Tuesday, January 5, 2016

Brief Pattern Change Expected Next Week

A change in the current weather pattern, albeit a brief one, is expected next week.

Image Valid: January 12
Ensemble forecasts see strong ridging building over Alaska and extreme northwest Canada, extending from northern Siberia all the way across the Arctic Circle to the waters immediately south of Greenland. Consequentially, this will force the tropospheric polar vortex to lower latitudes, and it just so happens that ensemble guidance sees a piece of this vortex will dip south into Canada, and will exert its influence on the United States.
It's important to note that the tropospheric polar vortex will not enter the United States, it will stay well north of the Canada/US border. Cooler air will still result, but extreme cold should not be expected right now.

Image valid: January 15
By January 15th, the pattern is progressive enough so that the piece of the tropospheric polar vortex shifts east into eastern Canada. We also see strong ridging dominating the Arctic Circle, which will drop the Arctic Oscillation (AO) index well into negative territory, as we'll see a little later on in this post. Note the strong trough modeled over the Gulf of Alaska- this will maintain a positive phase of the East Pacific Oscillation (EPO). While I won't discuss the EPO in this post, that trough in the Gulf of Alaska is what will return our pattern to a more seasonal, and possibly warmer than normal pattern after this brief change to chillier temperatures.

Above 3 images from CPC
Just a brief look at the teleconnection forecasts, we see the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) both expected to drop into negative territory. This is a sign for cooler than normal weather, and as we saw earlier in this post, ensemble guidance believes a brief shot of colder weather will hit the Central and East US. The Pacific-North American (PNA) index is expected to stay positive, possibly heading more towards  neutral territory in the medium-long range, but generally should remain positive. This is likely a result of that trough in the Gulf of Alaska. That trough will keep the pattern progressive, which means the ridge that forms over the West US as a result (the positive PNA indicator) will likely drift east and spread warmer weather east.

To Summarize:

- A brief flip to colder weather is expected next week, mainly between a January 10-15 period.
- Warmer weather is expected to return after January 15th.


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