We are now reaching the time of year when people around the northern 1/3 of the United States start talking about the “Indian Summer”. The term refers to a warm stretch of weather after the first, or killing frost, of autumn. This typically occurs sometime in mid to late October in Iowa, but can also occur well in to the winter months. We are looking at a stretch of warmer weather starting on Friday, and stretching through the weekend, that many would consider to be an “Indian Summer.”
So what happens in the atmosphere to cause this kind of influx of warm air? In the United States, there is a pattern that normally sets up with the jet stream and interaction between low and high pressures that causes the influx of warm air. It generally will start setting up with a block of high pressure setting up somewhere around the Great Lakes region, and slowly moving toward the east coast. This clockwise spin in the atmosphere (high pressure) pulls warm air from the gulf region northward, resulting in temperatures 10°-15° above normal. This also causes the northern jet stream to move off to the north, usually around the Canadian border, which usually means that the chance for storms usually goes down. The result is a warm, and generally stable air mass for 2-6 days. Below is a typical “Indian Summer” pressure set up.
What will cause an end to the “Indian Summer” you ask? Generally, a low pressure center will develop over the Rockies, building intensity slowly, and eventually gaining enough strength to form a deep trough in the jet stream. Once the trough(and low) have enough energy, they will start to displace the high pressure to the east, usually by advancing 2-3 cold fronts in a 36-48 hour period off to the east/northeast. When these fronts move through, there is usually a corridor of marginally severe weather associated with them. This normally occurs on the southeast side of the low. On the northwest and west side of the low, winter precipitation can occur, if the air behind is cold enough. The later in the year this occurs, the higher the chance for winter precipitation.
This particular “Indian Summer” event is occurring early enough in the autumn that most of Iowa will not be seeing any of the winter precipitation on the back side of the system, but rest assured that winter will be making its way toward the area before too much longer. I will have an updated “Winter Outlook” coming out Sunday.