Posted by: splillo | March 31, 2012

What we wanted all winter

As we move ahead into April, I want to take a moment to reflect on the historic month of March 2012.

Here's an animation of locations which exceeded record temperatures (courtesy of coolwx.com). Record highs were broken on many days during the month of March. The main "heat wave" started on March 14th and lasted though the 24th. I put heat wave in quotes here because the technical definition is at least three consecutive days exceeding 90 degrees. This was not achieved. But for this warm spell that was likely a 500-year event, I think the term heat wave is justified.

The records "pulse" up during the afternoon since that's when the highest temperatures are reached, however notice in this animation that some locations exceeded record high temperatures even during the night. During the daytime, incredibly large regions of the United States reached record levels.

Following this historic event, you may have noticed a bit of a change in the weather. We're back to winter. Or at least something closer to it. Just 48 hours after basking in 80 degree warmth, we were rudely thrown back into the 40's. In fact, these temperatures are normal for this time of the year, but in comparison it felt like a new ice age.

Moving ahead into April, we can not rule out more winter. Tomorrow is the 15th anniversary of the April Fool's Day Blizzard of 1997, which dropped over 20 inches of snow across much of southern New England. So significant snowfall in April is certainly not unprecedented. Currently, we are in fact heading toward a pattern that is conducive to more wintry conditions. This pattern is what we looked for all winter long and never got ... a negative NAO.

NAO stands for North Atlantic Oscillation. The negative phase is characterized by lower than normal pressure over the Atlantic ocean, and higher than normal pressure over Greenland (this is also referred to as the Greenland block). The low over the Atlantic forces the jet stream further south across the eastern United States, allowing colder air to pour into New England. This past winter the NAO was in the positive phase, yielding quite opposite conditions (warm and snowless). We wanted that negative NAO instead!

At least for the first half of April, a negative NAO should result in generally near or below normal temperatures and even opportunities for snow.


Responses

  1. [...] A break down of the negative NAO block in the Davis Straight (see my last post). This block is anchoring an upper level low over the Northeast which is responsible for the cool [...]


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