery from NOAA polar-orbiting satellites, the NASA Tropical Rainfall
Measuring Mission (TRMM), the NASA QuikSCAT, and Defense Meteorological
Satellite Program (DMSP) satellites, among others, were also useful in
tracking Karen.

Karen's maximum intensity was estimated operationally to be 60 kt,
corresponding to a strong tropical storm. Post-storm analysis of the
data, however requires a slight increase in the best track intensity. A
NOAA Hurricane Hunter mission measured peak 700 mb flight-level winds of
69 kt and SFMR surface winds of 62 kt shortly before 0000 UTC 27
September. These data support an intensity of 60 kt. By the time of the
aircraft observations, however, Karen's appearance on satellite imagery
was decidedly less-organized than it was 6 to 12 h earlier (Fig. 4).
Around 1200 UTC 26 September, the imagery showed significantly
better-defined banding features than the imagery from about 12 h later.
Moreover, a visible satellite picture taken near 1200 UTC 26 September
(Fig. 5) showed a faint eye-like feature. It is therefore estimated that
Karen was a 65-kt hurricane at 1200 and 1800 UTC on 26 September. This
is also consistent with Dvorak intensity estimates from TAFB at those
times, which were based on the measurement of a banding feature that
wrapped entirely around the center (Note: the possibility that a
post-storm upgrade would be required was noted in the National Hurricane
Center Tropical Cyclone Discussion product issued when the aircraft data
arrived).

NOAA data buoy 41041, located at 14.5ºN 46.0ºW (sensor elevation 5 m),
reported a maximum 1-minute wind of 43 kt at 0700 UTC 27 September, at
which time the center of Karen was located about 75 n mi to the south.
No ship reports of tropical storm force winds associated with Karen were
received.

c\. Casualty and Damage Statistics

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

d\. Forecast and Warning Critique

Karen's formation was well anticipated. The tropical wave and
accompanying broad area of low pressure that eventually developed into
Karen, and the possibility of slow development, were first mentioned in
the Tropical Weather Outlook (TWO) nearly 57 h before genesis. The next
TWO, issued nearly 51 h prior to genesis, indicated the potential for
tropical depression formation over the ensuing few days. Subsequent TWOs
leading up to Karen's formation were more emphatic about genesis.

A verification of official and guidance model track forecasts is given
in Table 2. Average official track errors for Karen were 52, 72, 96,
106, 142, and 186 n mi for the 12, 24, 36, 48, 72, and 96 h forecasts,
respectively. The number of forecasts ranged from 16 at 12 h to 2 at 96
h. There were no forecasts that verified out to 120 h. These track
forecast errors are a little higher than the average long-term official
track errors at 12 through 36 h and somewhat lower than the average
long-term official track errors at 48 through 96 h (Table 2).

Average official intensity errors were 6, 11, 13, 15, 22, and 25 kt for
the 12, 24, 36, 48, 72, and 96 h forecasts, respectively. For
comparison, the average long-term official intensity errors are 6, 10,
12, 14, 18, and 20 kt, respectively. Table 3 gives a verification of
official and guidance model intensity forecasts. There was somewhat of a
high bias in the official intensity forecasts, but the official
forecasts were generally correct in anticipating that Karen's
strengthening would be limited or halted by increasing shear. That
increase of shear ahead of the tropical cyclone was rather well
predicted a few days in advance by the NCEP Global Forecast System and
other global models.

Table 1. Best track for Hurricane Karen, 25-29 September 2007.

