What is a Greenland block?

There’s a lot of talk on the block. What about? Many forecasters are closely monitoring what’s known as the Greenland block.

Blocking high pressure is one of many teleconnections that affect our weather patterns, but what is the Greenland block, and how does it impact our weather in Atlantic Canada?

The Greenland block is a ridge of high pressure that sits near or over Greenland. The blocking often leads to troughing in the jet stream over eastern North America. It slows or stalls the progression of low-pressure systems trying to exit Atlantic Canada.

This blocking pattern is indicated during a negative phase of the North Atlantic Oscillation, or NAO, representing surface sea-level pressure differences between the Azores-bermuda high and the subpolar low.

A positive NAO will signal below-normal heights and low pressure across higher latitudes in the North Atlantic, with above-normal heights and high-pressure over the eastern seaboard and central North Atlantic.

The opposite occurs during a negative NAO – with above-normal heights and high-pressure across higher latitudes in the North Atlantic and below-normal heights and low pressure over eastern North America into the central North Atlantic.

What does that mean for us? A positive phase favours high-pressure with calmer weather over Atlantic Canada, while a negative phase often leads to persistent periods of unsettled weather with systems moving slowly or stalling before clearing completely.

The position and strength of this and other features determine the impact on our weather pattern, but a negative NAO is often a recipe for active weather for us.

With a strong Greenland block possible toward the middle of the month, we’ll want to keep a close eye on how it will impact our weather story.