Storm Harvey Update

Catastrophically for parts of Texas, Tropical Storm Harvey is not done yet:

“Catastrophic” flooding in the US state of Texas is only expected to worsen in coming days as waters rise following a storm of historic proportions.

A record 30in of rain (75cm) has already fallen on the city of Houston in the wake of Hurricane Harvey, turning roads into rivers.

But forecasters say that number could nearly double later this week.

http://www.bbc.co.uk/news/world-us-canada-41074919

Already some are attempting to link Harvey with global warming, for instance here. This is despite the US having gone a record amount of time without a major landfalling hurricane.

The real story is that Harvey has been stuck between two high-pressure cells since making landfall, thus dragging in large amounts of moisture from the Gulf, and continuously dumping it on a comparatively small area around Houston.

Current forecasts suggest about another 15 in to fall over the next three days, before things settle down.

As of Sunday evening, rainfall totals have been up to 27 in.

https://web.archive.org/web/20170828050322/http://www.wpc.ncep.noaa.gov/discussions/nfdscc1.html

There is nothing at all unprecedented about amounts like this. The all-time record for 24-hour rainfall in Texas was 42 in, set during Tropical Storm Claudette:

https://www.ncdc.noaa.gov/extremes/scec/records

In fact, there was a succession of catastrophic storms during the 1960s and 70s, when the Texas climate became much cooler, as the Texas Almanac records:

http://texasalmanac.com/topics/environment/significant-weather-1960s

According to NOAA, the rainfall from Amelia was actually much greater, amounting to 48 in at Medina, most over just four days, from Aug 2 to Aug 5.

Obviously most of the damage from Harvey has been due to the amount of rainfall, but it is worth taking a look at wind speeds as well, according to NCEP:

https://web.archive.org/web/20170828050322/http://www.wpc.ncep.noaa.gov/discussions/nfdscc1.html

As reported by satellites, Harvey peaked at a sustained speed of 115kt, or 132mph, just before landfall.

However, this appears to be at odds with the land based data, where the highest PEAK GUST was 132 mph. Wind gusts are typically about 1.3 times as high as 1-minute sustained speeds, according to NOAA, which suggests that Harvey’s sustained speeds at landfall were about 100 mph, making it a Cat-2 hurricane.

Note that hurricanes like Carla and Celia were much more powerful at landfall. Once more, this raises questions about the current practice of comparing satellite data with historical land data. This often results in claims that storms these days are more powerful than in the past.

In the meantime, let us pray that Texas stays safe.

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