Alberta Floods: Why is there so much rain?

Saturday, September 21, 2013, 5:33 PM -

A week ago, meteorologists at The Weather Network knew there would be the potential to be a big one, including severe thunderstorms and heavy rain, in Alberta. While we couldn't foresee the scale and impact of the floods, computer models did suggest a very unique weather pattern would set up that would eventually lead to the disastrous floods in Alberta. 

An upside-down weather pattern took shape across western North America earlier in the week. By ‘upside-down’ we’re referring to a jet stream pattern that causes warmer temperatures to the north than the south. Dramatically, this was exemplified on Thursday (when the flooding began) with Fort Simpson in the Northwest Territories being the hottest place in all of Canada at 31 C. Meanwhile, Calgary only reached 17°C.

Courtesy: College of DuPage Weather Lab

How did this upside-down weather pattern occur?

The jet stream is the roughly continuous corridor of strong high-altitude winds (at the cruising altitude of passenger aircraft) which moves from west to east across the mid-latitudes.  The jet stream pattern which dictates the weekly variations in our weather can be thought of as a skipping rope, with ups and downs (what meteorologists call ridges and troughs).  These ups and downs in the jet stream are responsible for our fair and foul weather respectively.

Sometimes, the jet stream pattern gets stuck or ‘blocked’ as we say in meteorology.  This is the setup that took shape across western North America this week.  The large ‘up’ or ridge in the jet stream which caused record warmth in Alaska and hot weather across parts of Canada’s north, blocked a strong dip in the jet stream from moving quickly from west to east.  In the image below, the ‘H’ represents High pressure, or the up in the jet stream, and the ‘L’ represents Low pressure, or the down in the jet stream.

Courtesy: College of DuPage Weather Lab

With the counter-clockwise winds around this low pressure system, a channel of very moist air was tapped from the Gulf of Mexico and pulled into Alberta. Like a fire hose, this atmospheric river of water vapour blasted up against the foothills and Rocky Mountains, rising, cooling, condensing and releasing tremendous amounts of precipitation.

Courtesy: College of DuPage Weather Lab

The key factors that led to the tremendous rainfall in Alberta can be summarized as follows:

1. An upside-down or ‘blocked’ jet stream pattern across western North America 
2. A strong area of low pressure that developed and got stuck near southern Alberta because of the jet stream pattern.
3. A moist channel of air from the Gulf of Mexico that was pulled up by the low pressure system and slammed up against the foothills and Rocky Mountains.

While the extent of the flooding has surprised everyone, the actual forecast of heavy rain was well predicted. The image below shows the Canadian computer model’s forecast from Wednesday, prior to the heavy rain starting. This model, and most other computer models that meteorologists use to make forecasts, accurately predicted that over 100 mm of rain would fall, and even suggested that epic totals above 150 mm would be possible.

Canadian computer model prediction of rainfall. Courtesy: Environment Canada

If the weather forecasts were relatively accurate, why was there little warning about the scale and severity of the flooding? The interface between meteorology and hydrology (weather forecasting vs. river forecasting) is a very difficult area of prediction and needs more attention both from a research and operational focus. For example, snow cover can be both a positive and a negative in a flooding situation. In this case, more snow pack may have resulted in less serious flooding because of its ability to act like a sponge, slowing the runoff of rain water.

Disasters like this demonstrate the destructive potential of moving water. Flooding, particularly flash flooding (where water rises very fast in just hours due to very heavy rain), is a serious severe weather risk in Canada. The stories and images from Alberta and British Columbia are a shocking reminder of how damaging the impact can be.