As fairly quiet weather continues across the Prairies, the big weather story recently has been Hurricane Harvey, which came ashore in Texas late on Aug. 26.
Harvey rapidly strengthened in the 12-hour period leading up to landfall and came ashore as a borderline Category 4 hurricane, with top winds of 210 kilometres per hour. It wasn’t the winds that ended up being the problem with this system, but rather the rains.
Hurricanes and tropical storms typically bring with them copious amounts of rain. Totals will often be in the 100- to 200-millimetre range with amounts sometimes pushing into the 300- to 400-millimetre range. What helps to determine just how much rain will fall is the speed that the system moves through. Typically, these systems pick up speed as they travel northwards and get picked up by the mid-latitude westerlies. This helps to limit just how much rain can fall on any given area.
This didn’t happen with Harvey.
Instead of picking up speed as Harvey moved inland, the mid- and upper-level steering currents around it collapsed, which essentially caused Harvey to stop moving. This set up the perfect conditions for a major history-making rain event to develop over southern and eastern Texas.
With Harvey stalled out right near the coast, it was still able to tap into the huge amounts of moisture over the Gulf of Mexico and pump it inland. Computer models, which did a very good job of predicting rainfall totals early on, predicted some regions would see as much as 1,250 millimetres of rain. In fact, an area east of Houston received 51.88 inches or 1,317 millimetres. That’s right, more than 1.3 metres of rain!
That beat the U.S. record of 1,220 millimetres of rain that fell from Tropical Storm Amelia in 1978.
The chart above shows some of the earlier rainfall totals measured over the Saturday and Sunday time period around the Houston region.
Let’s try to put these values into some kind of perspective (see charts below).
Total yearly rainfall for Winnipeg is on average about 500 millimetres, for Calgary it is around 400 millimetres. The largest single rain event that I know of was the 325 millimetres of rain that fell in High River over a 48-hour period in June of 2013. This was part of the historic flooding that occurred across parts of southern Alberta in that year.
Trying to dig out multi-day rain events is tough to do. Here is a list of single-day record rainfalls for the main centres I use across the Prairies followed by the largest monthly total rainfalls ever recorded. (Data source: Environment Canada.)
From this you can see that we really have no idea just how much rain fell across Texas from Harvey. There are places that received — in just one 24-hour period — three times more rain than we have seen in a record wet month!