Thursday 10:30 a.m.
Isaac's winds are weakening in terms of speed, but it remains a potent rain producer. It will move northward through the lower Mississippi Valley, then eventually turn more to the east. Its precise track will then determine where most of the rain falls. This video shows how this could unfold.
The tropics remain active, and an area of disturbed weather in the eastern Atlantic could become Leslie. It has already been picked up by the National Hurricane Center and is called Tropical Depression 12. At 11 a..m. EDT, it was at latitude 14.1 North and longitude 43.4 West. Computer models have offered various scenarios for this storm, and we'll be watching it. At 11 a.m., it was 2,475 miles east-southeast of Cape Hatteras, N.C. I used that point just to show how far away it is from the East Coast, not to suggest where it is going.
Meanwhile, a hazard has popped up today along Lake Michigan. The southerly flow behind the high now in the Northeast is going to be strengthening. The increase in heat will make swimming attractive, but the National Weather Service office in Grand Rapids considers the situation to hazardous. The waves will be higher than usual in summer and will be spaced a shorter distance apart than usual. The various hazards this poses are depicted on this map:
The full NWS release is here
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.