There’s been chatter the last few days of a potential weekend snowstorm, and we’re here to break down the setup and try to put your questions to rest. The reason there has been a fair amount of hype over this Friday – Saturday – Sunday timeframe (December 8th, 9th and 10th) is because of the upper-level pattern at hand that we’ve been warning about for days now.
The upper atmosphere screams for some type of big winter weather event to impact the eastern third of the US this weekend, but just as we see with every storm, it takes all players on the field working perfectly together in order to get something significant. This weekend we’ll have a rather large positive PNA (strong ridging out in the west), coupled with a very expansive east-based Positive EPO (massive ridging over Alaska and western Canada). The ridge is helping to pump a deep trough down over the eastern US, with heights extremely low all the way down into the southern states. Temperatures will be below-normal as a lobe of our Polar Vortex (PV) accompanies the deepening trough. Not only that, but with heights well above-average across the North Pole and Greenland, we’ll also have a negative NAO (good blocking that slows down the pattern and allows winter storms to really blossom) and a negative AO (ample supply of very cold air directly from northern Canada).
Looking at just the 500 mb anomalies alone, we are in good shape for some type of storm. But the timing of the pieces of energy and the players at the surface also have to be in order for something significant to come out of all of this…and we’re not quite there yet.
This is another look at our 500 mb pattern, but now showing the vorticity, or energy, involved. We have two main players of energy that we’re watching. The first is associated with a potent clipper diving south from Canada (Energy 1), and the second is from a coastal storm trying to develop off the eastern seaboard (Energy 2). The reason we’re not seeing computer model guidance print out a massive snowstorm is because the timing and location of Energy 1 and Energy 2 keep them separated enough for there not to be a phase in energy. If this proves through, these two storms will move through the region rather harmlessly.
In order for us to get some type of significant winter storm across the Northeast and New England, we need to either see our coastal storm associated with Energy 2 slow down (allowing the first piece of energy to catch up), or we need Energy 1 to speed up and make it close enough to Energy 2 for the storms to come together and give us something more significant. The way either of these scenarios becomes a reality is for our trough axis to tilt negatively. In other words, we need the deep cold front to negatively tilt, drawing both pieces of energy closer to each other.
This uncertainty makes the upcoming weekend worth keeping an eye on, especially considering it’s still nearly a week out. There is time for changes, and they don’t have to be very significant to get some type of snowstorm to impact either the I-95 corridor or areas further inland across the interior Northeast and New England. With that being said, we don’t expect that to happen at this time. It’s possible, but right now we wouldn’t call it likely.
Friday 1 PM
Saturday 1 AM
Saturday 1 PM
So what does this all mean when it comes down to the weather at the surface? Well, if nothing changes and the two pieces of energy remain separate because of the positive/neutral tilting trough axis, we’ll get two rounds of weak storm systems. The first one is our coastal storm, which will just barely graze the coast with some light rain and snow on Friday afternoon. The second is our clipper, which will bring something pretty similar but with colder air and probably some more snow. But again, nothing too significant.
However, what is interesting is that after the first system moves through, our trough does appear to eventually tilt negative, enabling the clipper to slow down and tap into some additional energy. This could mean that even if our coastal storm doesn’t produce, our clipper has a chance of transferring energy over to the coast and producing a “last-minute Nor’easter” for parts of the Northeast and New England. All guidance seems to think this is a realistic possibility come Sunday. This would mean a wet snow event would theoretically be possible from southern New England up through Boston and into Maine, Vermont and New Hampshire. Just how significant an event we see will depend on the timing and extent of the negatively tilted trough, as well as how much energy can be tapped in from our arctic Jet Steam to the north. The more energy and the quicker the tilt, the more significant of a storm we’ll have to deal with.
Sunday 1 AM
Sunday 7 AM
You can see from the guidance above (18z GFS via TropicalTidbits) the trough indeed tilts negative, allowing the energy to phase together and produce a late snow event for New England. Given that Sunday is a week out, we will have time to watch this situation develop and make a forecast accordingly. For now, what’s most important to know, is that there is real potential(s) for a snow event of some kind to impact either the entire Northeast or part of New England as we head into next weekend.
We’ll keep you updated throughout the week, and should have a much better idea of the situation at hand over the next few days—keep it here.