This time of year, the northeast is usually an icebox, with cold air pretty much locked in, right through February.  March is typically the time when the jet stream starts oscillating north and south, and periods of warmer air move in.  However, it will feel like March in the northeast this weekend, thanks to the jetstream moving way to the north, into Canada.  This will allow warmer air from the south to move into the region.  In fact, warmer than average temperatures will be felt from Chicago, all the way to the gulf coast. Pick your favorite city on Friday, and follow the temperatures through the weekend: Just about all locations east of the Mississippi River will be very mild.  Saturday will be the warmest day of the weekend in the northeast.  Temps in New York City will reach 52 on Saturday.  Boston will reach 51 degrees.  Average high temps in both cities are 38 and 36 degrees respectively.  Those 50's are going to feel terrific! But unfortunately, all good things must come to an end.  A cold front will move through the northeast on Sunday and Monday, bringing not only a chance for rain, but cooler temperatures.  The Jetstream will also move southward behind this cold front, allowing colder air to swing down from Canada.  By Tuesday and Wednesday, cold sunshine will be the rule, with temperatures falling a bit below average.  The 50's will be gone in New York, and the 30's will return.  New York City could even see snow showers Monday night into Tuesday morning.  As quickly as the 50's arrived, they will disappear, until the next time the jetstream decides to take a trip into Canada. For WeatherNation:  Meteorologist Matt Monroe