- Temperatures will be well above average in most of the lower 48 regions.
- The northwestern United States may trend near average or slightly cooler.
Above-average temperatures are expected to spread across the Lower 48 in September, meaning the first month of fall may not bring much relief from the summer heat, according to updated forecasts from The Weather Company, an IBM company. Business and Atmopheric G2.
What are the changes in the latest September forecast? The updated temperature forecast is noticeably warmer due to recent trends in computer model guidance.
Temperatures will generally be far from average across much of the Plains and the upper Midwest. Warmer-than-average conditions will extend across much of the East, as well as into the Rockies and Southwest. Areas along the East Coast and in Florida may be near average or slightly warmer.
The only area that can be a little cooler than average is the northwest up to Northern California.
Temperature forecast for September
(Atmosphere G2 and Weather Company)
What's driving the September forecast? The dominant upper-level pattern that models predict will feature an upper-level ridge of high pressure over parts of the Midwest, with a southward depression in the jet stream near the northwest.
This pattern will affect temperatures across the lower 48 for the first week of September. Highs and lows will be warmer than average, especially in the Midwest and Northeast starting Labor Day weekend. Record highs are possible in parts of the Plains, Midwest and Northeast during the first few days of the month.
“Given the latest guidance and the similarity of the forecast pattern to that of the top five September synopses, we have sharp increases in forecast temperatures across the central and eastern U.S., with some cooler changes across parts of the West,” said Todd Crawford, director of meteorology at Atmospheric G2. The forecast is for the third warmest September on record, and we may still be quite cold.
What does this mean compared to the average? High temperatures tend to cool slightly from the beginning of September until the end of the month. Average highs over the entire month show a moderate feel across the northern tier with 60s, 70s and even around 50s at higher highs in the west.
In the south, the highlands remain warm with temperatures rising to the 80s. Highs in the 90s are typical in parts of South Florida, South Texas and the Southwest, and average highs continue to be in the 100s.
This suggests that for most of the central and eastern United States, where the month is expected to be hotter than average, it will feel more like summer with probability high in the 80s and 90s.
Average highs in September
(Based on data from 1990-2020)
Mornings also tend to get cold during the first month of fall. Lows will drop into the 30s and 40s across much of the West, as well as across the Northern Tier and into parts of the interior Northeast.
Lows in the 50s and 60s are common across much of the East, Midwest, and Plains. Mild mornings continue in Florida and along the Gulf Coast, as well as parts of the Southwest.
Given the current forecast, the morning could remain very mild with lows widely in the 60s and 70s from the Plains to the east. Cool conditions in the northwest mean the morning could be a bit chilly.
Average September lows
(Based on data from 1990-2020)
Linda Lam is Weather.com's chief meteorologist. Growing up in Massachusetts she developed a passion for winter storms and tornadoes which led her to pursue a career in meteorology.
The Weather Company's primary journalistic mission is to report on breaking weather news, the environment, and the importance of science in our lives. This story does not necessarily represent the position of our parent company, IBM.
