With the new year comes new opportunities for interesting weather here in Maine and no time will be wasted in setting up an interesting pattern to start off the incoming year. The main event this post will focus on will be a mixed precipitation event forecast to move into the area Tuesday. Low pressure will approach from the west while high pressure will dig in to the NE. To add some extra fun to the mix, coastal low pressure is forecast to develop as energy is transferred across the Appalachians which will drag in colder air and possibly introduce some heavy precip Tuesday night into Wednesday morning.
To understand the weather in our corner of the world, you often need to zoom out. A lot. This is why I’ll begin with the entire northern hemisphere. The pattern is set up for cold air to move south due to high latitude blocking over the North Pacific and the North Atlantic. This means that the bubble of cold air surrounding the north pole is being pinched (think of pressing in on a balloon from opposite sides). When the cold pool is pinched, parts of it are forced to spill south and that is exactly what we’re seeing with a pocket of cold air centered over James Bay, Canada. This combined with tonight’s cold front and attendant shortwave energy (little wiggle in lines near New England) means that cold air will be in place in advance of our next storm system.
Sure enough, model data shows the cold locked in Tuesday morning. Notice the subzero temps inland and the sharp gradient associated with a coastal front. Remember the last time we had a sharp coastal front set up ahead of a developing coastal storm? I strongly believe that all model guidance warms temps too quickly during the day on Tuesday. While the immediate shorelines are likely to go above freezing before substantial precip arrives, anyone NW of route 1 needs to expect a period of mixed precipitation Tuesday night.
Here’s the synoptic scale setup early Tuesday afternoon as precip begins to arrive in the area. Cold high pressure is locked in over Quebec (not over Bermuda like other recent events) and a storm is transferring its energy across the Appalachian mountains to secondary low pressure off the Mid Atlantic coast. This setup screams Cold Air Damming that lasts. Other events have lacked the big cold high to the north and the coastal redevelopment. For more on how CAD works, check out one of last winter’s UpPortland columns that explains how that works.
Now that we know the Hemispheric scale pattern, and the synoptic scale pattern across the entire east coast, let’s focus in on New England for the mesoscale patterns. High res guidance does show mixed precip over the mountains Tuesday night but I believe the rain/mix line will be considerably farther SE than modeled. You can see the difference in the graphic. This line will begin moving SE as the developing coastal storm draws in colder air. Some bands of heavier precip may develop and act to speed cooling but the dynamics that we enjoyed Thursday night are not likely to be present with this event. This is due to the fact that the energy aloft for this upcoming event will be nowhere near as strong as it was on Thursday. Despite that, the low is forecast to deepen at a reasonably quick rate and thus we will have to worry about the possibility of some heavier bands working in early Wednesday morning.
The upper air situation will not be conducive for explosive deepening of the coastal low. The northern and southern stream disturbances will remain separate and unphased meaning that they can’t join forces to become a powerful storm like Thursday night. We also will get the pleasure of dealing with the southern disturbance which is the one with the warmth and the moisture and being more or less cut off from the northern disturbance which has the cold air supply. Despite this, some cold will be around before the storm as we saw with the temps Tuesday morning and that cold will likely hang around long enough to cause some issues inland.
Here’s what I think will be falling from the sky Tuesday afternoon through Wednesday morning. Almost everyone will see some frozen precipitation at the beginning and after a fairly short period of time, the immediate coast and CT river valley are likely to go over to rain. Most of the coastal plain will continue as freezing rain for much of the event while the mountains start and end with some snow. Exact accumulations are still uncertain but snowfall is unlikely to exceed 6″ even in the northernmost mountain towns. Ice amounts could get close to .25″ in the typical foothill areas that keep the ice around longer but most areas will see between a trace and .1″ of ice.
Precip moves out Wednesday morning before colder air invades for the second half of the week. Threats of a coastal storm late week appear to have diminished as the pattern becomes less amplified though it’s still worth keeping an eye on if you have plans for Friday as the low could easily drift back in our direction.