A storm centered in extreme southwestern Virginia at midmorning will be southeast of Cape Cod at sunrise tomorrow. With the storm adding moisture and a steady stream of ice cold air feeding in from eastern Canada, a substantial accumulation of dry, powdery snow will cover the I-95 corridor from northeastern Maryland to eastern Massachusetts with 6- to 12-inch blanket. Along the eastern New England coast, gale-force gusts will combine with the snow to create blizzard conditions. This map shows the pressure pattern across the Northeast at 10 a.m. ET.
This early-morning video shows how things should unfold during the next few days.
One tool I like to use in snow amount predicting sounds ridiculous. You look at a 200 mb map. At that high level, troughs are warm and ridges are cold, the opposite of the arrangement at 500 mb. I look for the warmest reading in a trough and the coldest temperature in the downstream ridge, then draw a line connecting them. I take the difference in readings and divide by two to get an accumulation estimate in inches. You can see why that seems ridiculous: how can a Celsius temperature calculation miles overhead give an estimate of snow accumulations in inches? I have found it helpful over the years. I won't use it for a specific accumulation forecast, but it is a guideline. Also, if we are thinking that a storm will bring a lot of snow, but the temperature spread from trough to ridge is small, it is a red flag suggesting we could go overboard on accumulations. Here is a map showing the arrangement this morning. Since we are making a forecast, I look at the downstream end of the line as being the area where this method may work best.
In this case, 7 inches is the predicted amount. Realize, of course, a tool like this cannot help with fine scale details within a storm, cannot handle changes in the flow pattern with time, cannot anticipate the effects of mountains and oceans, cannot help with the snow/rain line, etc., etc.
That moisture may move away for the afternoon, but thunderstorms will erupt farther west where morning and midday sunshine adds fuel to growing cumulus clouds. This forecast map from the main U.S. model suggests little or no rain in the I-95 corridor from Maryland to Maine tomorrow afternoon.
The surface pressure pattern looks chaotic today, with a multitude of trough lines. A few of these can be caused by glitches in the data, but any of the real ones could be all that's required to organize a short band of showers or thunderstorms. However, these features tend to change character with time, or they disappear and new ones pop up.
Here's a cool fact: even when Death Valley, California, has a temperature of 110 or 120 degrees, you only have to go up a little more than 3.5 miles to find temperatures at or below freezing.
It appears the dry comfortable air mass now in the Northeast will be replaced by a humid flow from the South Atlantic states for the coming weekend. An upper-air forecast map sequence in the video shows how this could happen. The following map shows the predicted flow from Florida to New Jersey Friday night.
This map shows the pressure analysis for the Northeast and Great Lakes. The gusty flow on the west side of the low pressure area adds a real autumn feel to the air.
Since individual lines and clusters of thunderstorms have limited life spans and change character constantly, forecasting whether it will or won't rain at any one time this weekend is difficult at best. One solution is to have your tablet or phone available with the AccuWeather.com app so you can see where all the storms are at the times when it concerns you the most.