Number range CHAPTERTropical Cyclone Report

Tropical Storm Erick

(EP082007)

31 July-2 August 2007

Eric S. Blake

National Hurricane Center

23 August 2007

Tropical Storm Erick was a short-lived tropical cyclone that moved
westward over the open waters of the eastern North Pacific Ocean.

a.  Synoptic History

The tropical wave that eventually led to the formation of Erick departed
western Africa on 16 July. Associated convection was limited to the ITCZ
for the next several days, then diminished on 19 July. The wave passed
through the Lesser Antilles on 22 July with some deep convection, but
thunderstorm activity remained disorganized for the next several days.
The system moved across Central America on 25 July, and a weak low
pressure area formed along the wave axis on 28 July in the eastern
Pacific. Easterly wind shear, however, prevented development of the low
for a few days as associated deep convection remained well west of the
low. Early on 31 July, thunderstorms formed closer to the center of the
low, and it is estimated that the system gained enough organization to
be classified as a tropical depression at 1200 UTC that day, located
about 925 n mi southwest of the southern tip of Baja California. 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.

Despite easterly shear, the depression intensified into a tropical storm
early on 1 August, but reached a peak intensity of only 35 kt. The
easterly shear never relented, and Erick weakened to a tropical
depression at 0000 UTC 2 August, while located about 1175 n mi
west-southwest of the southern tip of Baja California. Overnight
microwave and QuikScat images indicated that the surface low lost its
well-defined circulation as it became elongated from northeast to
southwest. The depression degenerated into a tropical wave by 0600 UTC 2
August, and its remnants continued moving westward. A weak low reformed
within the wave on 3 August before entering the central North Pacific
basin early on 4 August. The low dissipated several hundred miles
southwest of the Hawaiian Islands on 8 August.

b.  Meteorological Statistics

    Observations in Erick (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).
    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 Erick. The 35 kt estimated peak intensity of
    Erick is based on a blend of objective and subjective Dvorak
    satellite classifications.

c.  Casualty and Damage Statistics

    There were no reports of casualties or damages associated with
    Erick.

d.  Forecast and Warning Critique

    The tropical wave that eventually spawned Erick was mentioned in the
    Tropical Weather Outlooks for about two and a half days prior to
    genesis. The timing of the genesis of the system was not
    well-anticipated, however, and the system became a tropical cyclone
    earlier than expected. In fact, the Tropical Weather Outlook
    products did not mention the possibility of a tropical cyclone
    forming prior to the time of genesis in the final best track.

    A verification of official and guidance model track forecasts is
    given in Table 4. Average official track errors for Erick were 26
    and 49 n mi for the 12 and 24 h forecasts, respectively. These
    errors are lower than the average long-term official track errors of
    33 and 57 n mi for the 12 and 24 h forecast periods (Table 2).
    However, the numbers of forecasts verified were only four at 12 h
    and two at 24 h. The GFDI model provided the best guidance during
    the period, while the NGPI had a poor performance.

    Average official intensity errors were 2.5 and 5 kt for the 12 and
    24 h forecasts, respectively. For comparison, the average long-term
    official intensity errors are 6.3 and 11 kt, respectively. The
    official forecast errors were better than average, but on a rather
    small number of cases (Table 3). Despite these statistics, it would
    be incorrect to consider Erick to have been a well-forecast tropical
    cyclone, however. In addition to forming without warning, it also
    dissipated unexpectedly.

Number range CHAPTERTable 1. Best track for Tropical Storm Erick, 31
July-2 August 2007.

