A cold front is moving across the Midwest and Northeast this week, bringing lower temperatures and dry air to these regions. The air mass pushed into the Northern Plains and Upper Midwest Saturday night, September 12, and is expected to reach the Mid-Atlantic and Northeast by Tuesday morning, September 15.
The temperature shift arrives as the autumnal equinox occurs on Tuesday, September 15, at 6:04 p.m. EDT. According to meteorologist Tyler Ory, the cooling trend is driven by a pool of cold air that is expanding as the Northern Hemisphere loses daylight at a rate of more than 17 minutes per week. Major cities including Chicago and New York have already lost more than two hours of daylight since the summer solstice.
Daytime high temperatures are expected to stay in the mid-70s for many regions, while interior sections of the Northeast and Upper Midwest will see overnight lows fall into the 40s. In New York City, temperatures are forecast to drop to 60 degrees Monday night, with highs barely reaching the 70s on Tuesday. Boston and Providence are projected to remain in the 50s overnight and stay below 70 degrees through Wednesday morning. While the northern U.S. cools, the South and central U.S. remain under heat with temperatures reaching the 90s and 100s.
Humidity levels are expected to drop sharply along the East Coast as cold air piles against the eastern side of the Appalachian Mountains. Conversely, minor flooding is projected for the Kickapoo River at Steuben and the Upper Iowa River at Dorchester and Decorah by Tuesday, September 15. In the South, the first taste of fall is not expected until later in the week or next.
The cooldown comes as nighttime hours lengthen, allowing the atmosphere more time to cool. While light rainfall chances of 20-30% are forecast for parts of the Midwest between Monday and Tuesday, the primary change for most will be the end of summer humidity levels. The first full day of fall is Wednesday, September 16, following the equinox on Tuesday evening. A warming trend is projected to return by Friday and the following weekend, with temperatures rising back toward the low 70s.