Unprecedented June Heat in Northwest U.S. Caused by Extreme Jet Stream Pattern

By Jeff Masters
Published: 02:30 PM GMT die 29o June, anno 2015

A searing heat wave unprecedented for June scorched the Northwest U.S. and Western Canada on Saturday and Sunday. Temperatures soared to their highest June levels in recorded history for portions of Washington, Idaho, Montana, and British Columbia; both Idaho and Washington set all-time high temperature records for the month of June on Sunday. According to wunderground's weather historian, Christopher C. Burt, the 113°F measured in Walla Walla, Washington beat that state's previous June record of 112°F, set at John Day Dam on June 18, 1961. In addition, the 111°F reading at Lewiston, Idaho was that state's hottest June temperature on record. An automated station at Pittsburg Landing, Idaho hit 116°F, but that reading will have to be verified before being considered official. A slew of major stations set all-time June heat records on both Saturday and Sunday in Washington, Idaho, and Montana, and at least two tied their hottest temperature for any day in recorded history. A destructive wildfire hit Wenatchee, Washington overnight, destroying twelve buildings. Wenatchee set a new June record high of 109°F on Sunday, just one degree shy of their all-time record of 110°F set on July 17-18, 1941. Jon Erdman of TWC has full details of all the records set. Sunday will end up being the hottest day of the heat wave for most locations in the Northwest U.S. and Western Canada, but temperatures will still be 10 - 15°F above average most of the remainder of the week.

Figure 1. A wildfire burns in Wenatchee, Washington on Sunday, June 28, during the hottest June temperatures ever recorded there. Image credit: komonews.com

What caused the heat wave?
The planet as a whole has experienced its warmest January - May period on record this year, and it is much easier to set all-time heat records when your baseline temperature is at record warm levels. But all-time records require some unusual meteorology, and this week's heat wave was caused by an extreme jet stream configuration that featured a very sharp ridge of high pressure over Western North America and a compensating deep trough of low pressure over the Midwest United States. The ridge of high pressure allowed hot air from the Southwest U.S. to push northwards, and brought sunny skies that allowed plenty of solar heating of the ground. An extreme jet stream configuration also was in evidence over Western Europe, where a strong ridge of high pressure on Sunday brought the warmest June temperatures ever recorded to the Spanish cities of Madrid and Toledo. This sort of extreme jet stream pattern has grown increasingly common in recent decades, as I wrote about for the December 2014 issue of Scientific American (behind a pay wall for $6.) A study published last week by researchers at Stanford University found that unusually intense and long-lived high pressure systems of the kind responsible for heat waves have increased over some parts of the globe since the advent of good satellite data in 1979. In particular, they found that in summertime, these patterns had increased over Europe, western Asia, and eastern North America. As yet, scientists have not come to a consensus on what might be causing the jet stream to behave in such an extreme fashion, though one leading theory is that rapid warming of the Arctic that has led to record sea ice and spring snow cover loss might be responsible.

Figure 2. Departure of temperature from average at 2 meters (6.6') as diagnosed by the GFS model at 00 UTC June 28, 2015. A sharp kink in the jet stream (Figure 3) allowed warm air to flow northwards into the Northwest U.S. and Western Canada beneath a ridge of high pressure, bringing temperatures up to 20°F above average. A compensating trough of low pressure set up over the Midwest U.S., allowing cold air to spill southwards and cause an usually cool June day. Data/image obtained using Climate Reanalyzer™ (http://cci-reanalyzer.org), Climate Change Institute, University of Maine, Orono, Maine.

Figure 3. Winds at a height where the pressure is 250 mb show the axis of the jet stream, seen here at 00 UTC June 28, 2015. An unusually strong ridge of high pressure was over Western North America and Western Europe, leading to all-time June temperature records being broken in both places. Data/image obtained using Climate Reanalyzer™ (http://cci-reanalyzer.org), Climate Change Institute, University of Maine, Orono, Maine.

A few of my recent posts discussing extreme jet stream behavior
Buffalo Belted With Five Feet of Snow; Is Jet Stream Weirdness to Blame? my November 2014 post.
California Drought/Polar Vortex Jet Stream Pattern Linked to Global Warming, my April 2014 post.
Extreme Jet Stream Bringing U.S. Record Heat, Record Cold, and Flash Flooding, my July 2013 post.
Extreme Jet Stream Pattern Triggers Historic European Floods, my June 2013 post.
Are atmospheric flow patterns favorable for summer extreme weather increasing? my March 2013 post.
Where's spring? 2nd most extreme March jet stream pattern on record extends winter, my March 2013 post.
Extreme jet stream causing record warmth in the east, record cold in the west, my January 2013 post.
Arctic sea ice loss tied to unusual jet stream patterns, my April 2012 post.
Summer in March, 2012, draws to a close, my March 2012 post.
Our extreme weather: Arctic changes to blame? my December 2011 post.

Note: I originally posted that Cranbrook, British Columbia hit 98°F (36.8˚C), its hottest temperature of any day since records began in 1901. Weather Records researcher Maximiliano Herrera pointed out to me that Cranbrook has changed location 4 times, and has had five reporting stations. If we consider all of these locations, Sunday's 36.8˚C reading does not set a record. Here are the highest temperatures at the main Cranbrook station:

37.2˚C 7/18/1918
37.8˚C 7/4/1924
38.3˚C 7/12/1940
38.9˚C 7/16 and 7/17 1941
37.8˚C 7/27/1939
37.8˚C 7/12/1953
37.2˚C 8/17 and 8/18 1967

Jeff Masters

Winnipeg Sunset (TheBigAppleImages)
Last Night Route 90 Looking South At The Perimeter Hwy. Smoke In The Air
Winnipeg Sunset, by TheBigAppleImages
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About The Author
Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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