As Tropical Storm Harvey continued to pound southeastern Texas on Monday, the U.S. National Weather Service issued an ominous forecast.

Some parts of Houston are now expected to receive an almost inconceivable 1.3 metres of rain, in total, by the time the storm finally blows through.

To put that in perspective: It's nearly 17 times the amount of rain that fell on Calgary over a period of three days during the June 2013 flood.

It's not an apples-to-apples comparison, though, as Calgary's flooding was amplified by even heavier rainfall and snowmelt in the mountains upstream that accumulated and carried a surge of high water down the Bow and Elbow rivers.

In the Canmore area, more than 200 millimetres fell over a 48-hour period, and much of that precipitation rapidly made its way downstream to Calgary, about 100 kilometres to the east.

Compare that to the 1,270 millimetres of rain that Harvey is expected to dump on Houston, and you can see how quickly cars and trucks were replaced by boats and rafts on the Texas city's streets.

That's more rain than Houston normally receives in an entire year, and it would be the largest amount of rainfall ever recorded in Texas from a single storm.

"The breadth and intensity of this rainfall is beyond anything experienced before," the National Weather Service said in a statement.

Brock Long, the director of the Federal Emergency Management Agency, predicted the aftermath of the storm would require FEMA's involvement for years.

"This disaster's going to be a landmark event," Long said.

With files from The Associated Press