Number range CHAPTERTropical Cyclone Report

Tropical Storm Nana

(AL142008)

12-14 October 2008

Stacy R. Stewart

National Hurricane Center

28 November 2008

Nana was a short-lived, weak tropical storm that remained over the open
tropical Atlantic Ocean throughout its lifetime and threatened no land
areas.

a.  Synoptic History

    Nana originated from a tropical wave that moved off the west coast
    of Africa coast on 6 October. QuikSCAT satellite-derived surface
    wind data (not shown) indicated the wave was accompanied by a broad
    low pressure system. Convection associated with the wave was
    initially minimal, but by 8 October convection became organized into
    bands around the low center as the system moved westward at about 10
    kt. By 0000 UTC 11 October, thunderstorm activity had increased
    enough for Dvorak satellite classifications to be initiated.
    Convective banding continued to increase and become better organized
    over the next 36 h, and it is estimated that a tropical depression
    formed around 0600 UTC 12 October, when the system was centered
    about 690 n mi west of the Cape Verde Islands. The "best track"
    chart of the tropical cyclone's path is given in Fig. 1. The best
    track positions and intensities are listed in Table 1[^1], with the
    wind and pressure histories shown in Figs. 2 and 3, respectively.

    The depression moved west-northwestward toward a weakness in the
    subtropical ridge and quickly strengthened into a tropical storm
    just 6 h later. Nana continued its west-northwestward motion into a
    region of moderate to strong upper-level westerly winds. Nana is
    estimated to have reached its peak intensity of 35 kt and lowest
    estimated minimum pressure at 0000 UTC 13 October (Fig. 4) before it
    weakened back to depression status 12 h later about 870 n mi west of
    the Cape Verde Islands. The strong westerly vertical wind shear
    continued to displace the convection well to the east of the center,
    and Nana degenerated into a non-convective remnant low pressure
    system on 14 October. The remnant low turned northwestward ahead of
    a strong frontal system and dissipated around 1200 UTC 15 October
    about 820 n mi east-northeast of the Leeward Islands.

b.  Meteorological Statistics

    Observations in Nana (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). 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 Nana.

    The estimated peak intensity of 35 kt is based on QuikSCAT wind data
    obtained from a 2036 UTC 12 October overpass.

    There were no ship reports of winds of tropical storm force
    associated with Nana.

c.  Casualty and Damage Statistics

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

d.  Forecast and Warning Critique

    The genesis of Nana was well forecast. The incipient disturbance was
    first noted in the Tropical Weather Outlook (TWO) issued at 0000 UTC
    10 October. The possibility of tropical depression or tropical
    cyclone formation was first mentioned in the 0000 UTC 11 October TWO
    -- 30 h before actual genesis occurred. The experimental genesis
    forecasts were in the "high" (\> 50% probability) category 24 h
    prior to issuance of the first advisory.

    A verification of official and guidance model track forecasts is
    given in Table 2. Average official track errors for Nana were 33,
    42, and 68 n mi for the 12, 24, and 36 h forecasts, respectively.
    The number of forecasts ranged from 5 at 12 h to only 1 at 36 h.
    These errors are lower than the average long-term official track
    errors at 24 and 36 h (Table 2), albeit for a very small sample.

    Average official intensity errors were 2, 5, and 5 kt for the 12,
    24, and 36 h forecasts, respectively (Table 3). For comparison, the
    average long-term official intensity errors are 7, 10, and 12 kt,
    respectively.

No watches and warnings were associated with Nana.

Number range CHAPTERTable 1. Best track for Tropical Storm Nana, 12-14
October 2008.

