Number range CHAPTERTropical Cyclone Report

Tropical Storm Dalila

(EP072007)

22-27 July 2007

Daniel P. Brown

National Hurricane Center

29 August 2007

Tropical Storm Dalila was a northwestward-moving tropical cyclone that
reached its peak intensity near Socorro Island.

a.  Synoptic History

Dalila formed from a tropical wave that entered the eastern North
Pacific Ocean on 17 July. The wave spawned a broad area of low pressure
several hundred nautical miles south-southeast of the Gulf of
Tehuantepec on 19 July. As the low moved west-northwestward during the
next couple of days, the associated shower activity gradually increased.
Dvorak satellite classifications were initiated shortly before 0000 UTC
21 July and additional development during the next 24 h led to the
formation of a tropical depression at 0000 UTC 22 July, centered about
400 n mi south of Manzanillo, Mexico. 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.

Northeasterly shear inhibited significant strengthening and the cyclone
remained a tropical depression for the next two days. The depression
moved west-northwestward during the first 24 h of its existence, but
then turned northwestward on 23 July around the southwestern portion of
a mid-level ridge centered over northern Mexico. The shear decreased
late on 23 July, which allowed thunderstorm activity to form closer to
the circulation center. The depression strengthened and became a
tropical storm at 0000 UTC 24 July, while located about 350 n mi
southwest of Manzanillo. Thereafter, the shear continued to relax and
Dalila gradually gained strength, reaching a peak intensity of 50 kt at
0000 UTC 25 July, while centered about 50 n mi southeast of Socorro
Island. Dalila continued moving northwestward and its center passed over
or very near Socorro Island just after 0600 UTC 25 July. The next day
the tropical storm turned west-northwestward and began moving over
progressively cooler water which initiated a weakening trend. Dalila
weakened to a tropical depression at 0600 UTC 27 July and degenerated to
a remnant low 12 h later, while located about 400 n mi west of the
southern tip of Baja California. The remnant low moved
west-northwestward during the next couple days before slowing down and
turning west-southwestward on 29 July. It then drifted southward on 30
July and dissipated about 700 miles west of the southern tip of Baja
California around 1200 UTC that day.

b.  Meteorological Statistics

    Observations in Dalila (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 Dalila.

    The best-track intensity of Dalia on 22-23 July is slightly lower
    than the subjective Dvorak intensity estimates (Fig. 2). During this
    time the center was extremely difficult to locate in conventional
    infrared imagery and the estimated center location of some of these
    Dvorak fixes were likely too far west and farther embedded in the
    deep convection. This resulted in slightly inflated intensity
    estimates, which were somewhat exacerbated during this time period
    due to Dvorak rules that do not allow prompt weakening.

    Four QuikSCAT passes between 1313 UTC 23 July and 0115 UTC 25 July
    were helpful in determining the intensity of Dalila during this
    time. The pass at 1313 UTC 23 July detected tropical storm force
    winds within the tropical cyclone's circulation, however analysis of
    these data suggest that these vectors were rain-inflated. The
    estimated peak intensity of 50 kt was based on data from the 0115
    UTC 25 July QuikSCAT pass. Because Dalila was an atypically large
    tropical cyclone, the corresponding estimated minimum pressure is a
    little lower than what the standard Dvorak pressure-wind
    relationship yields.

    There were no surface observations available from Socorro Island
    during the passage of Dalila, and no ship observations of winds of
    tropical storm force in association with Dalila were received.

c.  Casualty and Damage Statistics

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

d.  Forecast and Warning Critique

    The Tropical Weather Outlook issued at 1700 UTC 20 July, about 31 h
    prior to development, was the first Outlook to describe the area of
    disturbed weather from which Dalila developed. However, the first
    Outlook to explicitly mention possible tropical depression formation
    was issued only about 7 h prior to genesis.

