RADAR CHECK: A mesoscale convective system is moving into extreme north Alabama as I write this at 3 p.m., and a severe thunderstorm watch has been issued for the Tennessee Valley until 7 p.m. The main threat will come from strong winds, and there is a chance the upper ridge will win the battle and the storms will fizzle out quickly as they move southward below the Tennessee River. But some risk of strong storms will persist through the evening over the northern third of the state.

Otherwise, the weather is hot and hazy, with temperatures generally in the low to mid 90s this afternoon; heat indices are over 100 in many places.

Heat levels remain elevated through the weekend. Expect a high between 97 and 101 degrees Friday. Factor in the high humidity, and the heat index will peak at 105-110 over the western two-thirds of the state, where an excessive heat warning is in effect. There is a heat advisory for east Alabama, where the heat index will rise into the 100- to 105-degree range.

FOURTH OF JULY WEEKEND: The ridge slowly weakens, but the weather remains hot Saturday and Sunday, with highs between 94 and 98 degrees. The heat backs down Monday and Tuesday, with a high close to 90 degrees. Each day we will mention the chance of a few random, scattered afternoon and evening showers and thunderstorms; odds of any one spot getting wet each day are around 30%. Most of the storms will come from 2 until 9 p.m.

REST OF THE WEEK: Showers and storms will increase in coverage over the latter half of the week as moisture levels rise and the air becomes more unstable; highs drop into the upper 80s, which is below average for early July in Alabama.TROPICS: A surface trough continues to produce disorganized showers and thunderstorms a few hundred miles south-southeast of Bermuda. Upper-level winds do not appear conducive for development of this system during the next couple of days while it moves generally northward at 5 to 10 mph. The chance of development is only 10%.

The rest of the Atlantic basin, including the Gulf of Mexico, is very quiet and tropical storm formation is not expected through early next week.

ON THIS DATE IN 1998: “The Corn Belt Derecho of 1998” hit Nebraska, Iowa, Illinois, Indiana and Kentucky. A derecho that originated in far southeast South Dakota moved across Illinois during the afternoon and evening and continued as far east as Ohio the next morning. Every county in central Illinois sustained some damage as these severe thunderstorms passed. Winds gusted at 60 to 80 mph, with some localized microbursts producing winds of more than 100 mph. Significant damage occurred in the microburst areas, including the towns of Morton, McLean, LeRoy and Tolono.

For more weather news and information from James Spann and his team, visit AlabamaWx.