Weather rollercoaster could end with Thanksgiving Nor’Easter

As most of you could probably tell by simply walking outside, another mid-winter airmass has filtered into the region today, and will set up another very cold night tonight. As high pressure settles into the region this evening and tonight, winds will subside to nearly calm levels. When this is combined with clear skies and a very dry atmosphere, ideal conditions for radiational cooling will be generated. This leads to lows in the upper teens in the interior, and lower 20s near the coast, with perhaps mid 20s in the warmest urban areas.

Afterward, the trend will be for moderation. This is because the high pressure will be sliding off the coast, giving way to a ridge and a return southwest flow. This will help to significantly warm temperatures aloft, but surface temperatures may be a bit more stubborn to warm up due to the presence of high pressure wedging the colder air. Still, though, the shift in wind direction and the ridge will be enough for temperatures to warm to around 40 or the low 40s tomorrow afternoon.

It won’t be until a warm front crosses the area tomorrow night that the weather will start to get a lot more interesting.

As mentioned before, cold air may initially remain trapped at the surface, with warm air aloft. When temperatures near the ground are below freezing, with a large layer of warmth (above freezing) aloft, precipitation melts in the warm layer aloft, but re-freezes at the ground in the form of freezing rain. Although the warm front is weak, there may be enough lift to trigger some precipitation — particularly north of NYC. Temperatures near the coast will probably be too warm for freezing rain, but the interior — particularly parts of the Hudson Valley — could hold onto surface cold long enough for a brief period of freezing rain late Saturday night into Sunday morning.

Once the warm front crosses, any inversion will break and the warm air aloft will very readily translate down to the surface. With a continued SW flow, temperatures on Sunday could reach the low to mid 50s. However, mid and high clouds will be on the increase during the afternoon, ahead of a developing storm system. Fortunately, this storm will be tracking well to our west and will send another warm front in our direction in an already warm airmass, which could lead to temperatures approaching record-high levels on Monday. But before then, we have a storm to deal with.

The storm itself will be plowing into the Great Lakes and will be dragging a strong warm front out ahead of it. What this does is supply a rich supply of SW winds in the middle of the atmosphere — straight from the Gulf of Mexico. Additionally, a high pressure system well offshore, combined with strong cyclonic flow from the storm system itself will allow winds closer to the surface to be due southerly and southeasterly — right from the Atlantic Ocean. This is a classic recipe for a period of heavy rain and strong winds — models are showing a SE low-level jetstream of 50 knots, which in heavy rain could mix down to the surface. The heaviest rain would fall between 1:00am and 7:00am Monday morning.

What will prevent a prolific flooding event is the quick duration of the rain — once the warm front crosses, precipitation will shut off and temperatures will soar well into the 60s on Monday. In fact, with enough sunshine, temperatures could reach the low 70s!

Tuesday will be another mild day, with temperatures reaching the mid to upper 5os, as a cold front crosses the region. The weather will again become interesting on Wednesday, as this frontal system may be slow to clear the coast, and a storm system may ride along the frontal boundary.

Although the airmass initially will not be that cold, there will be a strong trough moving into the Great Lakes, providing cold weather to our northwest. Thus, if the storm system becomes strong enough, it may be able to gain access to that cold airmass, and provide a snowstorm somewhere in the Wednesday-Friday timeframe. While we still have very low confidence in this actually occurring, the setup is intriguing enough to mention the possibilities.

Once the cold front crosses, there will be a strong ridge developing in the Western half of the US, with some additional ridging developing in Alaska. This helps to greatly amplify the pattern in the Pacific, and could lead to pieces of energy interacting with each other and tapping into the cold air.

The weather pattern in the Pacific is almost ideal if you are looking for a snowstorm, with a trough just off the coast helping to amplify an already strengthening ridge out west. The general idea is that a disturbance will slide down the ridge on Monday and into/near the Gulf of Mexico and tap into some moisture. Additionally, two other disturbances will be sliding down the ridge on Tuesday and might try to catch up to the first disturbance. If they can all interact in a certain way, then a Nor’Easter with potentially significant snow is possible. If they do not, then a snowstorm is unlikely.

Due to there not being anything to “block” the pattern and slow it down (perhaps a ridge in Greenland), as well as the aforementioned strong height gradient in the Atlantic, the flow becomes very fast as the storm tries to round the corner and head up the East Coast. At the same time, however, the flow becomes increasingly amplified — it’s essentially going to be a race between the weather pattern amplifying with the second and third disturbances reaching the first disturbance, and the first disturbance “escaping”. Due to the fast flow of the pattern in the Atlantic and the fact that the 1st disturbance already has a head start, the most likely outcome is that the first disturbance will get too far out ahead of the next two disturbances, and they will only minimally interact. This would mean either the storm stays offshore, or we get brushed by it and get some light rain changing to light snow on Wednesday night. 

Still, though, the ridge out west being as strong as it is, plus the fact that this storm is developing during the busiest travel day of the year means that this storm needs to be watched very closely. Some data actually rushes the first disturbance out of the way so quickly, that there is time for the 2nd and 3rd disturbance to interact and create a big snowstorm all on their own on Thanksgiving night into Friday. That is unlikely, but not out of the realm of possibility.

The storms also appear to be an either/or scenario, not two storms. In other words, to get a snowstorm, either 1) The first disturbance slows down enough to interact with the other two, and we get a snowstorm on Wednesday night, or 2) The first disturbance gets out of the way faster than what’s shown, giving room for the next two disturbances to phase and turn into a snowstorm on Thanksgiving night.

If the first disturbance remains ahead but does not truly interact with the other two disturbances, the resulting initial storm would be too weak to impact us, yet it would create enough havoc in the weather pattern to leave little room for the follow-up disturbances to amplify and give us a snowstorm. This is why the odds are still low at this point for a snowstorm — there are plenty of moving parts, and nothing to force them to all interact in a timely matter. The amount of energy, the three disturbances, and the strong ridge out west are enough, however, to keep us very interested in this potential.