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United States Extended Forecast Discussion

Weather Pattern Details
Weather Pattern Details
Brandon (Images courtesy of NCEP and Allan Huffmans model website)

Extended Forecast Discussion

Thanks for stopping by and I wanted to post some more thoughts on the pattern as we head through the rest of December. My previous post was posted before it was clear to a lot of meteorologists that a pattern change was coming and I am glad to see it has worked out so well. The progression I detailed seems to have come to fruition, with warm weather followed by increased storminess and eventually a cold pattern towards Christmas.

Previously I discussed the AO and its impact on the pattern and indeed it has changed the pattern dramatically to the north of the CONUS. We can look at the current and short term model trends and easiliy notice the dramatic blocking that has built into Greenalnd and will continue to build southwest into northern Canada.

There is generally a lag time on the impacts of these building blocks and the pattern in the US. Over the course of the next week there will be several waves coming through the eastern US, which will progressively bring in colder and colder air.

Currently the AO is still negative although it has risen somewhat from the steep decline over the last week. The GFS ensemble link below details where we’ve been on the AO and where we’re likely to go:

As you can see, the average for December is likely to be quite low, and will make 3 successive months with such readings. From a statistical standpoint this may be significant. When matching other Oct-Nov-Dec’s to the same standard, while also bringing a -PDO pattern for this same time period, you come up with the following winters(1950-51, 1952-53, 1962-63, 1968-69, 2010-2011). In the slideshow at the top, on the second image you can see the following January and February combined average 500mb pattern for these years. As you can see, the AO tanking this month is generally a good sign and an unfavorable pacific is not a death nail for winter.

Winter Storm

Taking all of this together, we can start to perhaps sniff out the possibilities of winter storms in the southeast. There will likely be several small to medium winter storms over the Northeast and New England over the course of the next 10 days, which will lay the ground work for a possible stormy period towards Christmas week in the Mid-Atlantic and Southeast. In the image section you can see some comments on the extended Euro ensemble mean and what it could mean down the road. Lastly with the GFS forecasting the MJO to move into phase 1, this tends to favor colder and stormier patterns, as well as changes in the EPO and PNA indices. This coinciding with the AO dip could prove to be more efficient in delivering true Arctic blasts as head down the road into January.


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