Major Nor'easter ahead

The potential exists for a major winter storm to affect much of the eastern third of the nation this weekend. Many factors to discuss.

1) Snow event tomorrow (Thursday). While the GFS has had an okay handle on the storm. The rest of the model guidence has not been up to speed at all. A major shift to the north this morning for a storm 36 hours away is not good if you want to build confidence on a longer term forecast.

Besides model verification, the further north track will encourage some stronger ridging in the southeast which would account for a more westerly track on Sunday.

2) Current pattern is progressive / lack of down stream blocking. We have a La Nina encouraging a strong Pacific jet stream. This is notorious for making forecasting a coastal storm difficult. Without an already established negative NAO, we have varying tracks the primary low can take. This opens the doors to many factors affecting the resulting track.

3) 50/50 low. The Euro and GFS have suggested the possibility of a polar vortex moving out of Quebec to act a a psuedo low over 50N 50W, however recent runs have taken the strong jet and whipped the storm east towards Greenland. This illiminates the possibility of a more amplified -NAO. So right now, expecting a more east based or neutral NAO. The timing of the low will be something to watch on the models the next few days.

4) Variability in the long term GFS. The track has been bouncing back and forth on the GFS. Looking at trends, my thinking is a southerly trend is about to begin. Where it goes from there is anybodies guess.

5) Canadian high pressure. This has helped produce a couple significant ice events for SNE in the past 3 days. High pressure located in southeast Canada has induced cold air damning across much of the mid Atlantic and northeast. This will continue through the weekend, promoting higher chances of precipitation type being snow inland and at least ice towards the coast.

6) OLGA! This year's December surpise could play a part in the storm this weekend. A tropical connection with the sub tropical jet as the primary starts heading north will increase amounts of moisture for the storm to work off of. The final timing of the connection will affect who gets in on the added moisture. The Euro has suggested immediate connectio with Olga from the start, with a large moisture plume across the east coast. The GFS has Olga joining up later, with main affects across New England. Olga could have two impacts: increased moisture supply and increased warmth.

7) Overall QPF and Temperature profile. At this time, precipitation type is not a concern for Keene. The entire boundary layer is below freezing with surface temperatures averaging around 20 for the entire event. Even the Euro, one of the warmest solutions, gives Keene all snow. I95 will likely see a change to rain or ice later in the storm with the way the models are currently looking. QPF looks to be at least an inch. Euro on board for upwards of 3 inches, while GFS has a stripe of 2 inches (located somewhere in the northeast depending on which model run you look at).

8) Snow ratios and snow accumulation. 850mb temperatures near -10C will allow for a dry snow event and considering good dendric growth, snow ratios could exceed 15:1. For snow accumulation, based on current model data, at least 18 inches is likely for much of central New England into NY state and PA. However, the models are likely to shift around a lot, so right now I'll leave you with a prediction for Keene of >12" likely.

9) Winds. Blizzard conditions? Low pressure will bomb out (somewhere around the coast +/- 500 miles based on the models) and combined with high pressure to the north, a very strong pressure gradient is likely into Sunday Night. The possibility exists for wind speeds to exceed tropical storm force along the coast, and at least wind advisory criteria inland. The possibility does exist for a portion of the region to experience blizzard conditions.

10) Timing. Likely to start Saturday evening and last at least 12 hours. Lack of blocking and 50/50 low will illiminate any prolonged snow event, but Sunday will likely see flakes in the sky all day long.

Many many factors - many things to go wrong, many things to go right. The NAO falling to just below 0SD is encouraging but a generally neutral NAO is not perfect for the set up. With the blocking out of the picture, the track has a lot of freedom. This is 3 to 5 days away, so approaching short term (within 3 days). Once the Thursday snow storm is through, some things will become clearer, so first accumulation forecast map will be issued sometime tomorrow. Expecting at least a foot from this, with somebody in the northeast getting over two feet of snow.