Per the long range outlook issued back on November 11, the cold arrived around Thanksgiving weekend and has not looked back.
Temperatures for the first 3 days of December have been well below normal across much of the US.
The main drivers of the colder pattern are: A favorable tropical Pacific (MJO in phase 7-8) and a bit of upper atmospheric warming over the North Pole.
The first 15 days of December look to stay below normal across northern Ohio for the first time since 2005! It's only happened two other times in the last century.
Its not just here in Ohio. Most of the country has had well below normal temperatures since Thanksgiving week.
Here are a few years that were similar to the current cold: Temperatures vs normal on the left. Upper level pattern on the right. Notice how widespread the cold was in each instance along with persistent low pressure around the Great Lakes/eastern US.
Interestingly, the cold during the first half of December has been continuous but it hasn't originated from over the North Pole. High pressure near Alaska has developed from time to time over the last week which has kept the central US trough/active pattern with frequent precip/snow. But it's strength has been fluctuating. The low over Hudson Bay is trapped in between the Alaskan high pressure ridge and the low over Greenland. Definitely no sustainable blocking pattern capable of tapping arctic air.
What has to happen for the colder pattern to become extremely cold?
The Greenland low needs to transition to a high pressure block along with a sustainable Alaskan block. That hasn't happened...yet.
Recently, a huge drop in the SOI (southern oscillation index) occurred. This indicates huge pressure pattern changes between Australia and Tahiti in response to the tropical changes (MJO moving into phase 8)
Notice on the long range outlook into the second half of December. BOTH blocks seem to develop:
WATCH FOR CONSISTENT HIGH PRESSURE OVER ALASKA
WATCH DEVELOPMENT OF HIGH PRESSURE OVER GREENLAND
WATCH LOW DEVELOP CENTRAL US/HUDSON BAY
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Here is the take home forecast message:
* Cold continues but nothing extreme for southern Great Lakes through mid-December.
* Active storm track continues with frequent snows across the central US/Great Lakes.
* Short-lived break around December 16-18
* Then cold reloads as blocking pattern establishes itself last 10 days of December/early January
Watch how the cold continues, a small break mid-month before the cold build around Alaska which heads south around the 20th. Get ready for a Arctic Christmas!
