We are mounting one heck of a 4th quarter comeback on our upcoming storm, the first big precipitation producer of 2017. There was a plan in place for much of the energy associated with this storm to track well into Canada, setting the stage for a influx of mild air and some rain. Subtropical or southern branch energy has been an undervalued asset during the last few weather systems and the models have struggled to catch on until late in the ball game. In this case, the southern branch component of this system will ultimately absorb most of the energy with this storm and by early Wednesday, a signifcant low pressure center will have developed off Cape Cod. All that said, we are still behind and don't have a lot of cold air to work with. Temperatures between 6000 and 10000 feet in the atmosphere were orginally expected to warm to as much as 45 Tuesday night into early Wednesday and we are down 8 degrees to about 37. A few more and we might at least have an elevation event to talk about. For now however the forecast is for a cold rain and some ice. This should develop during the day Tuesday with bulk of the precipitation falling Tuesday night. Temperatures will generally hover between 32 and 35 throughout the storm. It's getting close but we actually need this system to deepen a little more aggressively late on Tuesday if we want to see the snow.
Temperatures will work their way back down toward the freezing mark during the day Wednesday and we should see a few bursts of snow. The snow showers will continue through much of the day Thursday and within this time frame some elevation sensitive accumulation is certainly possible. I'll call it a few inches right now but we will reevaluate the situation in a few days.
Both major ensembles underwent a major shift regarding the weather for the upcoming weekend. We had discussed a possible storm in the previous posts but the polar jet appears much stronger now and seems intent on overwhelming the pattern with a decent outbreak of cold. Nothing wrong with that although we could use a little snow to go along with it. I think some might be possible during the weekend from a clipper like system or weak disturbance but a major storm appears a lot less likely under these circumstances. Still we will remain on the lookout, there remain a few indications that an east coast storm is still possible for somebody over the weekend. Right now, Vermont would not be the favored location.
In spite of the changes to short term and the upcoming weekend, the longer range outlook is still plagued with this negative PNA regime. The PNA or Pacific North American oscillation index, when negative, tends to favor both the cold and the snow across western North America. This has been an issue for a few weeks and seems likely to prevail through mid January. That said, other teleconnection indices have moved more favorably in our direction and will allow cold air to compete for territory. We've seen this situation before that sometimes we don't want the pattern to get "too" favorable because the storm track can often shift too far south. It certainly won't be in this case and I would again think it likely that we see a significant snow event before the middle of the month while also keeping the possibility for a brief thaw and some ice and rain.