It was warm over Long Island this weekend. "Over" as in about 2000 feet up, resulting in a dreary, foggy time. The weather pattern may stall a bit for the upcoming week, but eventually, as the air up there starts to move, colder temperatures are in the forecast later in the week.
It's called an inversion. That's the easier meteorological explanation. As a warm front tried to move northward, the warmer air just didn't make it to the surface (the lower ground level where we all are). With the cold air trapped underneath, the inverted temperature profile in the atmosphere just couldn't correct itself. Abundant moisture settled, and the result was a foggy mess for most of the weekend. One more night of dense fog is forecasted before a cold front clears it all out early Monday.
The cold front is expected to bring showers along with it. The radar Sunday night showed a good line of showers from lower Canada right down to the Deep South. Most of this rain will dry out before it gets to Long Island Monday morning. Clearing is expected by the afternoon and temperatures should top out around 50, as long as the afternoon sun fights off the colder air filtering in. It's the warmest temperatures expected in the next week, and maybe longer.
As the front stalls, disturbances ride along it originating around the Tennessee Valley. A mix of rain and snow is in the forecast as these waves pass, Monday night and again Tuesday night into Wednesday. The nature of the pattern makes the timing subject to change.
After the last wave pulls away, colder temperatures ride in behind it. Readings should drop to around or just below normal starting Thursday, as highs are expected to be in the 30s into next weekend. Beyond this, temperatures get even colder, according to the GFS model.