November: A very cold month so far will likely finish below average – even after temps moderate

blizzard

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.

2014-11-19-1019-NOVTMP

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.

2014-11-19-1019-PSTWKT

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.

The East Pacific Oscillation

The East Pacific Oscillation

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.

Typical weather pattern when the East Pacific Oscillation (EPO) is positive.

Typical weather pattern when the East Pacific Oscillation (EPO) is positive.

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.

Typical weather patterns when the East Pacific Oscillation (EPO) is in a negative state.

Typical weather patterns when the East Pacific Oscillation (EPO) is in a negative state.

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.

2014-11-17-0834-SSTAML

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.

Warm water (relative to average) in the NE Pacific continues to be the main driver in our weather pattern, forcing a ridge into Alaska and a southbound jet stream from the Arctic to the U.S.

Warm water (relative to average) in the NE Pacific continues to be the main driver in our weather pattern, forcing a ridge into Alaska and a southbound jet stream from the Arctic to the U.S. (December, 2013)

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.

Advertisements

4 thoughts on “November: A very cold month so far will likely finish below average – even after temps moderate

  1. Question: you said “The warmth off of the west coast favors a ridge of high pressure in that area,”

    Why wouldn’t the warmer SSTs lead to more evaporation, convection, lower pressures generally in that area, and more rain?

    • That’s a good point, but remember even though those SSTs are above average, they are still only 7 to 15C which is still way to cool for deep moist convection. Where they do have impact is adding to the moisture availability for Pacific storm systems such as we’ve seen for early to mid December. The contrast between the cooler SSTs in the NC/NWPAC (roughly bounded by 30N/140W..35N/155W..Northern Japan.. Okinawa.. 35N/180E..30N/140W) and the warmer waters to the south have helped fuel the enhanced Pacific jet this month. At the same time, the frequent storms in this flow have maintained southerly flow over the far east Pacific. This has produced onshore flow and prevented upwelling by pushing near surface water northeastward. (Offshore flow pushes near surface water away from the coast and pulls deep cold water upward near the coast for cooling). Overall, the pattern favors a retraction in the Pacific jet as we move into January and the formation of a ridge along the northwest coast of North America again. The element we have this year is a warmer pool of water in the subtropical/tropical central and eastern Pacific. This helps produce a split flow with a more active southern jet. This should mean that the PACNW will dry out again while the SW US sees more southern stream systems. We’ll also see times when the jet stream will break through the western ridge, perhaps cutting it off over Alaska/NW Canada. Don’t think we’ll see a complete repeat of 2013-14 because conditions are different, SSTs for one, more favorable signals for the Greenland Block for another. We’re likely to see much more back and forth this year and that may mean more snowstorm potential for some, especially for those further south..when colder air can work into these southern stream systems. How far north/west they track over the central/east U.S. will depend upon how strong the cold becomes over the Northeast/Great Lakes/Plains as we move through the rest of winter.

  2. That helps a lot. Thanks.

    It kind of seems like the low associated with Nuri, the resulting jetstream deflection, and the November dump of arctic air was pretty huge. Seemed to not only increase the Pacific Jet, but also drain a lot of pressure out of Siberia. Of course, it seems to be making up for that in a big way more recently.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s