+---------+---------+---------+---------+-----------+----------------+
| Da      | L       | Lo      | P       | Wind      | Stage          |
| te/Time | atitude | ngitude | ressure | Speed     |                |
|         |         |         |         |           |                |
| (UTC)   | \(N\)   | \(W\)   | (mb)    | (kt)      |                |
+---------+---------+---------+---------+-----------+----------------+
| 25 /    | 10.0    | 35.9    | 1006    | 30        | tropical       |
| 0000    |         |         |         |           | depression     |
+---------+---------+---------+---------+-----------+----------------+
| 25 /    | 10.3    | 37.0    | 1005    | 35        | tropical storm |
| 0600    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 25 /    | 10.6    | 38.0    | 1005    | 35        | \"             |
| 1200    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 25 /    | 10.8    | 39.2    | 1005    | 35        | \"             |
| 1800    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 26 /    | 10.9    | 40.4    | 1003    | 40        | \"             |
| 0000    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 26 /    | 11.2    | 41.5    | 997     | 50        | \"             |
| 0600    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 26 /    | 11.7    | 42.4    | 988     | 65        | hurricane      |
| 1200    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 26 /    | 12.3    | 43.3    | 990     | 65        | \"             |
| 1800    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 27 /    | 12.8    | 44.6    | 995     | 60        | tropical storm |
| 0000    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 27 /    | 13.2    | 45.7    | 998     | 55        | \"             |
| 0600    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 27 /    | 13.5    | 46.8    | 1002    | 55        | \"             |
| 1200    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 27 /    | 14.1    | 47.9    | 1005    | 50        | \"             |
| 1800    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 28 /    | 14.1    | 48.8    | 1005    | 50        | \"             |
| 0000    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 28 /    | 14.3    | 49.0    | 1007    | 40        | \"             |
| 0600    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 28 /    | 14.6    | 49.0    | 1008    | 35        | \"             |
| 1200    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 28 /    | 15.8    | 49.4    | 1008    | 35        | \"             |
| 1800    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 29 /    | 16.1    | 51.0    | 1008    | 35        | \"             |
| 0000    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 29 /    | 16.3    | 52.6    | 1008    | 30        | tropical       |
| 0600    |         |         |         |           | depression     |
+---------+---------+---------+---------+-----------+----------------+
| 29 /    | 16.8    | 54.2    | 1009    | 30        | low            |
| 1200    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 29 /    |         |         |         |           | dissipated     |
| 1800    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 26 /    | 11.7    | 42.4    | 988     | 65        | minimum        |
| 1200    |         |         |         |           | pressure       |
+---------+---------+---------+---------+-----------+----------------+

Table 2. Preliminary track forecast evaluation (heterogeneous sample)
for Hurricane Karen, 25-29 September 2007. 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.

+-----------+-------+-------+-------+-------+-------+-------+-------+
| Forecast  | For   |       |       |       |       |       |       |
| Technique | ecast |       |       |       |       |       |       |
|           | P     |       |       |       |       |       |       |
|           | eriod |       |       |       |       |       |       |
|           | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CLP5      | 57    | 72    | **74  | **85  | **139 | 202 ( |       |
|           | (15)  | (13)  | (     | (     | (     | 2)    |       |
|           |       |       | 11)** | 9)**  | 6)**  |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFNI      | 67    | 76    | **93  | 129 ( | 223 ( |       |       |
|           | (12)  | (10)  | (     | 6)    | 2)    |       |       |
|           |       |       | 8)**  |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDI      | **49  | 83    | 110   | 131   | 162 ( | 222 ( |       |
|           | (     | (14)  | (12)  | (10)  | 6)    | 2)    |       |
|           | 16)** |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| HWFI      | 60    | 93    | 113   | 125   | 186 ( | 281 ( |       |
|           | (16)  | (14)  | (12)  | (10)  | 6)    | 2)    |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFSI      | 65    | 97    | 132 ( | 146 ( | 161 ( | **161 |       |
|           | (13)  | (11)  | 9)    | 7)    | 3)    | (     |       |
|           |       |       |       |       |       | 1)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| AEMI      | 61 (  | 101 ( | 131 ( | 146 ( | 236 ( | 232 ( |       |
|           | 5)    | 5)    | 5)    | 5)    | 4)    | 1)    |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NGPI      | 57    | **66  | **90  | 113 ( | 199 ( | 341 ( |       |
|           | (14)  | (     | (     | 8)    | 4)    | 2)    |       |
|           |       | 12)** | 10)** |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| UKMI      | **50  | **53  | **76  | **78  | **100 | **78  |       |
|           | (     | (     | (     | (     | (     | (     |       |
|           | 15)** | 13)** | 11)** | 9)**  | 5)**  | 1)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMD      | **50  | **68  | **92  | 121   | 215 ( | 302 ( |       |
|           | (     | (     | (     | (10)  | 6)    | 2)    |       |
|           | 16)** | 14)** | 12)** |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMM      | 53    | **71  | **86  | **105 | 169 ( | 190 ( |       |
|           | (16)  | (     | (     | (     | 6)    | 2)    |       |
|           |       | 14)** | 12)** | 10)** |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMS      | 78    | 147   | 211   | 263   | 356 ( | 366 ( |       |
|           | (16)  | (14)  | (12)  | (10)  | 6)    | 2)    |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CONU      | **50  | **67  | **89  | **101 | **114 | **121 |       |
|           | (     | (     | (     | (     | (     | (     |       |
|           | 16)** | 14)** | 12)** | 10)** | 6)**  | 2)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GUNA      | 56    | 72 (  | 99 (  | 107 ( | **86  | **153 |       |
|           | (11)  | 9)    | 7)    | 5)    | (     | (     |       |
|           |       |       |       |       | 1)**  | 1)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | 54    | **64  | **70  | **69  | **120 |       |       |
|           | (13)  | (     | (     | (     | (     |       |       |
|           |       | 11)** | 9)**  | 7)**  | 4)**  |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 52    | 72    | 96    | 106   | 142 ( | 186 ( |       |
|           | (16)  | (14)  | (12)  | (10)  | 6)    | 2)    |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 35    | 61    | 86    | 112   | 162   | 221   | 290   |
| Official  | (     | (     | (     | (     | (     | (885) | (723) |
|           | 1852) | 1686) | 1519) | 1362) | 1100) |       |       |
| (         |       |       |       |       |       |       |       |
| 2002-2006 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

