Very wintry this week but milder pattern to emerge for back half of December

A dense 5 inches of snow begins what will be a very wintry week in Vermont. One of the stronger December outbreaks of arctic air will descend on the region during the middle of the week and along with the cold will come a few bursts of snow. Unfortunately, the news isn't as good for the rest of December as various teleconnection indices turn against us and will ultimately force us into more of a defensive posture. Will we have yet another Christmas thaw ? Maybe and maybe not, but we have been incredibly unlucky in this regard the last several seasons and the latest data pushes the risk of this higher yet again for this year. 

We have a polar vortex to contend with in the short term ! It is rapidly moving southward and will rotate its way just south of the Hudson Bay later this week. Several surface waves (for lack of a better word) associated with the incoming cold weather will focus the limited available moisture and bring bursts of snow to the region over the next few days. The changeable conditions will make short term forecasting a bit messy but it's a little easier to summarize in a blog. Additional snow accumulations will greet skiers Tuesday morning and following a several hour period of drier weather we should see an additional round of snow showers Tuesday night as the cold air begins to envelop the region. The snow will bring a light accumulation to the mountain by Wednesday morning but another round of snow showers and quite possibly a more organized snow squall will impact the region Wednesday night. The heaviest snow will be quick moving enough that big snow accumulations are unlikely but another 2-4 inches by Thursday is likely. At this point, temperatures will have plummeted into the single numbers and are unlikely to change to much during the day along with rather stiff winds. The last in this series of waves will could then bring a final burst of snow later Thursday. Another inch or two is possible here and readings will then make their first foray into sub-zero territory.  Friday will be dry and winds will gradually slacken but temperatures will start close to -10 and struggle to climb back to 5 during the afternoon. It will undoubtedly feel like winter in Vermont which is a nice feeling given that we really had no such week last year with this combined amount of both snow and cold. 

A good buddy of mine texted me about the remarkable turn of events that could transpire this upcoming weekend. Quantitatively speaking it could be quite remarkable but were it to happen, it is an all to notorious movie script.  The strength of an airmass (cold or warm) is often measured by referring to the associated mid-level temperatures in the troposphere. This is a general term but it usually refers to a level of the atmosphere just above the height of Mt Washington. When the cold weather peaks very early Friday morning, temperatures near the summit of Mt Washington are expected to be at or just below -15. That's a pretty impressive figure for mid-December. As this is happening, the storm that we had discussed in the last update will organize in the plains and advance toward the eastern Great Lakes. The track of this system is not what I had hoped for and although we could still see some changes with this particular part of the forecast, it appears as if New England will again have to endure another crazy temperature swing, one that characterizes the uniqueness of our weather. We joke with fellow skiers about this but certainly don't brag. There is a chance of seeing some rain with this storm, yes, but the more likely scenario is a snow to ice situation. The snow would begin Saturday and could include a period of moderate or heavy snow before changing to sleet and freezing rain Saturday evening. There is certainly a good chance that a good portion of Saturday could be powdery so keep that in mind. Temperatures will be at or below zero midnight on Saturday but will likely be close to freezing by midnight Sunday. On the summit of Mt Washington, temperatures could warm by as much as 50 degrees in the period between Friday morning and Sunday morning. Only in New England ! 

Cold will be reinforced at a much lesser intensity by Monday but unless there are some changes with the aforementioned storm, we are likely going to have endure a damp Sunday with readings creeping above the freezing mark, even at the summits. The cold air will send temperatures back below the freezing mark for a few days but the polar jet is expected to retreat and the overall weather pattern is expected to realign more unfavorably for us eastern US skiers.  The pattern shift is the result of the combined work of several teleconnection indices but none bigger than the EPO which describes the pattern in the eastern Pacific Ocean. I like to describe it as a jet stream tightening which encourages a more zonal and less amplified jet stream configuration. Although the cold and much of the snow will focus its attention on the west in the period leading up to Christmas Day, I remain hopeful that arctic air will remain close enough in eastern Canada to prevent a major thaw. I think of a Vermont "thaw" as 3 successive days of above-freezing temperatures. On Friday I deemed this unlikely but I would certainly up the chances of such an occurrence as of  this Monday evening. This part of the forecast will require some additional attention and I'll try and expand on this on Wednesday.