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.
There's a west to northwesterly flow of basically dry air from south-central Canada to the northern part of the Middle Atlantic states. The flow changes orientation farther south. A light northerly flow is helping slightly less humid air to seep southward.
In parts of the Northeast, a sky looking as blue and clear as the one in the picture will be quite a departure from the look of the rainy skies that were much more common earlier in the summer.
The steering winds aloft are changing, and the next phase looks like it will feature a northwesterly flow of cooler air this weekend and much of next week.
It's going to be hot hazy and humid in the much of the Northeast today through Thursday. The recalescent, sudorific, canicular, pyrogenic frying heat gains our attention.
A line of persistent showers and thunderstorms stretched from Long Island Sound to Cape Cod. Some areas in this zone had more than 3 inches of rain this morning, and there can be highway flooding in localized heavy showers this afternoon and evening. This radar shows the rain zone just after 10 a.m. ET.
This map shows the predicted upper-air flow for tomorrow night. Our timing estimates suggest showers and some thunderstorms could affect the I-95 corridor from Portland and Boston to New York City (and perhaps Philadelphia) on Sunday.