Monday, July 30, 2007

More Heat, Less Humidity, Some Light


Warm, humid, widely scattered thunderstorms. Following yesterday's much-needed, if not exactly drought-busting rains (National 0.98", Dulles 0.54"), today's storm activity has been considerably lighter and mainly confined to east of I-95. National reported thunder for several hours and even some frequent lightning at mid afternoon, but no rain as of post time. A north to northeasterly wind indicates that the nearly stationary front is over the Eastern Shore and southeastern Virginia, but humidities are still quite muggy with dewpoints within a degree or two of 70°.

A few isolated storms are possible into the evening hours, then gradual clearing overnight. For the hotter and drier outlook through the rest of the week and into the weekend, scroll on down to Jason's post below.

Tropical Topics

The National Hurricane Center is keeping an eye on a tropical wave about 950 miles east of the southern Windward Islands. This area shows some signs of organization and has the potential to develop more in the next couple of days.

Some more light is being shed on the issue of a possible hurricane/global warming link described in Chris Mooney's just-published book, "Storm World". The discussion continues with the publication today of a peer-reviewed paper, "Heightened Tropical Cyclone Activity in the North Atlantic: Natural Variability or Climate Trend?" in the Philosophical Transactions of the Royal Society of London. This is a formal publication of data presented in a forum at the annual meeting of the American Meteorological Society in January. The analysis by Greg Holland of the National Center for Atmospheric Research and Peter Webster of Georgia Tech indicates that there has been a significant increase (see chart to the right) in Atlantic Basin tropical cyclone activity in the last century.

In press reports (see AP, via WaPo), however, an NHC "official" is quoted as saying that the research is "sloppy science". The reason this is controversial is (1) the sample size is so small, even over 100 years, and (2) there have been major improvements in observational technology over that time, most notably aircraft reconnaissance beginning in 1944 and satellite surveillance in 1970. Chris Landsea of the National Hurricane Center has some very interesting charts in a paper published in the May 1 issue of EOS, the Transactions of the American Geophysical Union. He shows, for example, that in the record-breaking season of 2005, storms were observed over nearly all of the Atlantic Ocean. In the previous record year of 1933, however, no storms were observed over the central Atlantic, where data was presumably much more limited.

Holland and Webster are not unaware of this possible effect and take it into account in their analysis. One point they make is that storms in the open ocean were less likely to be missed in the pre-satellite era because unsuspecting ships would accidentally encounter them. The most convincing argument, however, is that there are indeed several jumps in the storm frequency, but they occur at different points than the changes in technology.

This debate is likely to continue for a long time. Of all the possible connections to global warming, tropical cyclones are probably the most difficult to definitively analyze.

Chart by Steve Deyo, © UCAR

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