    A verification of official and guidance model track forecasts is
    given in Table 2. Average official track errors (with number of
    cases in parentheses) for Dalila were 32 (21), 48 (19), 69 (17), 99
    (15), 142 (11), 165 (7), and 187 (3) n mi for the 12, 24, 36, 48,
    72, 96, and 120 h forecasts, respectively. The official track errors
    through 72 h are comparable to the average 5-year (2002-2006)
    official track errors while errors at days 4 and 5 are lower than
    the 5-year mean. The shallow and medium Beta and Advection Model
    (BAMS and BAMM) errors were lower than the NHC track forecasts for
    most time periods. The official track errors were lower than most of
    the individual dynamical model guidance; however, the consensus of
    these models (GUNA and CONU) and the Florida State Super Ensemble
    (FSSE) provided forecasts that were slightly better than the NHC
    track predictions through 96 h.

    Average official intensity errors (Table 3) for Dalila were 4, 6, 5,
    8, 11, 15, and 17 kt for the 12, 24, 36, 48, 72, 96, and 120 h
    forecasts, respectively. Despite official intensity errors that were
    lower than the 5-year means, the NHC forecasts were biased high,
    especially at 96 and 120 h. Intensity guidance from the Decay-SHIPS
    model showed a similar high bias and the interpolated GFDL model
    (GFDI) was the only intensity predictor that was consistently better
    than the official intensity forecast.

Number range CHAPTERTable 1. Best track for Tropical Storm Dalila, 22-27
July 2007.

+----------+----------+----------+----------+---------+--------------+
| D        | Latitude | L        | Pressure | Wind    | Stage        |
| ate/Time |          | ongitude |          | Speed   |              |
|          | (°N)     |          | (mb)     |         |              |
| (UTC)    |          | (°W)     |          | (kt)    |              |
+----------+----------+----------+----------+---------+--------------+
| 22 /     | 12.5     | 104.6    | 1008     | 25      | tropical     |
| 0000     |          |          |          |         | depression   |
+----------+----------+----------+----------+---------+--------------+
| 22 /     | 12.6     | 105.3    | 1008     | 25      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 22 /     | 12.7     | 105.9    | 1007     | 30      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 22 /     | 12.9     | 106.6    | 1007     | 30      | \"           |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 23 /     | 13.2     | 107.3    | 1007     | 30      | \"           |
| 0000     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 23 /     | 13.8     | 108.0    | 1007     | 30      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 23 /     | 14.5     | 108.5    | 1006     | 30      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 23 /     | 15.1     | 108.9    | 1006     | 30      | \"           |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 24 /     | 15.8     | 109.3    | 1005     | 35      | tropical     |
| 0000     |          |          |          |         | storm        |
+----------+----------+----------+----------+---------+--------------+
| 24 /     | 16.4     | 109.7    | 1004     | 35      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 24 /     | 16.9     | 110.0    | 1002     | 40      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 24 /     | 17.5     | 110.3    | 999      | 45      | \"           |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 25 /     | 18.1     | 110.6    | 995      | 50      | \"           |
| 0000     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 25 /     | 18.7     | 110.9    | 996      | 50      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 25 /     | 19.3     | 111.3    | 998      | 45      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 25 /     | 19.8     | 111.8    | 1000     | 45      | \"           |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 26 /     | 20.3     | 112.3    | 1000     | 45      | \"           |
| 0000     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 26 /     | 20.9     | 112.9    | 1000     | 45      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 26 /     | 21.5     | 113.7    | 1000     | 45      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 26 /     | 22.0     | 114.8    | 1002     | 40      | \"           |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 27 /     | 22.4     | 115.7    | 1003     | 35      | \"           |
| 0000     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 27 /     | 22.7     | 116.6    | 1005     | 30      | tropical     |
| 0600     |          |          |          |         | depression   |
+----------+----------+----------+----------+---------+--------------+
| 27 /     | 23.0     | 117.5    | 1006     | 25      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 27 /     | 23.3     | 118.2    | 1008     | 25      | low          |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 28 /     | 23.6     | 118.9    | 1009     | 25      | \"           |
| 0000     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 28 /     | 23.9     | 119.6    | 1009     | 25      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 28 /     | 24.2     | 120.3    | 1009     | 25      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 28 /     | 24.5     | 121.0    | 1009     | 25      | \"           |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 29 /     | 24.6     | 121.5    | 1009     | 25      | \"           |
| 0000     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 29 /     | 24.6     | 121.9    | 1009     | 25      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 29 /     | 24.6     | 122.2    | 1009     | 25      | \"           |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 29 /     | 24.4     | 122.5    | 1009     | 25      | \"           |
| 1800     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 30 /     | 24.1     | 122.5    | 1009     | 20      | \"           |
| 0000     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 30 /     | 23.8     | 122.5    | 1009     | 20      | \"           |
| 0600     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 30 /     |          |          |          |         | dissipated   |
| 1200     |          |          |          |         |              |
+----------+----------+----------+----------+---------+--------------+
| 25 /     | 18.1     | 110.6    | 995      | 50      | minimum      |
| 0000     |          |          |          |         | pressure     |
+----------+----------+----------+----------+---------+--------------+

