It’s been a cold start to the month of December, and all indications are for the cold to stick around through the end of this month. One of the main reasons for this chill across the East is because of what’s happening in the West, which is a positive PNA pattern, or Pacific/North American pattern. This type of pattern creates extreme ridging over the West. According to basic physics, what goes up must come down, so while a ridge sits over the West, a trough forms over the East. Based on the PNA forecast from the European model, it continues to show a positive PNA phase through mid-December, then it may actually turn negative for a brief time. That may sound like a complete pattern change but there other factors that play a role into the current pattern across the nation. Therefore, the general theme of a ridge over the West and a trough over the East will persist.

EURO PNA forecast

Once the PNA turns negative for a brief time, the pattern will look like this:

EURO 500 mb heights on Dec. 16th

What your looking at above is the geo-potential heights. Basically, the warmer colors hinting at ridging and above average temperatures. Notice how the ridge doesn’t look as impressive with the negative PNA phase but it is still present.

Cold East:

In the East, a trough will dominate the weather. This week, temperatures will be up to 30 degrees below average. Those greatest negative temperature anomalies will occur on Wednesday as a piece of the Polar Vortex dives southward from the Arctic into the Mid-Atlantic.

EURO 850 mb temperature anomalies

For the rest of this week, temperatures will generally be ten to twenty degrees below average east of the Mississippi. This weekend and early next week, temperatures may actually be above average, especially in the Northeast due to the negative PNA and more zonal flow for a brief time.

Now looking long-range to the rest of December, the EPS ensemble mean continues the theme of a chilly East and mild West. This weather will likely be active in the East during the remainder of the month. That includes Christmas, so the chances for a white and snowy Christmas are above average. What we do know is that it will be cold, but will we be able to get a snow-producing storm around this holiday? It’s still too early to know.

EPS 850 mb temperature anomalies 5-day average

Mild West:

The complete opposite is happening across the West. We’ve been talking about how a ridge of high pressure is dominating the weather pattern for the Western U.S., and that will certainly continue through the remainder of the month.

This week, temperatures will be as high as thirty degrees above average along the West coast through the Mississippi River. It’s also worth noting that some of the heat will intrude into the East this coming weekend and early next week for a brief period as the positive PNA breaks down, making for a more zonal flow or flatter jet stream. With this zonal flow, maybe a storm system sneaks into the Pacific Northwest this weekend before the ridge begins to build back up next week.

temperature anomalies this wednesday

This ridge of high pressure will continue to sit over the West through at least the end of December. I am worried that the wildfire risk for Southern California will remain high. This week, winds are forecast to be calmer, so the risk isn’t as high this week. Santa Ana winds along with the low humidity will likely make a return by the final third of the month, however.


Jackson is Head of Content and Social Media at WeatherOptics. He is currently a student at the University of Miami, studying Meteorology and Broadcast Journalism. Dill produces forecast articles for the website and helps to manage the content schedule. He has also led the growth of WeatherOptics’ social media accounts, working to keep them aligned with the company’s evolving vision.

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