9 thoughts on “Cyclone Nora magically avoids weather stations”

  1. Still no gusts over 100kmph measured at 3:54am. Winds currently at around 40kmph (10:30am Qld time) but still shows Nora as a Cat 2.
    (I believe the winds are measured by satellites – probably the same sats that measure temps which are promptly dismissed).

  2. The US JTWC warning position for TC Nora at 18:00Z on 24 March (4:00 AM on 25 March local time) was 15.5S 141.8E, about 6.5 km E-SE  of KowanyamaJTWC data shows the 1 min sustained wind as 65 KN (120 kph) with gusts to 80 KN (148 kph).  Using the BoM TC categories graphic to convert for BoM's preferred standard 10 min sustained wind it comes to approx. 57 KN (106 kph).  Hence converting from JTWC (US) terms to BoM standards 106 kph wind with 148 kph  gusts.
    The BoM TC graphic at No 27 shows Nora's track passed about 5 km to the west of Kowanyama and the BoM weather station there shows wind conditions peaked at 4:00 AM local time this morning with a sustained wind of 54 kph (29 KN) with gusts to 100 kph (54 KN).  This suggests that at 4:00 AM Nora was probably a tropical low and didn't rate as category 1, although when you consider the wind gust speed of 100 kph then Nora could be considered category 1?  But over land the ratio of gust to sustained wind speed becomes higher than over open sea so perhaps defer to barometric pressure, which at 984.2 hPa was probably low enough for Nora to be rated a category 1 TC….?
    If you consider the US JTWC 4:00 AM data then Nora rates as a mid category 2 (in BoM terms) or a low category 1 in US (Saffir-Simpson) terms.  However, the accuracy of the JTWC 4:00 AM position fix is quoted as within 40 NM which suggests caution in comparing the 4:00 AM data from BoM Kowanyama and that of the JTWC 4:00 AM warning, i.e. JTWC could have been measuring events up to 80.5 km (6.5 + 74) distant from the BoM weather station.

    So where does Nora now go…… will she bounce left or right?

  3. In my earlier comment the BoM TC categories graphic link is not very clear – just click on the link then click on item 2 under the heading Definitions.

  4. Thanks, Bob.
    ‘Q: How is tropical cyclone intensity determined?
    JTWC uses several tools and techniques to estimate tropical cyclone intensity, including subjective Dvorak estimates, objective fix data, and observations. Over most of the JTWC AOR, the Dvorak technique is the primary means to estimate tropical cyclone intensity. The Dvorak technique is based on the analysis of cloud patterns in visible and infrared imagery from geostationary and polar-orbiting satellites. The Dvorak technique results in a decimal number, called a T-number, which in turn corresponds to an intensity estimate.’
    Interesting way to measure intensity.
    Four uses of the word ‘estimate’ and one use of ‘subjective’.
    I presume the only ‘objective fix data’ are land/ship anemometers and maybe the odd weather balloon (which are all they had back before mid-1970s).

  5. Thanks again, Bob.
    A few sentences stand out.
    ‘The larger the difference, the more intense the tropical cyclone is estimated to be.’
    ‘Note that this estimation of both maximum winds and central pressure assumes that the winds and pressures are always consistent.’
    ‘Lastly, while the Dvorak technique is primarily designed to provide estimates of the current intensity of the storm, a 24 h forecast of the intensity can be obtained also by extrapolating the trend of the CI number. Whether this methodology provides skillful forecasts is unknown.’
    These words worry me.
    ‘estimated’, ‘estimation’, ‘assume’, ‘estimates’, ‘extrapolating’, ‘Whether…..unknown’.
    Wow, a cyclone intensity is based on that.

  6. So I guess when cyclone intensity is quoted as "fact" in our nightly MSM TV news recognise it for what it is, err, maybe somewhere near, um, err…  A case in point, tonight on our central Victorian 5:00 PM 9 News we saw a graphic still showing TC Nora spinning her way westward across the Gulf of Carpentaria toward Arnhem Land  while in reality the remnants of Nora is currently dumping on the east coast of Cape York Peninsular.  Clearly BoM was wrong in its predicted track for Nora, JTWC was closer to the mark, 9 News is probably just slack in upgrading its graphic?

  7. I see our ABC running protection for the BoM here – ignoring facts from wind gauge data.
    Cyclone damage predictions and what’s wrong with the current category system
    still claiming Yasi as a Cat5 when the damage was slight.
    Cyclone Yasi has been exaggerated by Govts and beaten up by media 4Feb2011
    We badly need a Senator who can probe the BoM at Estimates hearings. So much to do.

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