Number range CHAPTERNumber range CHAPTERTable 2. Preliminary track
forecast evaluation (heterogeneous sample) for Tropical Storm Dalila,
22-27 July 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    | For   |       |       |       |       |       |       |
| range     | ecast |       |       |       |       |       |       |
| CHAPTE    | P     |       |       |       |       |       |       |
| RForecast | eriod |       |       |       |       |       |       |
| Technique | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CLP5      | 44    | 79    | 121   | 171   | 264   | 401 ( | 535 ( |
|           | (21)  | (19)  | (17)  | (15)  | (11)  | 7)    | 3)    |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFNI      | 38    | 76    | 107   | 132   | 186 ( | 195 ( | 365 ( |
|           | (18)  | (16)  | (14)  | (12)  | 9)    | 5)    | 1)    |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDI      | **31  | 59    | 90    | 117   | 163   | 204 ( | 298 ( |
|           | (     | (18)  | (16)  | (14)  | (10)  | 6)    | 2)    |
|           | 20)** |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFSI      | **26  | **44  | 73    | 110   | **124 | **156 |       |
|           | (     | (     | (16)  | (14)  | (     | (     |       |
|           | 21)** | 18)** |       |       | 8)**  | 4)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| AEMI      | **30  | 56    | 94    | 132   | 179   | 231 ( | 228 ( |
|           | (     | (19)  | (17)  | (15)  | (11)  | 7)    | 3)    |
|           | 21)** |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NGPI      | 32    | 51    | 73    | 99    | **137 | 173 ( | 353 ( |
|           | (21)  | (19)  | (17)  | (15)  | (     | 7)    | 3)    |
|           |       |       |       |       | 11)** |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| UKMI      | 41    | 66 (  | 90 (  | 101 ( | **110 | **112 |       |
|           | (11)  | 9)    | 7)    | 5)    | (     | (     |       |
|           |       |       |       |       | 3)**  | 1)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMD      | 43    | 74    | 106   | 138   | 194   | 241 ( | 270 ( |
|           | (21)  | (19)  | (17)  | (15)  | (11)  | 7)    | 3)    |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMM      | **30  | **45  | **64  | **82  | **107 | **134 | **111 |
|           | (     | (     | (     | (     | (     | (     | (     |
|           | 21)** | 19)** | 17)** | 15)** | 11)** | 7)**  | 3)**  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMS      | **25  | **41  | **59  | **79  | **111 | **142 | 240 ( |
|           | (     | (     | (     | (     | (     | (     | 3)    |
|           | 21)** | 19)** | 17)** | 15)** | 11)** | 7)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CONU      | **27  | **44  | **66  | **93  | **121 | **134 | 226 ( |
|           | (     | (     | (     | (     | (     | (     | 2)    |
|           | 21)** | 19)** | 17)** | 15)** | 10)** | 6)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GUNA      | **26  | **43  | **60  | **83  | **107 | **121 |       |
|           | (     | (     | (     | (     | (     | (     |       |
|           | 20)** | 17)** | 15)** | 13)** | 8)**  | 4)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | **24  | **32  | **39  | **58  | **81  | **53  |       |
|           | (     | (     | (     | (     | (     | (     |       |
|           | 15)** | 13)** | 11)** | 9)**  | 5)**  | 1)**  |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 32    | 48    | 69    | 99    | 142   | 165 ( | 187 ( |
|           | (21)  | (19)  | (17)  | (15)  | (11)  | 7)    | 3)    |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 33    | 57    | 79    | 99    | 140   | 188   | 233   |
| Official  | (     | (     | (     | (897) | (655) | (465) | (311) |
|           | 1349) | 1192) | 1039) |       |       |       |       |
| (         |       |       |       |       |       |       |       |
| 2002-2006 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