+-----------+----------+----------+---------+---------+--------------+
| Date/Time | Latitude | L        | P       | Wind    | Stage        |
|           |          | ongitude | ressure | Speed   |              |
| (UTC)     | (°N)     |          |         |         |              |
|           |          | (°W)     | (mb)    | (kt)    |              |
+-----------+----------+----------+---------+---------+--------------+
| 31 / 1200 | 12.9     | 122.5    | 1008    | 25      | tropical     |
|           |          |          |         |         | depression   |
+-----------+----------+----------+---------+---------+--------------+
| 31 / 1800 | 12.9     | 123.4    | 1007    | 30      | \"           |
+-----------+----------+----------+---------+---------+--------------+
| 01 / 0000 | 13.0     | 124.2    | 1005    | 35      | tropical     |
|           |          |          |         |         | storm        |
+-----------+----------+----------+---------+---------+--------------+
| 01 / 0600 | 13.0     | 124.9    | 1004    | 35      | \"           |
+-----------+----------+----------+---------+---------+--------------+
| 01 / 1200 | 13.1     | 125.7    | 1005    | 35      | \"           |
+-----------+----------+----------+---------+---------+--------------+
| 01 / 1800 | 13.2     | 126.7    | 1005    | 35      | \"           |
+-----------+----------+----------+---------+---------+--------------+
| 02 / 0000 | 13.3     | 127.7    | 1006    | 30      | tropical     |
|           |          |          |         |         | depression   |
+-----------+----------+----------+---------+---------+--------------+
| 02 / 0600 | \-       | \-       | \-      | \-      | dissipated   |
+-----------+----------+----------+---------+---------+--------------+
| 01 / 0600 | 13.0     | 124.9    | 1004    | 35      | minimum      |
|           |          |          |         |         | pressure     |
+-----------+----------+----------+---------+---------+--------------+

Number range CHAPTERNumber range CHAPTERNumber range CHAPTERTable 2.
Preliminary track forecast evaluation (heterogeneous sample) for
Tropical Storm Erick, 31 July-2 August 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.

+-----------+--------+-------+-------+------+-------+-------+-------+
| Number    | Fo     |       |       |      |       |       |       |
| range     | recast |       |       |      |       |       |       |
| CHAPTE    | Period |       |       |      |       |       |       |
| RForecast | (h)    |       |       |      |       |       |       |
| Technique |        |       |       |      |       |       |       |
+===========+========+=======+=======+======+=======+=======+=======+
|           | 12     | 24    | 36    | 48   | 72    | 96    | 120   |
+-----------+--------+-------+-------+------+-------+-------+-------+
| CLP5      | 39 (   | 71 (  | 88 (  |      |       |       |       |
|           | 5)     | 3)    | 1)    |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| GFNI      | 43 (   |       |       |      |       |       |       |
|           | 1)     |       |       |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| GFDI      | **22 ( | **29  |       |      |       |       |       |
|           | 3)**   | (     |       |      |       |       |       |
|           |        | 1)**  |       |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| GFSI      | 47 (   | 88 (  | 147 ( |      |       |       |       |
|           | 5)     | 3)    | 1)    |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| AEMI      | 37 (   | **43  | 32 (  |      |       |       |       |
|           | 5)     | (     | 1)    |      |       |       |       |
|           |        | 3)**  |       |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| NGPI      | 69 (   | 144 ( | 246 ( |      |       |       |       |
|           | 3)     | 2)    | 1)    |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| BAMD      | 37 (   | 72 (  | 64 (  |      |       |       |       |
|           | 5)     | 3)    | 1)    |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| BAMM      | 41 (   | 79 (  | 93 (  |      |       |       |       |
|           | 5)     | 3)    | 1)    |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| BAMS      | 30 (   | 59 (  | 59 (  |      |       |       |       |
|           | 5)     | 3)    | 1)    |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| CONU      | 44 (   | 90 (  | 188 ( |      |       |       |       |
|           | 5)     | 3)    | 1)    |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| GUNA      | 29 (   |       |       |      |       |       |       |
|           | 1)     |       |       |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| FSSE      | **20 ( |       |       |      |       |       |       |
|           | 2)**   |       |       |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| OFCL      | 26 (   | 49 (  |       |      |       |       |       |
|           | 4)     | 2)    |       |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+
| NHC       | 33     | 57    | 79    | 99   | 140   | 188   | 233   |
| Official  | (1349) | (     | (     | (    | (655) | (465) | (311) |
|           |        | 1192) | 1039) | 897) |       |       |       |
| (         |        |       |       |      |       |       |       |
| 2002-2006 |        |       |       |      |       |       |       |
| mean)     |        |       |       |      |       |       |       |
+-----------+--------+-------+-------+------+-------+-------+-------+

