December - January outlook

The negative NAO that was projected on the models a couple weeks ago hasnt played out to its fullest. Nevertheless, with the passage of the cold front yesterday, we are back into a colder regime. The pattern that we were stuck in of "cold & dry / warm & wet" is nearly over. We have ourselves three decent snow and ice threats in the next week, each of which will be discussed in depth in the winter weather outlook section. But for a summary:

1) Sunday 12/7: Clipper moves into NY with scattered snow showers. Coastal low develops to our south and moves to the east of the region. This storm will deepen quickly, but not fast enough to pull west enough and will only glance New England with an I95 corridor hit where up to 3 to 4 inches could fall. Further west, widespread 1 to 2 inch amounts will be possible.

2) Wednesday 12/10: Very dynamic set up with models showing 80kt southerly flow at 850mb! This would translate to surface winds up to 50mph. In addition to the wind, Precipitation moving in late Tuesday will begin as snow, transitioning to an icy mix Wednesday morning as mid level warmth advances. Strong cold front passage will then send temperatures tumbling back from the upper 30's on Wednesday into the teens Wednesday night followed by highs 25 to 30 and lows 10 to 15 through the weekend.

3) The GFS has been advertising a significant nor'easter for the end of next week (Friday - Saturday). Obviously it is too early for details, but considering established low level cold with high pressure to our north (finally), we could secure a plowable snow event with this one.

Looking further ahead, still on track for the artic reloading around the 15th to 20th, with generally a +NAO configuration. However this will not limit snow potential, and given the right set up, could offer some significant storm systems similar to last December.

After that, a moderate -NAO will develop for Christmas time with storm potential around the 22nd.

Now getting into January... I've applied the roll-forward method to the 500mb height configuration of this past November:



This matches very well with the forecast that I made in early October using September H5 data:


The analogs I used from September for my November forecast were: 1949, 1956, 1960, 1961, 1963, 1967, 1972, 1974, 1989, 1995, 1999, 2007

Now focusing on the November data, the new analogs I used are (in chronological order): 1956, 1962, 1967, 1969, 1971, 1981, 1989, 1995, 2000, 2007. I did weight some of these years more than others. Common analogs from September include: 1956, 1967, 1989, 1995, and 2007.



Two things pop out quickly: an east based negative NAO, and a distinctly negative PNA. Looking though the analogs individually, the majority have some form of a negative NAO, and a negative PNA. For the first half of January, the PNA region gets mixed signals, but there is a strong consensus for a negative state for the second half. While the second half has mixed Atlantic signals, the first half there is a consensus for strong -NAO. Generally, it appears the NAO block weakens and/or shifts east by the 15th. The only two analogs that did not agree with a -NAO start were January 1990 and 2008.



With this in mind, I think that we'll see a cold end to December continue into early January.



As the pattern shifts, with positive height anomalies retrograding through Alaska and to the Aleutians, I think a there is the potential for a significant winter storm in the transition around the 12th to 14th.