A FREEZE WATCH has been issued for all of northern Illinois beginning Tuesday night, lasting through Wednesday morning.

Skies will slowly clear as high pressure builds in from the north Tuesday evening. This will allow temperatures to fall into the upper 20s and low 30s under mostly clear skies. Areas of frost are also likely Wednesday morning. Be sure to cover up or bring in any plants or vegetation that may be sensitive to the cold.

This cooler than normal pattern will carry us through the end of April and beginning of May. Temperatures will warm by the end of the week, climbing into the middle and upper 60s Thursday and Friday, but will drop right back down by the weekend. The reason, another cold core low-pressure system over the Great Lakes.

The chilly pattern as of late – and going forward – is the result of a blocking pattern in the jet stream. This is known as a ‘Greenland Block’. High pressure becomes stationary near Greenland causing the jet stream to ‘back up’ to the west. A ridge of high pressure builds over the west coast of the United States, forcing the jet stream to buckle over the Midwest and Great Lakes causing the colder air to spill south.

Blocking patterns are hard to break and can last for several days. We will see the pattern weaken slightly by the middle of the week, but it looks to take hold once again by the weekend. Longer range outlooks continue to show a higher probability for below normal temperatures over the Great Lakes and East, as well as out West.

There remains a higher probability for above average precipitation over the Plains and out west, with below average precipitation over the Great Lakes.