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.
The first freeze of the season is when the temperature hits 32 degrees. As you can see on the map to the right, that usually happens during the first part of October for most of northern Illinois.
A Frost Advisory has been issued for Green, Rock and Walworth counties in southern Wisconsin through 6am this morning. Frost has developed across Wisconsin as temperatures have fallen into the middle 30s in many locations. Clouds will increase by daybreak along with an increase in southerly winds helping to warm temperatures early this morning. Until then, it's a very chilly start this morning!
Let's start with Illinois. While the state saw some improvement in the moderate drought department, it was all to our south. A good chunk of the Stateline continues to fall under the extreme drought category.
Wisconsin continues to see slightly worsening drought conditions. The southern part of the state continues to suffer from extreme drought conditions. Extreme drought conditions are also starting to spread north.
It was a frosty start this morning for some. Temperatures dipped into the mid and upper 30s with a few low 40s just to the northwest. 
Fall officially begins Saturday morning at 9:49 am and boy-oh-boy are temperatures going to reflect that this weekend.
High pressure looks to dig in to our north for the next several days. This does two things. It not only keeps temperatures in a seasonable range for us (upper 60s to low 70s), but also keeps rain chances to our south with a slow moving front.
While rain won't be a concern for us, that front will make for a soggy week in southern Illinois and much of the Ohio Valley.
Here's a look at the current 7-day forecast and just how nice it will be this week. Enjoy!
No shock here. While some summers may have been hot with plenty of rain or dry with cooler temperatures, 2012 ranked in the top four for both, making this summer the worst in terms of being both hot and dry.
A Freeze Watch has been issued for Jo Daviess, Carroll, Whiteside and Stephenson counties in northwest Illinois along with Green county in Wisconsin. These will go into effect Saturday night and last through Sunday morning. Freeze Watches extend through the Midwest and I wouldn't be surprised to see an extension in the watch to cover the remainder of Wisconsin as well as northern Illinois by tomorrow.
It was a very frosty start this morning for some as the mercury dipped below freezing across northern Illinois. Overnight lows Monday morning were as cold as 28° near Sterling and Dixon and 31° here in Rockford. With the low temperature dropping below the freezing mark this morning it marks the first September freeze in Rockford since September 29th, 2003 and the earliest freeze in Rockford in 17 years, since September 22nd, 1995. While this wasn't considered a killing freeze (temperature has to drop below 28°) it may have been in Sterling if the low temperature is actually correct. Looking ahead temperatures will warm for the last week of September with highs in the 70s and overnight lows in the 40s.