+---------+--------+--------+---------+---------+-------------------+
| Da      | La     | Lon    | P       | Wind    | Stage             |
| te/Time | titude | gitude | ressure | Speed   |                   |
|         |        |        |         |         |                   |
| (UTC)   | (°N)   | (°W)   | (mb)    | (kt)    |                   |
+---------+--------+--------+---------+---------+-------------------+
| 12 /    | 15.5   | 36.6   | 1007    | 30      | tropical          |
| 0600    |        |        |         |         | depression        |
+---------+--------+--------+---------+---------+-------------------+
| 12 /    | 16.0   | 37.1   | 1006    | 35      | tropical storm    |
| 1200    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 12 /    | 16.4   | 37.7   | 1005    | 35      | \"                |
| 1800    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 13 /    | 16.5   | 38.4   | 1004    | 35      | \"                |
| 0000    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 13 /    | 16.6   | 39.0   | 1005    | 35      | \"                |
| 0600    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 13 /    | 16.8   | 39.6   | 1006    | 30      | tropical          |
| 1200    |        |        |         |         | depression        |
+---------+--------+--------+---------+---------+-------------------+
| 13 /    | 17.2   | 40.3   | 1007    | 30      | \"                |
| 1800    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 14 /    | 17.6   | 41.5   | 1007    | 30      | \"                |
| 0000    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 14 /    | 18.0   | 42.7   | 1007    | 30      | \"                |
| 0600    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 14 /    | 18.3   | 44.0   | 1008    | 25      | remnant low       |
| 1200    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 14 /    | 18.6   | 45.0   | 1008    | 25      | \"                |
| 1800    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 15 /    | 19.4   | 46.1   | 1009    | 25      | \"                |
| 0000    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 15 /    | 20.5   | 47.1   | 1010    | 25      | \"                |
| 0600    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 15 /    | 22.0   | 48.0   | 1011    | 20      | \"                |
| 1200    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 15 /    |        |        |         |         | dissipated        |
| 1800    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+
| 13 /    | 16.5   | 38.4   | 1004    | 35      | minimum pressure  |
| 0000    |        |        |         |         |                   |
+---------+--------+--------+---------+---------+-------------------+

Number range CHAPTERNumber range CHAPTERNumber range CHAPTERTable 2.
Track forecast evaluation (heterogeneous sample) for Tropical Storm
Nana, 12-14 October 2008. Forecast errors (n mi) are followed by the
number of forecasts in parentheses.

+-----------+-------+-------+-------+-------+-------+-------+-------+
| Number    | For   |       |       |       |       |       |       |
| range     | ecast |       |       |       |       |       |       |
| CHAPTE    | P     |       |       |       |       |       |       |
| RForecast | eriod |       |       |       |       |       |       |
| Technique | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| CLP5      | 54 (  | 99 (  | 205 ( | 412 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFNI      | 79 (  | 166 ( |       |       |       |       |       |
|           | 3)    | 1)    |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFDI      | 45 (  | 75 (  | 46 (  | 166 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| HWFI      | 66 (  | 90 (  | 26 (  | 131 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GFSI      | 49 (  | 105 ( | 117 ( |       |       |       |       |
|           | 7)    | 2)    | 2)    |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| AEMI      | 27 (  | 27 (  | 61 (  | 165 ( |       |       |       |
|           | 6)    | 3)    | 2)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NGPI      | 69 (  | 142 ( | 185 ( | 257 ( |       |       |       |
|           | 6)    | 4)    | 2)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| UKMI      | 41 (  | 75 (  |       |       |       |       |       |
|           | 3)    | 1)    |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| EGRI      | 41 (  | 75 (  |       |       |       |       |       |
|           | 3)    | 1)    |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| EMXI      | 55 (  | 104 ( | 158 ( | 234 ( |       |       |       |
|           | 3)    | 2)    | 2)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMD      | 108 ( | 201 ( | 303 ( | 529 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMM      | 45 (  | 74 (  | 129 ( | 253 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| BAMS      | 36 (  | 66 (  | 92 (  | 97 (  |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| LBAR      | 82 (  | 152 ( | 239 ( | 435 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| TVCN      | 43 (  | 78 (  | 78 (  | 200 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GUNA      | 36 (  |       |       |       |       |       |       |
|           | 3)    |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | 17 (  | 90 (  |       |       |       |       |       |
|           | 2)    | 2)    |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 33 (  | 42 (  | 68 (  |       |       |       |       |
|           | 5)    | 3)    | 1)    |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 34.0  | 58.2  | 82.2  | 106.2 | 154.2 | 207.5 | 272.5 |
| Official  | (     | (     | (     | (     | (996) | (787) | (627) |
|           | 1742) | 1574) | 1407) | 1254) |       |       |       |
| (         |       |       |       |       |       |       |       |
| 2003-2007 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

Table 3. Intensity forecast evaluation (heterogeneous sample) for
Tropical Storm Nana, 12-14 October 2008. Forecast errors (kt) are
followed by the number of forecasts in parentheses.

