Number range CHAPTERTropical Cyclone Report

Unnamed Subtropical Storm

4-5 October 2005

Jack Beven and Eric S. Blake

National Hurricane Center

10 April 2006

As part of its routine post-season review, the Tropical Prediction
Center/National Hurricane Center (TPC/NHC) on rare occasions identifies
from new data or meteorological interpretation a previously unnoted
tropical or subtropical cyclone. The TPC/NHC re-analysis of 2005 has
revealed a short-lived subtropical storm near the Azores Islands, which
increases the record count of tropical/subtropical storms during 2005 to
28.

a.  Synoptic History

An upper-level low formed just west of the Canary Islands on 28
September and moved generally westward for the next two days. A large
but transient burst of convection developed near the low on 30
September, accompanied by the formation of a surface trough. The complex
system moved generally westward to west-northwestward into a more
unstable air mass, which allowed sporadic convection to develop. Surface
observations and satellite cloud-motion winds indicate a broad surface
low formed within the trough late on 3 October about 400 n mi southwest
of São Miguel Island in the Azores. Convection increased in association
with the surface low, and it is estimated that the system became a
subtropical depression around 0600 UTC 4 October (Table 1 and Figure 1).

The depression turned northeastward in the warm sector ahead of an
approaching cold front associated with a large non-tropical low
northwest of the Azores. Additional development occurred, and it is
estimated that the cyclone became a subtropical storm around 1200 UTC 4
October. The system reached an estimated peak intensity of 45 kt as the
center passed through the eastern Azores later that day. The cyclone
turned north-northeastward and merged with the approaching cold front
early on 5 October. Late that day, the remains of the subtropical storm
were absorbed by the non-tropical low. That low would evolve into
Hurricane Vince a few days later.

b.  Meteorological Statistics

Observations in the subtropical cyclone (Figs. 2 and 3) include
satellite-based Hebert-Poteat technique intensity estimates from the
Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis
Branch (SAB), along with surface observations from the Azores and nearby
ships. Microwave satellite imagery from NOAA polar-orbiting satellites
and the NASA QuikSCAT satellite were also useful in tracking this
system.

There are several pieces of evidence suggesting that this system had
enough tropical characteristics to be considered a subtropical storm
instead of a non-tropical low. The first is cloud patterns resembling
those used in the subtropical cyclone intensity estimates of the
Hebert-Poteat technique, which reflects that the system had sufficiently
organized convection (Fig. 4). A second is the non-frontal nature of the
surface low as depicted on TAFB surface analyses (not shown).
Additionally, surface data in the Azores did not reveal any significant
temperature changes from the passage of the cyclone, as all observed
temperature changes in the area occurred west of the cold front. The
final piece is data from the Advanced Microwave Sounding Unit (AMSU) on
the NOAA polar-orbiting satellites. Figure 5 shows AMSU temperature
imagery at 1517 UTC 4 October. Both the channel 5 image (corresponding
to approximately 550 mb) and the channel 7 image (corresponding to
approximately 200 mb) show a warm core, with the strongest signal in the
channel 5 image.

The presence of the warm core at 200 mb suggests the possibility that
the storm may have become fully tropical. However, the warm core is
indistinct in channel 8 imagery (corresponding to approximately 100 mb;
not shown), and satellite cloud-motion winds (not shown) indicate the
presence of an upper-level trough over the storm near the time of peak
intensity rather than the upper-level anticyclone that would be expected
for a tropical cyclone. Additionally, the radius of maximum winds based
on surface observations was large, about 75 n mi. These factors argue in
favor of a subtropical rather than a tropical classification.

There were no ship reports of gale-force winds from this system.
However, a ship with the call sign VRZN9 reported a pressure of 999 mb
and 22-kt winds at 1800 UTC 4 October.

