Tuesday, May 16, 2017

Mid-May Weather Pattern Created Extremes Around Northern Half Of World

Wildfires in Greece this week amid record early season heat
Photo by Aris Messinis, AFP/Getty Images
An especially wavy jet stream in the Northern Hemisphere is causing some pretty extreme weather in some areas of the world this week.

The jet stream - that upper atmospheric river of fast moving air that controls weather patterns, is experiencing wild bulges to the north in some areas, and deep plunges southward in others.

Depending on where these ridges and dips are and move the weather can change quite a bit.

As noted yesterday, this is affecting us here in Vermont: It snowed in some high elevations Sunday. By Thursday, a few towns in the warmer valleys of the Green Mountain State will be around 90 degrees.

As noted in Weather Undergrounds Category 6 blog, the weather elsewhere has taken some strange turns as well. Greece and parts of northern Africa are having some of their hottest May weather on record. It was as high as 105 degrees in one town in Greece, and Athens reached 92 degrees, its hottest day for so early in the season.

Brush fires broke out in Greece, killing one person.

Meanwhile, in parts of northern Europe, winter hangs on. Norway had its snowiest May since the 1960s. Parts of eastern Finland received six to 10 inches of snow on Mother's Day. In  Russia, bursts of heavy snow hit Moscow on May 11.

A snowy scene in Moscow on May 11
Meanwhile, back in the United States, parts of the Rocky Mountains, especially in areas of Idaho and Montana, a winter storm watch is in effect through Thursday because of expected heavy snow.

Part of the reason for the wild weather extremes and the wavy jet stream is something called a "negative NAO."  

NAO stands for "North Atlantic Oscillation."  Sometimes it's positive, sometimes it's negative, and there's a complicated index that tells you how positive and how negative it is. If it's really positive or really negative, weather can go off the rails.

A negative NAO can mean less than normal low atmospheric pressure near Iceland and less than normal high pressure in the subtropics. The weak pressure gradient helps give the jet stream more room to meander more to the north or south.

In the first part of May it the "NAO" was really  negative. That meant weather patterns that favored colder weather in the Northeastern United States and in northern Europe and hot weather in soutern Europe.

The NAO is trending toward zero, neither negative nor positive, so the weather is calming down somewhat. (By the way, when the NAO is positive, it tends to get stormy in western Europe, warm in the United States and frigid up in northern Canada.)

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