Mid-September Heat Wave

The heat from this past weekend was unusual for this time of year, but it was not as epic as the building heat wave this week has the potential to be. The high temperatures for Fri-Sun were 87, 88 and 86, respectively. After a briefly cool, lake-breeze driven day today, a massive downstream ridge will build over the eastern half of the United States (as we've been predicting) and will bring strong southerly flow and very warm temperatures from Wednesday through next Monday (at least). Here's a look at the temperature for graph for Thursday, September 14 through Monday September 23:

As you can see, we have a chance to break a number of record high temperatures later this week. For reference, the average high temperature at O'Hare Airport this time of year is between 70 and 75. The models are forecasting highs approaching 90. The record highs all hover between 90 and 92.

Why the epic heat?

For reference, here is the setup, with the giant red blob in eastern North America representing a dome of high pressure and anomalously warm temperatures:

The diving jet stream in the western United States is responsible for this dome of high pressure (we call the diving jet stream a "trough" and the high pressure a "ridge"):

After a mild summer, our very warm 2017 in Chicago continues.