Table 3. Preliminary intensity forecast evaluation (heterogeneous
sample) for Tropical Storm Erick, 31 July-2 August 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.

+---------+-------+--------+--------+--------+--------+-------+-------+
| Number  | For   |        |        |        |        |       |       |
| range   | ecast |        |        |        |        |       |       |
| C       | P     |        |        |        |        |       |       |
| HAPTERF | eriod |        |        |        |        |       |       |
| orecast | (h)   |        |        |        |        |       |       |
| Te      |       |        |        |        |        |       |       |
| chnique |       |        |        |        |        |       |       |
+=========+=======+========+========+========+========+=======+=======+
|         | 12    | 24     | 36     | 48     | 72     | 96    | 120   |
+---------+-------+--------+--------+--------+--------+-------+-------+
| SHF5    | 4.6 ( | 6.7 (  | 7.0 (  |        |        |       |       |
|         | 5)    | 3)     | 1)     |        |        |       |       |
+---------+-------+--------+--------+--------+--------+-------+-------+
| GHMI    | 4.7 ( | **4.0  |        |        |        |       |       |
|         | 3)    | ( 1)** |        |        |        |       |       |
+---------+-------+--------+--------+--------+--------+-------+-------+
| DSHP    | 4.0 ( | 5.0 (  | 5.0 (  |        |        |       |       |
|         | 5)    | 3)     | 1)     |        |        |       |       |
+---------+-------+--------+--------+--------+--------+-------+-------+
| FSSE    | 6.0 ( |        |        |        |        |       |       |
|         | 2)    |        |        |        |        |       |       |
+---------+-------+--------+--------+--------+--------+-------+-------+
| ICON    | 4.3 ( | 5.0 (  |        |        |        |       |       |
|         | 3)    | 1)     |        |        |        |       |       |
+---------+-------+--------+--------+--------+--------+-------+-------+
| OFCL    | 2.5 ( | 5.0 (  |        |        |        |       |       |
|         | 4)    | 2)     |        |        |        |       |       |
+---------+-------+--------+--------+--------+--------+-------+-------+
| NHC     | 6.3   | 11.0   | 14.6   | 16.9   | 18.9   | 18.5  | 19.3  |
| O       | (     | (1192) | (1039) | (896)  | (655)  | (465) | (311) |
| fficial | 1349) |        |        |        |        |       |       |
|         |       |        |        |        |        |       |       |
| (20     |       |        |        |        |        |       |       |
| 02-2006 |       |        |        |        |        |       |       |
| mean)   |       |        |        |        |        |       |       |
+---------+-------+--------+--------+--------+--------+-------+-------+

Number range CHAPTERNumber range
CHAPTER![](media/image1.wmf){width="7.622222222222222in"
height="5.93125in"}

Number range CHAPTERFigure 1. Best track positions for Tropical Storm
Erick, 31 July-2 August 2007.

![](media/image2.wmf){width="6.930555555555555in"
height="4.819444444444445in"}

Number range CHAPTERFigure 2. Selected wind observations and best track
maximum sustained surface wind speed curve for Tropical Storm Erick, 31
July-2 August 2007.

![](media/image3.wmf){width="6.930555555555555in"
height="4.819444444444445in"}

Number range CHAPTERFigure 3. Selected pressure observations and best
track minimum central pressure curve for Tropical Storm Erick, 31 July-2
August 2007.