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

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.

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.

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