Friday 9 a.m.
A low pressure area that was centered over Detroit at 8 a.m. EDT will move northeastward today. It's a storm that looks much more like a winter storm than what we often see in the summer. On the west side of the storm, gusty north to northeast winds make it feel chilly to people from Green Bay to Chicago, especially considering how hot it has been most of the summer. The air is so cool that it has interacted with the warm waters of Lake Michigan to spawn water spouts, something we predicted earlier this week.
East of the storm center, there is a southerly flow of the kind of humid air typically experienced by residents of Charleston, S.C., and Savannah, Ga. As of mid-morning, a line of heavy showers and thunderstorms stretched from east-central New York to just west of Philadelphia and thence southward to the eastern shore of the Maryland portion of Chesapeake Bay. A lesser band of showers was moving through eastern Ohio. It could evolve into a band of thunderstorms later today as it moves to central Pennsylvania... where there will be time for some surface heating to occur first.
Tomorrow looks like a pleasant, sunny day for Chicago, and it should be pretty nice all the way to western or central Pennsylvania. From northeastern Ohio or northwestern Pennsylvania into the traditional snow belts of New York state, lake-effect showers may be common tomorrow. This video has more:
This radar from just after 8 a.m. EDT shows that rain was still west of the I95 corridor in New Jersey and Pennsylvania. However, you can see why the outdoor scene can change very quickly as the band of thunderstorms moves east. Heavy rain accompanies the leading line of showers and thunderstorms.
Sam the Dog has just heard some thunder, so he wants to do what we all should do: go inside to be sheltered from an approaching storm.
A weak low pressure area may form along the first cold front as it moves southward and then stalls tomorrow. The map below shows where thunderstorms could break out tomorrow near the eastern Great Lakes. More extensive thunderstorm development is likely with the stronger cold front. That activity is shown in the Plains on this map.
Air comes downhill off the Rockies and warms at a rate of 5.6 degrees F for every thousand feet of descent. The warmed air then continues east. Moisture from the Pacific doesn't make it much past the Rockies and a southerly flow of moist air from over the Gulf of Mexico won't readily reach the northern half of the country. This map shows the flow.
This enhanced infrared satellite picture from midmorning shows the thickest clouds near the Atlantic and Gulf coasts, as well as near the Pacific coast from northern California through Oregon and Washington. Vast areas are dry.
Winds are light from the Interior Northeast out through the Great Lakes region. However, a storm off the North Carolina coast has been generating stronger winds. The difference across the region shows up well in this surface pressure map from 9 a.m. ET. The winds are strongest where the lines (isobars) are closest together.
Typically, late September is in the height of hurricane season. However, things are quiet for the moment. This map by Dan Kottlowski, AccuWeather Hurricane Expert, shows the situation. Keep in mind that the big storm of 2012, Sandy, was still more than a month away on this date two years ago.
This upper-air forecast map for next Saturday shows the flow that would foster a warming trend later in the week and for next weekend.