After a couple of wet and warm days, the upper air pattern will change again by Friday as a trough redevelops over the East Coast. This means a return to cold air, but it won't be so bad. Highs will be in the 30s--a bargain for mid-January, climatologically the coldest time of the year in NYC.

What is a trough? It's an area where the air is colder and therefore more compressed. Pressure falls with height. When meteorologists speak of "high heights" and "low heights," they are talking about the height at which the air pressure is 500 millibars. If the 500mb point is higher, that means the air is less dense and warmer; if the 500mb point is lower, that means everything is being compressed down and the air is colder.

This nine-panel map, which I look at every day, shows one computer model's prediction of these 500mb heights 10 days into the future. (The map says MRF, but it is actually the newer GFS computer model.) You can see that by day 4, heights along the Northeast coast fall significantly and a trough builds down from Canada while a ridge re-establishes itself over the West Coast. These computer models are not guarantees of pinpoint temperature predictions, but they do a fairly good job of showing the general pattern over the next week.