Steve M expected, way back in early August, that there would be seven or fewer Atlantic hurricanes in 2006. To date, there have been five, with one (Ernesto) which might be downgraded to a tropical storm in reanalysis.

I am declaring Steve a winner on his wager, even though there are about 10 days left in the official season. He has beaten the US National Hurricane Center, Bill Gray, me, most private forecasting firms, my local TV weatherman, my neighbor and (I bet) the RealClimate regulars.

The reason I’m doing this 10 days early is because the Caribbean is about to experience a large intrusion of cool, dry air. That is somewhat unusual for mid-November. Orlando, Florida (DisneyWorld) is forcast to dip to +2C, Miami to +8C and Habana, Cuba to +14C. Miami is even issuing a “wind chill advisory”, I guess for the beachgoers.

This dry, cool air greatly inhibits the main breeding ground (western Caribbean) for late-season storms.

We’ll make a (virtual) champagne toast to Steve on December 1′st.

The Australian Bureau of Meteorology has a table here that lists the estimated minimum pressure of each storm in its region, stretching back to circa 1900. It’s not the same a 6-hourly estimates, but it’s still of some use.

I looked at the minimum pressures of storms on this list from 1970 thru 1979, which is essentially pre-satellite for intensity estimates. It is also a period used by Webster. Here is the distribution:

1000-1010 mb: 2 storms
990-999 mb: 34
980-989 mb: 42
970-979 mb: 37
960-969 mb: 19
950-959 mb: 15
940-949 mb: 8
930-939 mb: 8
920-929 mb: 3
910-919 mb: 2
900-909 mb: 0

Here are the storms for 1990-1999, presumably a predominately satellite (Dvorak) era:

1000-1010 mb: 0 storms
990-999 mb: 17 storms
980-989 mb: 14
970-979 mb: 13
960-969 mb: 13
950-959 mb: 16
940-949 mb: 7
930-939 mb: 7
920-929 mb: 10
910-919: 7
900-909: 1

Plotted, those distributions look quite different to me. Either (a) global warming has affected the distribution of storm pressures/intensities, (b) the use of different estimation techniques has affected the distributions or (c) some other shift occurred in the data, such as inclusion of storms from a different region. My guess is (b) or (c). If it is (a), then it seems likely that we should find similar shifts in other storm basins. I will be checking for (c).

(Note: my storm counts are by eyeball-tally, so my counts could be off by a few, but that would not affect the patterns.
Note: The lower the minimum pressure, the stronger the storm. Category 4 and 5 storms are probably 940 mb and lower. Weak storms are 980 mb and higher.)

It will be a very long time before we can discuss tornado frequency (or hail or straight-line winds) in the context of climate change. That’s too bad because it is an interesting question. Tornado winds are not measured; they are inferred from structural damage. On top of that, the F-scale is an uncalibrated scale. Extremely large tornadoes that hit nothing get ranked as F0, even if they had the potential to do tremendous damage. Storm spotters have also helped increase the number of reported tornadoes. A “harmless” tornado passing through a meadow may have been missed 25 years ago if it didn’t bother anyone; very few tornadoes are missed now. Wind speeds from thunderstorms are rarley measured; they are estimated–big difference. Hail stones are compared to coins or spherical objects (by spotters), then assigned an estimated diameter. There are more spotters near cities, so rural severe weather is under-reported. You wanna talk inhomogeneities? Start here.

The experts attribute the rise in recorded tornadoes to improvements in detection and reporting, and not due to actual increases in tornadoes. What a difference improved techniques make in the historical record. This is also true in the world of hurricanes.

I am glad that (so far) tornadoes have not entered the global-warming arena.

All that does is move the question upstream. The question is, where did the Australians, Fijians and Reunioners get their pressure values?

I’ve e-mailed the Australian BOM asking if they can explain the pressure-estimation methodology used in Australia before 1980. The other major party is Reunion (France) but my French is quite poor, so I am trying to think of a third party to ask.

What I expect to learn is that forecasters made best-guesses of central pressure and intensity pre-1980, based on scattered data from the edge of the storm. The exception would be if the storm overran an island having a barograph. Best-guesses are a poor technique (but better than nothing, usually) and become a real problem if they are grafted onto other techniques like satellite estimates and then used in a search for small trends.

How small of a data change is being sought? Per Steve’s graphs, there are about 100 storm-days a year in the Southern Hemisphere, with about 10 of those being category 4 or 5. Ten percent. What we’re looking for is evidence of about a 25 to 50% increase in cat 4 and 5, or 3 to 5 extra intense storm-days a year, in a mostly-remote 8,000-mile stretch of ocean. Daunting.

Neumann’s article contain many warnings and caveats about the SH storm intensity data, even in the satellite era. The data was OK for his purposes (calculating hurricane risk at given locations) but my guess is that he’d caution against using it to search for changes in cat 4 and 5 frequency. I wonder if he was asked.