Mid Summer Heat at the End of September

The official start of Fall may begin later this week, but temperatures are expected to feel anything but fall-like.

Highs Sunday were a little more seasonable following the passage of a cold front.  Temperatures Monday will remain in the mid to upper 70's ahead of a warm front that is expected to bring quite a bit of warmth back into the Midwest.

The jet stream pattern this week will be 'stuck' in a blocking pattern with high pressure centered over the Great Lakes and East, and low pressure over the West.  Hurricane Jose is moving up the East Coast, and while not a direct impact on land, is still affecting the weather pattern over the lower 48.  A strong ridge of high pressure won't move much to the east due to Hurricane Jose and this will continue to push temperatures well above average over much of the Midwest, Great Lakes and Southeast.  Meanwhile, cooler than normal conditions are expected over the west with rain and snow.

The middle of the country will remain the 'active zone' with low pressure systems developing over the southern Plains and lifting into the Upper Midwest.  We will see a few showers and thunderstorms this week in northern Illinois and southern Wisconsin, but the chance for those remain isolated and low.

To better understand why exactly our weather pattern has taken such a drastic change from where we started out the month, we have to look roughly 18,000 feet above our heads.

The upper level pattern in the atmosphere resembles the negative phase of the Pacific North American teleconnection, or PNA.  According to the Climate Prediction Center, the PNA is 'one of the most prominent modes of low-frequency variability in the Northern Hemisphere extratopics'.  Basically, it has an impact on the weather that occurs over the United States and identifies where height anomalies occur in the atmosphere.

Think of the atmosphere like an apartment building.  Just like an apartment building has different floors, the atmosphere consists of different height levels.  When the air mass over a certain region warms, that air mass expands causing the height levels over that area to rise.  The opposite happens when the air mass cools.  These above average and below average anomalies correlate well with what's occurring over the United States, and could explain why our temperatures are warming like they are.  Heights levels the Midwest and Great Lakes have been rising above average, while falling below average out west.  While the high heat might not last, the above average warmth (remember our average high by the end of the month is the low 70's) could last through the end of September.