By Paul Homewood
Thankfully it’s stopped raining in Houston, so time for a round up on Storm Harvey.
Total rainfall from the storm peaked at 51.88 in at Cedar Bayou, just outside Houston.
This beat the previous record rainfall from a single storm over the contiguous US. The previous record was 48 in from Storm Amelia in 1978, also in Texas. However, most of that fell over just four days, from Aug 2nd to 5th.
Rainfall from Harvey was less intensive, as it was spread over five days.
Neither event was close in intensity to Storm Claudette, which dumped 43 in on Alvin, Texas in July 1979 in a single 24-hour period.
Texas Hurricane Trends
In 2010, David Roth of the NWS published his “Texas Hurricane History”, which gave this list of Texas landfalling cyclones:
Since 2010, there have been three tropical storms, Alex, Hermine and Bill, and no hurricanes (prior to Harvey of course).
The 1940s was by far the busiest decade.
Roth goes on to explain why the weaker tropical storms tend to carry the greatest flooding risk:
He also includes the full list of hurricanes:
Based on this list, and including Harvey, we can tabulate the number of major hurricanes by decade:
Clearly there is no trend towards more powerful hurricanes, and Harvey was the first Cat4 since Gilbert in 1988. (I have seen it incorrectly reported that Harvey was the most powerful Texas hurricane for “more than 50 years”).
Roth also has an account of one the worst Texas floods on record, San Antonio’s Great Flood of 1921:
The rainfall recorded over 18 hours in Williamson County was far greater than anything experienced around Houston during Harvey.
Jet Stream Behaviour
The usual suspects have attempted to claim that global warming has made Harvey more intense than it would have been otherwise, for instance Mann and Trenberth here.
However, comparison with earlier storms, such as San Antonio, Amelia (1978) and Claudette (1979) discredits such claims.
As we know, the only reason why so much rain fell on Houston was because Harvey was stuck between two high pressure systems for five days. So step forward Michael Mann to claim that this weather event was due to climate change!
“The kind of stalled weather pattern that is drenching Houston is precisely the sort of pattern we expect because of climate change,” climatologist Michael Mann explained in an email to ThinkProgress.
Earlier this year, Mann co-authored a study explaining how human-caused warming is changing our atmosphere’s circulation, including the jet stream, in a way that leads to “increase in persistent weather extremes” during the summer.
“I agree with Mike [Mann] that the weak steering currents over the south-central US coincident with Harvey are consistent with our expectations for a warmer world, which of course includes effects of a very warm Arctic,” Jennifer Francis, a climate scientist at Rutgers University, told ThinkProgress.
Francis is a leading expert on how global warming and the related Arctic amplification affect the jet stream and extreme weather. A study she co-authored that appeared in the Journal of Climate in June, concludes, “Over the central United States during summer, the weaker and wavier flow” of the jet stream favors “more intense summer weather.”
A 2012 study led by the National Oceanic and Atmospheric Administration (NOAA), and coauthored by Francis, concluded global warming was driving changes in extreme weather in North America. “Our research reveals a change in the summer Arctic wind pattern over the past six years,” lead author James Overland of NOAA explained at the time. “This shift demonstrates a physical connection between reduced Arctic sea ice in the summer, loss of Greenland ice, and potentially, weather in North American and Europe.”
“Enhanced warming of the Arctic affects the jet stream by slowing its west-to-east winds and by promoting larger north-south meanders in the flow,” NOAA said in a press release. “The researchers say that with more solar energy going into the Arctic Ocean because of lost ice, there is reason to expect more extreme weather events, such as heavy snowfall, heat waves, and flooding in North America and Europe but these will vary in location, intensity, and timescales.”
Only one problem with Mickey Mann’s theory!
HH Lamb found exactly the same phenomenon back in the 1960s and 70s, when the Arctic was getting much colder and the sea ice was expanding. In 1982, he wrote:
Such worldwide surveys as have been attempted seem to confirm the increase of variability of temperature and rainfall [since 1950].’’
In Europe, there is a curious change in the pattern of variability: from some time between 1940 and 1960 onwards, the occurrence of extreme seasons – both as regards temperature and rainfall has notably increased.
These variations, perhaps more than any underlying trend to a warmer or colder climate, create difficulties for the planning age in which we live. They may be associated with the increased meridionality of the general wind circulation, the greater frequency of blocking, of stationary high and low pressure systems, giving prolonged northerly winds in one longitude and southerly winds in another longitude sector in middle latitudes.
Over both hemispheres there has been more blocking in these years… The most remarkable feature seems to be the an intensification of the cyclonic activity in high latitudes near 70-90N, all around the northern polar region. And this presumably has to do with the almost equally remarkable cooling of the Arctic since the 1950’s, which has meant an increase in the thermal gradient between high and middle latitudes.
And he was not the only one. CC Wallen of the WMO wrote:
The principal weather change likely to accompany the cooling trend is increased variability-alternating extremes of temperature and precipitation in any given area-which would almost certainly lower average crop yields.
During cooler climatic periods the high-altitude winds are broken up into irregular cells by weaker and more plentiful pressure centers, causing formation of a “meridional circulation” pattern. These small, weak cells may stagnate over vast areas for many months, bringing unseasonably cold weather on one side and unseasonably warm weather on the other. Droughts and floods become more frequent and may alternate season to season, as they did last year in India. Thus, while the hemisphere as a whole is cooler, individual areas may alternately break temperature and precipitation records at both extremes.
In other words, these proper scientists found the exact opposite of what Mann and Francis are claiming now.
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August 31, 2017 at 11:09AM