Monday 8 a.m.
Dry, chilly air is in place across the Northeast today. A reinforcing shot of cold air is advancing southward through the western Great Lakes region. Meanwhile, a weak low pressure area is moving east-northeastward from Oklahoma. That system will strengthen a bit and head toward Virginia, then move out to sea tomorrow night. This video and the map that follows it show Monday morning's thinking on what will happen. These systems often pack surprises, so please check back for later information at accuweather.com.
This snowfall map was prepared before daybreak Monday. Please check back for updated stories as the snow area actually develops.
Now, here's a strange forecasting tool. When we talk about the flow aloft, we often are looking at the flow pattern about 3.5 miles overhead. We look at areas of rather straight flow and also at troughs and ridges. Troughs are cold and ridges are warm at that altitude.
Higher in the atmosphere, about 7 miles overhead, the barometric pressure is about 200mb, which is only 20% of the pressure down here where we live. At the 200 mb level, troughs are warm and ridges are cold. The amount of temperature difference between the bottoms of the troughs and the crests of the ridges is related to the amount of upward motion going on with the system involved. Upward motion within a storm means air is rising and cooling, eventually reaching the saturation point. Major snowstorms and blizzards can have a difference of more than 20 degrees between the trough and the ridge. Looking at today's 200 mb map, we see only a 5-degree spread in the disturbance that can cause snow in the Middle Atlantic states tomorrow. That's a relatively weak system.
At the top of this section, I promised a strange forecasting tool. Here it is: if other factors favor snow, you draw a line from the warmest temperature at the bottom of a 200 mb trough to the coldest spot in the downstream ridge. Make note of the temperature difference in degrees Celsius between those two points. Now, divide the number in half, and that's the snow accumulation forecast in inches. It seems like nonsense, right? It doesn't make much sense to use a difference in degrees Celsius to give a snow accumulation in inches: the two items are in different units! All I can say is that I was shown this tool many years ago and it works surprisingly well. If the values don't change (but they often do change as a storm evolves), the snow accumulation in the middle of the snow band tomorrow would be about 2 or 3 inches. Pro site members can get their own plots of the 200 mb level and try the tool for themselves. Remember, however, it is just a rough estimate, and much can change.
Looking at next week, the GFS ensemble spaghetti plot of upper air winds shows how much agreement there is among members of the ensemble (same model running multiple times using slightly different starting assumptions). The maps are from next Tuesday, Nov. 25, and Thanksgiving Day, Nov. 27. There is good agreement on the first map, but a lot of spread two days later.
The location of lake-effect snow bands is tightly controlled by geography, topography and wind. From this pressure analysis, we see why the wind favored heavy snow staying south of the hardest hit Buffalo snow belts earlier today.
If this timing works out, there would be good travel weather for the Northeast Corridor on Wednesday while snow showers cross the Great Lakes and reach the northern and central Appalachians.
This map from one of my tweets yesterday (accuElliot) showed the wind direction most favorable for heavy lake-effect snow in and near Buffalo. Just a minute change in direction greatly affects the location of the heaviest snow, almost as if you were operating a fire hose. The snow is so deep (more than 4 feet in spots and deepening) that officials were considering the use of high lift equipment to extract vehicles.
It suggests rain in the I-95 corridor and snow from the mountains of West Virginia and Pennsylvania to southwestern Maine. Other models and ensemble versions will be examined this weekend as we narrow down the uncertainties associated with this fast-moving storm. Whatever the form of precipitation, you can count on another shot of cold air behind it. Lake-effect snow will be common as well.
I can see how slippery spots can develop from Philadelphia's northern and western suburbs on northeastward. The highest amounts of snow may be from Boston's western and northern suburbs on through Portsmouth, Portland and Augusta. Some spots could get 3-4 inches. Be ready for a slow commute tomorrow morning. If these problems develop, they could occur rather suddenly.