Table 3. Preliminary intensity forecast evaluation (heterogeneous
sample) for Hurricane Karen, 25-29 September 2007. 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.

+-----------+-------+-------+-------+-------+-------+-------+-------+
| Forecast  | For   |       |       |       |       |       |       |
| Technique | ecast |       |       |       |       |       |       |
|           | P     |       |       |       |       |       |       |
|           | eriod |       |       |       |       |       |       |
|           | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| SHF5      | 9.1   | 16.5  | 21.9  | 20.7  | *     | 28.5  |       |
|           | (15)  | (13)  | (11)  | ( 9)  | *21.5 | ( 2)  |       |
|           |       |       |       |       | (     |       |       |
|           |       |       |       |       | 6)**  |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GHMI      | 7.6   | 11.7  | 16.3  | 16.6  | 23.7  | 34.5  |       |
|           | (16)  | (14)  | (12)  | (10)  | ( 6)  | ( 2)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| HWFI      | 7.6   | **8.5 | *     | *     | *     | 31.5  |       |
|           | (16)  | (     | *10.3 | *12.3 | *18.8 | ( 2)  |       |
|           |       | 14)** | (     | (     | (     |       |       |
|           |       |       | 12)** | 10)** | 6)**  |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| SHIP      | 7.8   | 11.9  | 13.3  | 18.0  | 28.8  | 41.0  |       |
|           | (16)  | (14)  | (12)  | (10)  | ( 6)  | ( 2)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| DSHP      | 7.8   | 11.9  | 13.3  | 18.0  | 28.8  | 41.0  |       |
|           | (16)  | (14)  | (12)  | (10)  | ( 6)  | ( 2)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | 9.1   | 14.5  | 16.7  | 17.9  | *     |       |       |
|           | (13)  | (11)  | ( 9)  | ( 7)  | *18.8 |       |       |
|           |       |       |       |       | (     |       |       |
|           |       |       |       |       | 4)**  |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| ICON      | 6.8   | *     | 12.9  | *     | 23.0  | 40.0  |       |
|           | (16)  | *10.1 | (12)  | *13.9 | ( 6)  | ( 2)  |       |
|           |       | (     |       | (     |       |       |       |
|           |       | 14)** |       | 10)** |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 5.6   | 11.4  | 12.9  | 14.5  | 21.7  | 25.0  |       |
|           | (16)  | (14)  | (12)  | (10)  | ( 6)  | ( 2)  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 6.4   | 9.8   | 12.0  | 14.1  | 18.3  | 19.8  | 21.8  |
| Official  | (     | (     | (     | (     | (     | (885) | (723) |
|           | 1852) | 1686) | 1519) | 1362) | 1100) |       |       |
| (         |       |       |       |       |       |       |       |
| 2002-2006 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

![](media/image1.wmf){width="7.798611111111111in"
height="6.000694444444444in"}

Figure 1. Best track positions for Hurricane Karen, 25-29 September
2007.

![](media/image2.wmf){width="7.097222222222222in"
height="4.819444444444445in"}

Figure 2. Selected wind observations and best track maximum sustained
surface wind speed curve for Hurricane Karen, 25-29 September 2007.
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.

![](media/image3.wmf){width="7.097222222222222in"
height="4.819444444444445in"}

Figure 3. Selected pressure observations and best track minimum central
pressure curve for Hurricane Karen, 25-29 September 2007.

![](media/image4.jpeg){width="4.404861111111111in"
height="4.404861111111111in"}![](media/image5.jpeg){width="4.404861111111111in"
height="4.404861111111111in"}

Figure 4. Infrared satellite image of Karen at 1145 UTC 26 Spetember
2007 (left), and 2345 UTC 27 September (right). Images are from the
Naval Research Laboratory Tropical Cyclone page.

![](media/image6.jpeg){width="6.997222222222222in"
height="6.997222222222222in"}

Figure 5. Visible satellite image of Karen at 1145 UTC 26 September
2007. Image is from the Naval Research Laboratory Tropical Cyclone page.