Major winter storm aims for the Northeast

A cold high pressure area covers the Midwest and most of the Northeast. A low pressure area from the southwest will head straight for the high-pressure area. As warm and increasingly moist air from the south climbs over the retreating cold air, clouds and precipitation and will break out rapidly tonight and tomorrow. We could actually see two pulses of precipitation. The first will be caused by the advance of the warm air over the cold air, and the second will be associated with the actual low pressure area and upper-air trough that support the whole event. The heaviest snow tomorrow seems likely to be centered over Pennsylvania with final amounts depending not only on the snow but on when the snow changes to sleet and freezing rain. Any delay of the change means there'll be more snow, while any speed up of the change will mean less snow. Regardless of the total snowfall, tough travel conditions are likely to affect places from Illinois and Indiana to New England as the storm advances.

One old-school snow forecasting technique involves a look at the 200 mbar chart. At that level, troughs are warm and ridges are cold. You find the warmest temperature at the bottom of the trough and the coldest reading in the downstream ridge, then draw a line connecting the two. On the map below, there is a 16-degree-Celsius difference. You then divide that difference in half and come up with a rough estimate of snow accumulation. It seems ridiculous to use Celsius temperatures at a level thousands of feet overhead to predict inches of snow at the ground. Obviously, this method will not be precise, but it can be helpful. If you see a large spread in temperature, consider the possibility of heavy snow. However if you are predicting heavy snow and there is very little difference between the temperature in the trough and the ridge, consider the possibility there will be less snow than you predict. The map below is from this morning. This evening's map should confirm or change downwind snow expectations.