It’s been a cold month so far with all of the U.S. with the exception of parts of California, southern Nevada, Arizona and northwest New Mexico, which have been milder than average. Alaska has been much warmer than average. The core of the cold has been across the center of the nation between the Appalachians and the Rockies, but an arm of the cold core extended westward through the Columbia River basin to northern Oregon.
Click on the map to make it more readable.
Just within the past week, you can see how far below average these temperatures have been. Keep in mind these are in degrees C and not F. Click on the map to make it more readable.
One of the indexes looked at to show milder or colder trends is the East Pacific Oscillation or EPO. The EPO had been positive in October and early November before turning sharply negative in early November.
Positive and negative values of the EPO are typically associated with certain large scale Pacific weather patterns. In the positive mode, which existed prior to Veteran’s Day, a deep low in the Aleutians and Gulf of Alaska helped anchor a dip in the jet stream out in that area with a mild ridge of high pressure over the lower 48.
After Veteran’s Day, and thanks in part to warming brought about by the remains of Super Typhoon Nuri, a ridge was formed in the Gulf of Alaska which caused the formation of a deep North American trough. That jet stream configuration brought early season Arctic air southward on a cross-polar trajectory and right into the central U.S. The typical negative EPO weather pattern is shown below..similar to the cold we’re just coming out of.
Global Sea Surface Tempatures (SST) also play a role. This map shows the difference from average in ocean temperatures. We’ve had the warm water off the west coast of North America since this time last year. It assisted in helping keep large scale weather patterns in place which favored cold in the central and eastern U.S. last winter, spring and most of summer. Below is a recent map of the SST.
Compare that to almost a year ago. There are a few changes…you’ll notice the warm area has shifted from the Gulf of Alaska to the Pacific coast from Alaska to Mexico. The tropical pacific is also warmer from south of Hawaii to Central America. Colder water has developed south of Alaska and the north Atlantic is much warmer off the east coast.
The warmth off of the west coast favors a ridge of high pressure in that area, which favors dry and warm weather out there and colder conditions from the Rockies east. The warmth in the Pacific won’t last. Long term trends, across several decades, favor cooling of the sea surface temperatures, all part of a natural up and down cycle in ocean temperatures. The cooling now developing in the north Pacific is likely a harbinger of an eventual end to our recent trend of cold and snowy winters. Should it spread and continue to develop, we’ll be in for mild, dry and rather uneventful winters for several years.