Table 3. Preliminary intensity forecast evaluation (heterogeneous
sample) for Tropical Storm Dalila, 22-27 July 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 |       |       |       |       |       |       |
| CHAPTE    | P     |       |       |       |       |       |       |
| RForecast | eriod |       |       |       |       |       |       |
| Technique | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| SHF5      | 5.6   | 8.5   | 8.4   | 9.4   | *     | 18.0  | 26.0  |
|           | (21)  | (19)  | (17)  | (15)  | *11.2 | ( 7)  | ( 3)  |
|           |       |       |       |       | (     |       |       |
|           |       |       |       |       | 11)** |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDI      | 3.9   | **6.2 | 6.4   | **6.9 | **8.2 | **9.0 | **7.0 |
|           | (20)  | (     | (16)  | (     | (     | (     | (     |
|           |       | 18)** |       | 14)** | 10)** | 6)**  | 2)**  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| SHIP      | 4.9   | 8.4   | 8.1   | 10.0  | *     | 16.4  | 23.7  |
|           | (21)  | (19)  | (17)  | (15)  | *11.1 | ( 7)  | ( 3)  |
|           |       |       |       |       | (     |       |       |
|           |       |       |       |       | 11)** |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| DSHP      | 4.9   | 8.4   | 8.1   | 10.0  | *     | 16.4  | 23.7  |
|           | (21)  | (19)  | (17)  | (15)  | *11.1 | ( 7)  | ( 3)  |
|           |       |       |       |       | (     |       |       |
|           |       |       |       |       | 11)** |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | 5.1   | 7.2   | 6.1   | 8.3 ( | **8.6 | 16.0  |       |
|           | (15)  | (13)  | (11)  | 9)    | (     | ( 1)  |       |
|           |       |       |       |       | 5)**  |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| ICON      | 4.2   | 6.9   | 7.4   | 9.9   | *     | 17.5  | 21.5  |
|           | (20)  | (18)  | (16)  | (14)  | *10.4 | ( 6)  | ( 2)  |
|           |       |       |       |       | (     |       |       |
|           |       |       |       |       | 10)** |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 3.8   | 6.3   | 5.3   | 8.0   | 11.4  | 15.0  | 16.7  |
|           | (21)  | (19)  | (17)  | (15)  | (11)  | ( 7)  | ( 3)  |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 6.3   | 11.0  | 14.6  | 16.9  | 18.9  | 18.5  | 19.3  |
| Official  |       | (     | (     | (896) | (655) |       |       |
|           | \(1   | 1192) | 1039) |       |       | \(    | \(    |
| (         | 349\) |       |       |       |       | 465\) | 311\) |
| 2002-2006 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

Number range CHAPTER

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

Number range CHAPTERFigure 1. Best track positions for Tropical Storm
Dalila, 22-27 July 2007.

![](media/image2.wmf){width="8.36736111111111in"
height="5.6819444444444445in"}

Number range CHAPTERFigure 2. Selected wind observations and best track
maximum sustained surface wind speed curve for Tropical Storm Dalila,
22-27 July 2007.

![](media/image3.wmf){width="8.374305555555555in"
height="5.677083333333333in"}

Number range CHAPTERFigure 3. Selected pressure observations and best
track minimum central pressure curve for Tropical Storm Dalila, 22-27
July 2007.