Chills tonight could be felt again by the weekend

A very cold and dry air mass will be upon us tonight as high pressure originating from Canada moves through the mid-Mississippi River Valley.  There are a few things I want to discuss in this post with the first concerning this afternoon.

Sunny skies will be found this morning but by mid to late morning you'll begin to notice those puffy clouds building in the sky.  These are what we call fair weather cumulus clouds.  As the warm sun heats the ground the air particles at the surface will begin to rise.  As they rise they'll enter into a fairly cold atmosphere only a few thousand feet above.  Once that rising air cools enough clouds begin to form.  Sometimes there can be a threat for sprinkles or showers but I feel with the very dry atmosphere in place it will be hard to get anything to fall, let along reach the surface.

Temperatures this morning near the core of the cold air fell into the low 30s as Frost Advisories and Freeze Warnings were issued for parts of the upper Midwest and northern Great Lakes.  We're not expecting temperatures to drop that significantly but it is going to get cold tonight.  Under clear skies and light winds meteorologists will look at the dew point temperature to get an idea of just what the overnight low could 'potentially' drop to.  With dew points upstream in the 30s it's likely that temperatures tonight across northern Illinois and southern Wisconsin will at least drop into the upper 30s, if not mid 30s.  The big question tonight will be if there could be any frost development by Wednesday morning.  It's possible that there will be patchy frost but I feel widespread frost may be just a little hard to come by just yet.  First, the center of the high will be just to our south and then quickly slide southeast by Wednesday morning.  This means that our winds will likely go calm (or near calm) between about 8pm/9pm and roughly 2am before switching around to the southwest.  It's during this time when temperatures will likely nose dive into the 30s and patchy frost may form.  If cloud cover moves in or the winds shift around to the southwest sooner the frost potential will decrease.  Another thing to keep in mind is that current soil temperatures are still in the low 60s.  Click here for a list of the 4 inch and 8 inch soil temperatures.  Our average first frost usually occurs in the first week of October, so we're not that far away.

While Tuesday night will be a cold night it's not going to be the only cold one this week.  Another shot of cold air looks to arrive this weekend with Saturday night's low potentially dropping into the 30s once again.  How appropriate considering it will be the first night of fall.  Either way you may want to think about covering up the plants or even spraying a little water on them to keep the heat in overnight.