A storm system organizing over the central U.S. and tracking eastward Friday afternoon and evening will bring widespread heavy rain to the Tennessee Valley after midnight Friday night and into Saturday.
The heavy rain will begin increasing between midnight and 2 AM west of I-65 - around Florence, Muscle Shoals, Tuscumbia, Russellville, Red Bay, Waterloo, and Moulton. The heavy rain will spread eastward, increasing along I-65 and I-565 between 2 AM and 4 AM - around Huntsville, Madison, Athens, Decatur, Fayetteville, Hazle Green, Hartselle, and Lacys Spring. Rain will fill in over Sand Mountain between 4 AM and 6 AM Saturday. These are times when you should expect the rain to start. Rain will fall, heavily at times, through much of Saturday.
A secondary surge of heavy rain will arrive in the Shoals between 9 AM and 11 AM, around Huntsville and I-65 around 11 AM to 1 PM, and around Sand Mountain between Noon and 3 PM. This line of heavy storms can come with isolated 50-60 mph wind gusts, which is nearing the threshold for storms to be classified as severe. A brief spin-up tornado cannot be ruled out. Though a risk is present for a severe thunderstorm or two, the overall risk is very low. The heavy rain earlier in the morning may help to limit how energetic the atmosphere is, which would limit or choke the severe risk before it starts. The change of wind above the ground is favorable for severe storms if storms can form within the heavy rain.
Moisture wrapping around the western edge of the storm system will bring spotty light rain occasionally through Saturday evening. A rapidly cooling atmosphere will mean that some of that rain can change to snow after about 7 PM Saturday. Snow will remain possible through about 3 AM Sunday. By the time the sun rises Sunday, the chance for snow will be over.
Rainfall amounts of 1-2 inches are possible. Rivers can rise, and some could be near flood levels by early next week. Snowfall amounts will be less than a quarter-inch, but most of the snow that falls will melt rather than accumulate. No significant accumulation is likely, but colder surface could see a light dusting.
Even away from the storms, wind will be significant. It will increase after midnight Friday night to 15-25 mph with gusts to as high as 35 mph. The strongest winds overall will relax after about 9 AM.
Very cold air will follow this storm system. Temperatures will drop nearly 30 degrees from Saturday afternoon through Sunday afternoon. Monday morning will chill into the high teens and low 20s. Wind chills may drop into the single digits Monday morning.