Major storm to affect the Northeast tomorrow

1. The surface analysis shows a high pressure area near Great Lakes, and it has sent cold, dry air well south and over to the East Coast. Meanwhile, in response to the approach of a trough in the upper atmosphere, a low pressure area is forming over the Southeastern states. That storm will move northeastward to be just east of New York City late tomorrow night. With cold air in place throughout the Northeast, precipitation is likely to start a snow in most areas.

However, in the I-95 corridor, east winds will bring milder air in from the Atlantic to change the snow over to rain. There is likely to be a transitional period with a mixture of snow sleet and rain. Accumulations will depend on how long the snow and sleet will fall. One problem with a storm like this is that precipitation is likely to become heavy. If snow is falling at the rate of 1 to 3 inches per hour, it is easy to see how a timing error on the changeover can't cause a huge error in the accumulation forecasts. In any case, rain will dominate during the later part of the storm, and there is concern for flooding.

In interior areas, the combination of snow, sleet and freezing rain will cause widespread traffic delays and a heavy buildup of snow and ice in the Appalachians. Where precipitation remains all snow (best chance in northern New England), more than a foot can accumulate. The storm will then head out to sea on Friday.

The following map shows our overall snow accumulations:

When we make forecasts for specific locations, we need to look at a variety of computer models. As you know, there is often a large spread in the solutions. This can be shown on a chart of the type below. As you can see, there is a wide range of accumulations, so by discussing the situation and analyzing other charts and maps we try to narrow down the accumulation forecasts.