Lake-effect snow hit Chicago this morning. This happens when cold winds at low cloud height are from the northeast. When the flow is from the west or northwest, the Michigan and/or Indiana snow belts come alive. These maps show the setup from this morning.
This map shows a low pressure center along a cold front that will cross the Eastern states today. From southern Pennsylvania southward, a few thunderstorms can develop this afternoon and evening, but everyone in the Middle and North Atlantic state should get some rain.
One way of displaying a forecast is by using a meteogram. Items like temperature, wind and precipitation are distributed across a chart so you can see what is supposed to happen and when. This cart is a meteogram for New York City. Three details are highlighted.
Thunderstorms broke out last night ahead of the warm front that will bring milder air to the Northeast on Thursday. Hundreds of lightning strokes occurred from Nebraska and Missouri into western Illinois. This picture shows a shaft of lightning about a mile from my home a couple of years ago.
Video: all the excitement of watching paint dry. This six-hour time lapse from Saturday, March 21, shows a parade of clouds overhead and the progressive melting of snow that fell heavily in central Pennsylvania the day before.
This map shows the storm causing today's area of snow, but with west to east flow, shows why the weather right behind the storm should be milder tomorrow. The cold front in the northwest corner arrives Sunday.
The map shows the snowfall forecast as of mid morning Thursday. In midwinter, windy cold weather often follows a snowfall. In this case, we expect Saturday afternoon to be nice mild in the areas that get snow tomorrow. However, another cold air mass will arrive Sunday and Sunday night.
While thinking about the idea of more cold and snow, I was reminded of the 19+ inches snowfall in Philadelphia in early April 1915 and the heavy wet snow storm in the Appalachians in late April 1928 that dumped more than 30 inches in spots.
On this pressure analysis map, specially dressed for St Patrick's Day, you can see the cold front and how the isobars are crammed much more closely together behind the front than ahead of it. Typically, the closer the line spacing, the stronger the wind.
As of midmorning Monday, the cold front was still quite a distance away from cities from Chicago to Boston, but it will be south of Ohio and Pennsylvania by late tomorrow. The warmest weather so far this season is arriving ahead of the cold front.