500mb heights compared to normal from April 26 through June 23, encompassing all of the current negative NAO with data. (ESRL/NOAA/author)

For almost two months now, a bulging zone of high pressure has stagnated near Greenland. This consequential weather pattern is poised to persist for the longest duration in modern records.

The stubborn pattern is making all kinds of weather more extreme. In May, it was connected to severe thunderstorms, tornadoes and flooding in the central United States. In June, it delivered a record-challenging early-season melt event over Greenland. This week, in concert with other weather systems, it is torching Europe in a potentially historic heat wave.

This high-pressure zone over Greenland is often referred to as a “blocking” pattern because it slows the flow of weather systems circulating around the Northern Hemisphere. When present, punishing weather extremes can affect the same areas for extended periods.

Scientists evaluate the presence of a Greenland block and its intensity and duration through an index known as the North Atlantic Oscillation (NAO). A negative NAO is often an indicator of a Greenland block, as it is now. A positive NAO signals low pressure over Greenland and generally less extreme weather over the Northern Hemisphere continents.

A negative NAO has now persisted for 61 days. Today will be day 62.


Observations of the North Atlantic Oscillation. (NOAA)

As Mika Rantanen, a meteorologist in Finland, tweeted last week, this is one of the lengthiest runs of a negative NAO on record and currently ranks third-longest. The only two longer stretches are 64 days from Dec. 22-Feb. 23 in 1963 and 68 days from June 6 to Aug. 11 in 2011.

Given that forecasts keep the negative NAO going through at least the end of the month, this run certainly has a chance to rival 2011′s record stretch. Even if doesn’t, it will be notable for being the sixth top 10 stretch since 2010 in a record back to 1950.

“The high latitude blocking has been very impressive,” wrote Judah Cohen, a scientist at Atmospheric and Environmental Research who closely tracks this pattern, in an email.

Cohen sees some opportunity for the negative NAO to break in the next month, but that prediction is uncertain. Compared to winter, summer patterns in the mid-and-high latitudes are much less energetic and more prone to stagnation.

“[O]nce the pieces fell into place for a long-lived negative NAO, there is almost no off button in summer that exists in winter,” Cohen wrote.


Ensemble forecasts for the North Atlantic Oscillation in the weeks ahead shows it rise into the end of the month then perhaps dipping again. (StormVistaWxModels.com)

This locked-in pattern has had many consequences. Below is a small sample of extreme weather and climate events at least partly related:

  • Greenland has melted at unprecedented rates this year. A negative NAO is frequently called a Greenland block because of its location. Where there’s high pressure in the warm months, temperatures are elevated.
  • The negative NAO doesn’t cause storminess, but it tends to cause the jet stream to dip into the Lower 48 states and incite storminess. Since late spring, a series of storms has triggered severe weather, including tornadoes and flooding.
  • Cold and snowy conditions are common to the west of the Greenland block, where the jet stream dips and chilled air surges south. Near the summer solstice, the central and northern Rockies picked up one to two feet of snow while Colorado’s snowpack is “off the charts” for the time of year. Ice was reported to be 20 feet thick on parts of the southern part of Hudson Bay into mid-June when the area is usually ice-free.
  • Rapid City in southwest South Dakota is in the midst of one of the coolest years since its records began in 1943. The average temperature of 35.3 degrees through June 25 is just behind the coldest mark of 35.2 in 1950. Oklahoma City finally hit 90 degrees in recent days. Before that, it was one of only seven years since 1891 to make it to June 19 with no 90s. Denver has reached only 86 degrees so far, which is the second-coolest maximum temperature to date.
  • Europe is baking in an extreme early-season heat wave. Part of the driver is the stopped-up North Atlantic pattern. The negative NAO is joining forces with high pressure over the continent to intensify the event.

In many ways, the pattern in the Lower 48 has played out as one might expect with a negative NAO. When present, cool air tends to dominate across much of the United States both in April and May. That was the case.


Temperature differences compared to normal in June show much of the country cooler than normal, a common theme of late. (PRISM Climate Group)

Impacts of a negative NAO tend to ease into June, but still generally favor cool weather in the north central U.S. When a negative NAO is present in July and August, traces of cooler than normal conditions have lingered historically although the overall signal dwindles.

A number of climate scientists now say this kind of blocking pattern is becoming more common as the Arctic rapidly warms. The opposite pattern configuration, the positive NAO, is more common with a cold Arctic.


A negative NAO generally promotes cooler than normal air in much of the country in spring, and the north-central parts into early summer. (Mark Ellinwood)

Winter lovers in the central and eastern United States often think of the negative NAO is a plus for cold and snow. For instance, Washington’s snowiest winter on record in 2009-10 was heavily driven by a negative NAO, as are many East Coast snowstorms.

But Cohen says it’s unlikely the current NAO will last that long. “This summer’s record-long negative NAO is not a reliable indicator of a negative winter NAO,” he wrote.

Regardless, as the countdown to the record enters its final week, this negative NAO will surely be remembered for its effects, and it will undoubtedly be studied for a long time to come.