Very heavy rain occurred in the Houston, Texas urban area on Memorial Day 2015. Several people drowned and property damage was substantial in a small part of the city. Was this a random weather event or was it part of a trend toward more-frequent heavy rain, possibly driven by global warming?
The US Historical Climatology Network (USHCN) provides convenient online data from climate monitoring stations scattered across the US. Unfortunately, there are no USHCN sites in Houston but there are three climate sites nearby: Liberty, Danevang (love that town’s name) and Brenham, located east, west and northwest of Houston, respectively. Here are the mapped sites, with Houston visible as the lighter-colored region in the middle:
I looked at daily rainfall data for the three sites for the period 1951 thru 2014, a 64-year period. This covers the time when atmospheric CO2 strongly increased.
First, for background, let’s look at annual rainfall at the three sites. Below is a plot of annual rainfall at the three sites (combined) since 1951. Southeast Texas has an east/west gradient in annual rainfall, with eastern areas getting more rain on average. Liberty, roughly 100 miles east of the other sites, receives on average about 30% more rain than do Brenham or Danevang. The three sites (combined) average about 46 inches a year, fairly typical for the eastern US. The plot of annual rainfall below shows considerable interannual variability.
Of interest is that there’s no clear trend in annual rainfall. Totals generally rose after the dry 1950s but have trended downwards since the 1980s. This lack of a trend is confirmed by rain data for the “Upper Texas Coast” from NOAA’s Climate at a Glance website. The data shows variation but no trend for the last 120 years.
OK, what about heavy rain events? For this review I defined a “heavy rain” as 4 inches or more in a day. For Houston, that amount in a day typically causes street flooding and motorist inconvenience but poses no risk to life or property. I summed the heavy rain events (Liberty + Brenham + Davevang) for each day since 1951 and then totaled the events over three year periods. A plot of total heavy rain events over the past 36 months gives an indication whether the frequency of heavy rain is growing, shrinking or is trendless. Here’s the plot since the early 1950s:
Here’s the same data except starting in the mid-1980s:
There’s no clear trend. There might have been an increase as the region emerged from the relatively-dry 1950s but any increase disappeared in recent decades. It’s doubtful that a linear trend would be evident in any case, as the atmosphere tends to oscillate over these time scales rather than trend in a straight line.
The review used 4-inch or greater rainfall days. What if the threshold was 2 inches instead of 4 inches? Here’s the plot”
How about 6+ -inch rainfall days? The plot is below. As expected, there are few such heavy rain events, a fact that complicates any search for trends. A linear trendline slopes upward but there might have been a step upwards about 1980 (the time of the PDO shift to its warm phase) with little change thereafter.
If the 65-year trendline is accepted as real then it means that the 6-inch rains per site have increased by 0.1 – 0.15 per year, or about one additional event per 9 years. The increase largely occurred over 30 years ago.
Now for a side issue: if overall rainfall increases but the distribution (big, medium, small) of rainfall events is unchanged, then it seems reasonable to expect that the count of each sized event would increase. So, what if an adjustment was made to account for the change in total rainfall? Below is a plot of the count of 4-inch rain events in the prior 36 months divided by the total rainfall in the prior 36 months:
And 6-inch rains adjusted for total precipitation:
There are no clear trends in heavy-rain, especially in the last 30 years. The practical effects of any increases, if they exist, are weak. Perhaps, over a longer period, increasing CO2 will drive increasing heavy rain events in the Houston area. There’s no clear evidence of that happening today.