Overnight, the kink in the jet stream that will spark our a possible winter storm this weekend for the east came ashore on the Pacific coast of the United States. The packet of energy is seen in the bending of the jet stream just off the coast of Washington state (see the 7pm Tuesday evening map, below).
To get a blizzard for the mid-atlantic states, this kink needs t0 settle in on the east coast, gathering up other packets of jet stream energy and making the air spin so strongly the jet stream spins into a circle as seen in this frame from the forecast 500mb jet stream for Saturday evening.
The strength and location of the center of the spin (an upper air low pressure system) will determine how much snow will fall and where the rain/snow line will be. Think of a wobbling top – this upper air storm is wobbly and the small wobbles are hard for the computer models to predict.
So, if you are looking at the predictions of snow amounts for the weekend and the time when the snow will start – realize how precarious this forecast is for a storm who’s energy is still on the other side of the country.