I thought it would be fun to print out a few monthly climate data sheets from the National Weather Service and show them to Adam Caskey this afternoon. I hid the location of one and asked him based of the recorded temperatures through the month for him to guess where the location was. Based on the temperatures in the teens, 20s and 30s overnight and highs in the 30s to low 40s during the day, he guessed somewhere in the Central Plains such as Tulsa, Oklahoma. Turns out I was showing him Anchorage, Alaska.
Anchorage is running over 10.5 degrees above normal this month as well as much of the state. Fairbanks, which has seen temperatures as low as -41°F twice this month is still running over 12 degrees above normal. Meanwhile, the D.C. area is running slightly below normal currently, looking to end the month a few degrees below normal. This is all due to the pattern aloft, steering milder air over the Western U.S. and Alaska and keeping the Midwest and East Coast in a cold pattern.12Z GFS 250mb Jetstream forecast for Thursday evening
Looking at the GFS pattern above, this gives you a hint why temperatures have been like this for much of the month. Taking a look at the U.S., you can see a wave extending from Canada through the Midwest eastward to New England.
This "U" shape in the jet stream is called a trough in the atmosphere. The West Coast north to Alaska has been experiencing just the opposite for much of the month, which is called a ridge.850mb Temperature Normalized Anomalies (Courtesy: WeatherBell Models)
The "troughiness" for the eastern half of the country has made for much colder than average temperatures as cold air has been continually steered out of Canada whereas milder air has been stuck and centered over the Pacific and north into Alaska.
The graphic above gives an idea of this with temperatures 2 to 3 standard deviations above the mean near Alaska and 2 to 3 below the mean in the D.C. area. With the persistent jet over much of North America, there was really only a period of 5 days in the month when the pattern changed.
When that happened, temperatures in D.C. soared into the 50s and 60s from the 11th-15th, whereas Fairbanks experienced 5 days with low temperatures below -16°F, two of which hit -41°F!12 GFS 850mb temperatures for Thursday evening
I thought this depiction of 850mb temperatures would help drive this point home to give you a nice idea of how these airmasses have been interacting. Looking at the graphic of the 250mb jet stream at the top of this post corresponds nicely to where the areas of cold and mild air are located. The ridging out west can easily be seen as well as the trough over the eastern half of the U.S.
Looking forward in time, unfortunately this pattern doesn't appear like it will be breaking down in January. There are hints the pattern may change and become more zonal (jet stream travels more west to east in nature) by the beginning of February. Time will tell if that forecast persists.
Until then, expect Friday to experience high temperatures nearly 20 degrees below average, the weekend to feature another round of snow showers with temperatures only in the 30s, and next week to start out with 3 straight days only in the 20s. Who else is waiting for January to hit the road?