After a brutally cold start to the Winter season across the eastern half of the United States, many are much happier this week with temperatures bouncing back between average and above average over the last 8-14 days.
Take a quick look at what the start to January 2018 looked like (week 1) vs the second week (week 2):
Notice just how anomalously cold the first week of January was from east of the Rockies all the way to the east coast, including the Deep South. So cold in fact, that the anomalies are literally off the charts across the ENTIRE Northeast, Mid-Atlantic and parts of the Midwest, bottoming out somewhere near 25 degrees below average.
A much different look to the overall pattern the following week, with solidly above average temperatures for most areas east of the Mississippi River, and a small cool-pool across the Midwest and northern Plains. This new pattern setup has now more or less established itself, with waves of warmer air and precipitation followed by cooler and drier air over and over. We tend to refer to this pattern as a transient one, which means its not dominated overwhelmingly by just warm or cold air. In this particular case however, the eastern third of the United States will likely end up slightly on the warmer side of things in the end, with shorter-term shots of chilly air.
Through the end of January, we expect this type of transient pattern to continue with slightly more prolonged periods of warmer and wetter weather followed by brief shots of cooler and drier weather. The latest long range GFS model depicts this nicely:
Other than the ups and downs in temperature that we’re likely to see through the end of the month, what we really need to monitor is whether any of the cold shots line up with higher chances of precipitation before we see a colder and snowier pattern shift during the month of February.
Right now, it looks like there are actually a few opportunities.
The first comes early next week when the 500 mb goes into a brief favorable setup for the Northeast with strong ridging across the west and a trough along the east coast.
You’ll also notice some blocking to the south of Greenland, but it’s largely eastern based, so not completely ideal for an east coast snowstorm. With the pattern being transient as well, it leaves very small window of opportunity for a storm to develop and impact the Northeast region. With that said, it’s by no means impossible.
Above are the last four runs of the GFS for Monday night (29th), and you can see how clear the trend is back to the north and west. The most recent run (18z) actually now has snow impacting the NYC to Boston to Bangor corridor with light to moderate snow. Parts of eastern Maine even pickup some moderate accumulations out of this system verbatim.
This is something we certainly have to keep an eye on over the next few days, as the threat is now only 126 hours out and moving in a direction that supports a snowstorm for at least part of the Northeast/New England region.
Beyond that, there’s another longer range threat for the end of next week, where we get yet another small window of opportunity within the transient pattern. Ridging this go around actually appears to be even more expansive, extending all the way north of Alaska and giving us a strong negative EPO signal. This is the same negative EPO that largely dominated the pattern during the first half of the Winter and allowed for extremely cold air to seep down into the US. While that won’t be the case necessary this time, we certainly have cold air to work with.
Blocking is also stronger, but again fairly transient and east-based. The trough also isn’t ideally placed, rather expansive and on the shallow side, meaning our system won’t dig quite as much and won’t become that strong. Again, this is just one run of one model verbatim, but its certainly another period we have to keep an eye on for potential snow, especially for the interior Northeast.
Wrapping things up, while this pattern is a pretty large shift from what we saw during the first half of Winter, there will be opportunities for snow across the eastern third of the US. In fact, in some ways this pattern could be even more ideal than the former with no deep cold air shunting storms to the South like we saw before. Rapid changes from warm to cold before switching back to warm often help setup small windows of opportunities that could potentially produce big storms. Time will tell.