GFS model simulation of jet stream pattern and omega block over western Canada. (WeatherBell.com)

Unseasonably hot weather in Alberta, Canada, is fueling the worst wildfire disaster in the country’s history. An extreme weather pattern, known as an omega block, is the source of the heat.

An omega block is essentially a stoppage in the atmosphere’s flow in which a sprawling area of high pressure forms. This clog impedes the typical west-to-east progress of storms. The jet stream, along which storms track, is forced to flow around the blockage.

At the heart of the block in Canadian’s western provinces, the air is sinking and much warmer than normal. Such a clog can persist for days until the atmosphere’s flow is able to break it down and flush it out.

Centers of storminess form on both sides of the block, and the resulting jet stream configuration takes on the likeness of the Greek letter omega. In this case, cool and unsettled weather is affecting the eastern Pacific Ocean and eastern North America, including much of the U.S. East Coast.

As the Fort McMurray wildfire rapidly spread Tuesday, temperatures surged to 90 degrees (32 Celsius), shattering the daily record of 82 degrees set May 3, 1945. Dozens of other locations in Alberta also had record high temperatures.

More records are likely to fall today. Temperatures are forecast to climb well into the 80s today at Fort McMurray, about 30 degrees warmer than normal. The average high is in the upper 50s.


Temperature difference from normal in western Canada. (WeatherBell.com)

The combination of heat and lack of recent rain and snow have combined to create tinderbox conditions. “Each month this year has been drier than average in Fort McMurray, and the city was 1.09 inches below average on precipitation for the January through April time period,” Weather.com reports.

Winds gusting over 20 mph have fanned the flames.

Fortunately, the block is forecast to break down by Thursday, allowing cooler air into the region and the chance of  showers. However, warm and dry weather is forecast to develop Friday through the weekend.

Mild, dry conditions through the winter and spring probably helped set the stage for this fire disaster. As Mashable’s Andrew Freedman points out, below-average snowfall dried out the landscape earlier than normal. “Typically, melting snow cover in early May would keep springtime temperatures far lower,” he wrote.

Since January, from Alaska through the western Canadian provinces and into the Pacific Northwest, a persistent ridge of high pressure has led to warmer-than-normal and sometimes record-breaking conditions.


Average upper-level weather pattern since January shows high pressure (warm pool) extending from Alaska through western Canada into the Pacific Northwest. (National Oceanic and Atmospheric Administration)

Freedman notes that the warmth present in western Canada is part and parcel of the long-term climate-warming trend. “Climate data shows that Fort McMurray has seen an increase in the number of days with high temperatures above 25 degrees Celsius, or 77 degrees Fahrenheit, since 1950,” he reports. “This number has jumped from an average of 21 such days in 1950 to an average of 35 such days in 2010.”