+-----------+-------+-------+-------+-------+-------+-------+-------+
| Number    | For   |       |       |       |       |       |       |
| range     | ecast |       |       |       |       |       |       |
| CHAPTE    | P     |       |       |       |       |       |       |
| RForecast | eriod |       |       |       |       |       |       |
| Technique | (h)   |       |       |       |       |       |       |
+===========+=======+=======+=======+=======+=======+=======+=======+
|           | 12    | 24    | 36    | 48    | 72    | 96    | 120   |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OCD5      | 3.6 ( | 5.8 ( | 6.7 ( | 8.0 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| GHMI      | 3.3 ( | 3.2 ( | 5.3 ( | 6.0 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| HWFI      | 6.4 ( | 9.6 ( | 14.0  | 19.0  |       |       |       |
|           | 7)    | 5)    | ( 3)  | ( 1)  |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| LGEM      | 4.7 ( | 3.2 ( | 5.3 ( | 8.0 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| DSHP      | 4.3 ( | 2.8 ( | 4.7 ( | 6.0 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| FSSE      | 3.0 ( | 1.5 ( |       |       |       |       |       |
|           | 2)    | 2)    |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| ICON      | 4.1 ( | 3.8 ( | 4.7 ( | 7.0 ( |       |       |       |
|           | 7)    | 5)    | 3)    | 1)    |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| OFCL      | 2.0 ( | 5.0 ( | 5.0 ( |       |       |       |       |
|           | 5)    | 3)    | 1)    |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+
| NHC       | 6.7   | 10.0  | 12.3  | 14.3  | 18.2  | 19.7  | 21.8  |
| Official  |       | (     | (     | (     | (996) | (787) | (627) |
|           | \(1   | 1574) | 1407) | 1254) |       |       |       |
| (         | 742\) |       |       |       |       |       |       |
| 2003-2007 |       |       |       |       |       |       |       |
| mean)     |       |       |       |       |       |       |       |
+-----------+-------+-------+-------+-------+-------+-------+-------+

Number range CHAPTER![](media/image1.jpeg){width="7.864583333333333in"
height="6.06875in"}

Number range CHAPTERFigure 1. Best track positions for Tropical Storm
Nana, 12-14 October 2008. Track positions during the remnant low stage
are based on analyses from the NOAA Tropical Prediction Center's
Tropical Analysis and Forecast Branch (TAFB).

![](media/image2.wmf){width="8.49236111111111in"
height="5.701388888888889in"}

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

> 12-14 October 2008. Estimates during the remnant low stage are based
> on analyses from the NOAA Tropical Prediction Center's Tropical
> Analysis and Forecast Branch (TAFB). Dashed vertical lines correspond
> to 0000 UTC.

![](media/image3.wmf){width="8.490277777777777in"
height="5.720138888888889in"}

Number range CHAPTERFigure 3. Selected pressure observations and best
track minimum central pressure curve for Tropical Storm Nana, 12-14
October 2008. Estimates during the extratropical stage are based on
analyses from the NOAA Tropical Prediction Center's Tropical Analysis
and Forecast Branch (TAFB). Dashed vertical lines correspond to 0000
UTC.

![](media/image4.wmf){width="8.995833333333334in" height="4.58125in"}

Number range CHAPTER Figure 4. Microwave satellite images of Tropical
Storm Nana near its peak intensity and lowest estimated minimum pressure
\--

a\. SSM/I S 2157 UTC 12 OCT 2008; b. AMSU-B 2331 UTC 12 OCT 2008 (images
courtesy of U.S. Naval Research Laboratory).

[^1]: A digital record of the complete best track, including wind radii,
    can be found on line at <ftp://ftp.nhc.noaa.gov/atcf>. Data for the
    current year's storms are located in the *btk* directory, while
    previous years' data are located in the *archive* directory.