This system brought gale-force winds to the eastern portions of the
Azores Islands. Santa Maria Island reported a 10-min average wind of 43
kt at 2100 UTC 4 October with a peak gust of 51 kt. The station reported
a minimum pressure of 1006 mb at that time. Ponta Delgada on São Miguel
reported 10-min average winds of 33 kt at 2230 UTC 4 October with a gust
to 46 kt. The station reported a minimum pressure of 999 mb at 2100 UTC
that day.

c.  Casualty and Damage Statistics

    There were no reports of damage or casualties associated with this
    system.

d.  Forecast and Warning Critique

    No official forecasts were issued for the subtropical storm, thus no
    verification is available. Operationally, it was treated as a
    non-tropical low. Post-storm analysis, including AMSU data that were
    not available in real time, indicated that the system had sufficient
    tropical cyclone characteristics to be considered a subtropical
    storm for 12-18 h.

    *Acknowledgements*

    Derrick Herndon and Chris Velden of the Cooperative Institute of
    Meteorological Satellite Studies at the University of Wisconsin
    provided post-storm analysis satellite data. David Roth of the
    Hydrometeorological Prediction Center in Washington, DC, also
    provided useful information on the system.

Table 1. Best track for the unnamed Subtropical Storm, 4-5 Oct 2005.

+---------+---------+---------+---------+-----------+----------------+
| Da      | L       | Lo      | P       | Wind      | Stage          |
| te/Time | atitude | ngitude | ressure | Speed     |                |
|         |         |         |         |           |                |
| (UTC)   | (°N)    | (°W)    | (mb)    | (kt)      |                |
+---------+---------+---------+---------+-----------+----------------+
| 04 /    | 33.8    | 31.8    | 1012    | 30        | low            |
| 0000    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 04 /    | 34.8    | 30.2    | 1009    | 30        | subtropical    |
| 0600    |         |         |         |           | depression     |
+---------+---------+---------+---------+-----------+----------------+
| 04 /    | 35.9    | 28.5    | 1004    | 35        | subtropical    |
| 1200    |         |         |         |           | storm          |
+---------+---------+---------+---------+-----------+----------------+
| 04 /    | 37.1    | 26.7    | 997     | 45        | \"             |
| 1800    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 05 /    | 38.8    | 25.0    | 997     | 45        | \"             |
| 0000    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 05 /    | 41.0    | 23.3    | 1000    | 40        | extratropical  |
| 0600    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 05 /    | 43.6    | 23.0    | 1002    | 40        | \"             |
| 1200    |         |         |         |           |                |
+---------+---------+---------+---------+-----------+----------------+
| 05 /    |         |         |         |           | absorbed by    |
| 1800    |         |         |         |           | larger low     |
+---------+---------+---------+---------+-----------+----------------+
| 04 /    | 37.1    | 26.7    | 997     | 45        | minimum        |
| 1800    |         |         |         |           | pressure       |
+---------+---------+---------+---------+-----------+----------------+

![](media/image1.wmf){width="8.010416666666666in"
height="6.291666666666667in"}

Figure 1. Best track positions for the unnamed subtropical storm, 4-5
October 2005.

![](media/image2.wmf){width="8.991666666666667in"
height="6.110416666666667in"}

Figure 2. Selected wind observations and best track maximum sustained
surface wind speed curve for the unnamed subtropical storm, 4-5 October
2005.

![](media/image3.wmf){width="8.991666666666667in"
height="6.110416666666667in"}

Figure 3. Selected pressure observations and best track minimum central
pressure curve for the unnamed subtropical storm, 4-5 October 2005.

![](media/image4.png){width="8.46875in" height="6.111111111111111in"}

Figure 4. METEOSAT-8 visible image of the unnamed subtropical storm at
1500 UTC 4 October.

Surface observations are overlaid on the satellite imagery, and a cold
front is analyzed to the west.

![](media/image5.png){width="4.5in" height="4.748611111111111in"}

Figure 5. AMSU temperature imagery from NOAA-18 overpass of the unnamed
subtropical storm at 1517 UTC 4 October. Channel 5 image from
approximately 550 mb is on the left, while a channel 7 image from
approximately 200 mb is on the right. Imagery courtesy of the
Cooperative Institute for Meteorological Satellite Studies at the
University of Wisconsin, Madison, Wisconsin. The storm center was near
36.5N 27.6W at this time.