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Reports from the National Data Buoy Center’s Stations During the Passage of Hurricane Flossie in the Eastern and Central Pacific

08 – 16 August 2007

Although Flossie did not set any extreme records at any of NDBC’s stations, it was unique in that it is the first time that a single tropical cyclone passed within 300 nautical miles (NM¹) of NDBC’s three types of moored buoy ocean observing systems – weather buoys (51004, 51002, and 51003), deep-ocean tsunameter (51407), and the Tropical Atmosphere-Ocean Array (TAO station 51006) (Figure 1). Initially a powerful tropical cyclone, it remained small and then weakened rapidly as it moved south of Hawai’i² .

By passing nearly over the top of station 51002, Flossie set extreme records for sea-level pressure, wind speed, and gusts (Figure 2) for August at station 51002 (Period of Record: September 1984 – August 2007). 51002’s minimum sea-level pressure of 1004.6 hPa was the lowest pressure for any month since January 1996. The 5.4 m wave height at 51004 was just shy of the August record of 5.5 m for that station (Figure 6).

1 The significance of 300 NM is that the tropical cyclone advisories request 3-hourly observations from ships within 300 NM of a tropical cyclone center. Track information is based on the positions given in the advisories.
2 Central Pacific Hurricane Center’s Preliminary Summary for Flossie (see http://www.prh.noaa.gov/cphc/summaries/2007.php#flossie

Figure 1: Flossie's Preliminary Track and NDBC Stations

Figure 2: Wind and Pressure at 51002

Figure 3: Wave Heights and Dominant Period at 51002

51002 also takes near-surface current measurements. During the passage of Flossie, 51002 reported the highest eastward speeds of the year (Figure 4) of more than 40 cm/sec.

Figure 4: Eastward Near-Surface Currents (SCMUV1), positive values are towards the east.

Link to Station Page: http://www.ndbc.noaa.gov/station_page.php?station=51002

Figure 5: Winds and Pressure at 51004

Figure 6: Wave Heights and Dominant Periods at 51004

Link to Station Page: http://www.ndbc.noaa.gov/station_page.php?station=51004

Figure 7: Winds and Pressure at 51003

Figure 8: Wave Height and Dominant Period at 51003

Link to Station Page: http://www.ndbc.noaa.gov/station_page.php?station=51003

The first NDBC station within 300 NM of Flossie was the TAO Station 51006 at 9°N 140°W. The closest point of Flossie’s approach to 51006 was approximately 219 NM to the north at 1100Z on 11 August 2007. NDBC receives hourly wind speed data from TAO Stations (Figure 9). The wind speeds are two-minute averages.

Figure 9: Wind Speed at TAO Station 51006

Lastly, Flossie passed 128 NM to the south the NDBC deep-ocean tsunameter at Station 51407, which is 140 NM southeast of Honolulu, at approximately 0900Z on 15 August 2007, although the water-column heights are unremarkable (Figure 10) given the small size of Flossie. The tsunameter measures pressure on the sea floor then converts the pressure to the height of the water column.

Figure 10: Estimated Water-Levels (MLLW) from Tsunameter 51407

Explanation of Terms

• Wind Speed: Wind speeds are reported in meters per second to a resolution of 0.1 m/s. For the plot legends:
• WSPD1: The average wind speed over an 8-minute period at the anemometer height. All anemometer heights on this report are at 5 meters.
• GUST1: The peak 5-second gust during the 8-minute sampling period.
• Pressure: Pressure is sampled at a rate of 1.0 Hz and reported to a resolution of 0.1 hectoPascal (hPa). For the plot legends:
• BARO1: Sea-level Pressure. Coastal-Marine Automated Network (C-MAN) stations average the pressure over 2-minutes. The buoy stations average the pressure over 8-minutes.
• Wave: The wave sampling period for all stations in this report is 20 minutes.
• WVHGT: The significant wave height that represents the average of the highest one-third of the waves during the wave sampling period.
• DOMPD: The Dominant Period that represents the wave period (time between consecutive passes of the wave crests) of the waves with the most energy.
• Currents: SCMUV1: The east-west current velocity. Positive values are towards the east and negative values towards the west.
• MLLW: Mean Lower Low Water derived from the observations from 10 August – 30 December 2007.
• Closest Point of Approach (CPA): Time, bearing (degrees true), and distance (nautical miles) from the station to Hurricane Flossie at CPA are computed using the positions from the National Hurricane Center's and Central Pacific Hurricane Center’s advisories that have been interpolated to hourly values for positions.

Acknowledgement: Dawn C. Petraitis provided hourly TAO data.