THE FIRST FROST....

Fall is known for pumpkins, Halloween, the changing leaves, the shorter days... but it's also known for frosts. Something farmers don't like too soon, especially when they're not done harvesting. It appears the first frost (and even freeze) will be likely in the Midwest later this week.

Let's start with the basics... Here's the definition of frost from the National Weather Service:

"Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects falls below 32°F. As with the term "freeze," this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a "killing frost." Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a thermometer level temperature in the mid-30s."

So temperatures don't need to be at freezing (32 degrees) for frost to occur, but temperatures will be in the 30s. Here's a look at the average date of the first 32 degree temperature of the fall in the Midwest -

For most of my local area the first frost/freeze is pretty typical around this time of year (October 1-20), but is likely even earlier in Minnesota and the Dakotas (last September). So this is not out of the ordinary.

These cold temperatures won't arrive until the end of the week. To start the week high pressure will be in control and temperatures will be near normal.

A change happens after Friday when a storm moves in and sends temperatures below normal through the middle of October. Each of those nights could lead to frost. Frost forms under clear skies with light winds and temperatures down in the 30s. First here's a look at the storm that will bring in this cool air.

Showers and thunderstorms will be moving through Thursday and Friday in Minnesota, Iowa, Illinois, Wisconsin and Missouri. As cold air moves in snow will begin to fall in the Dakotas, Nebraska and portions of Minnesota. Cold air gets yanked in and here's a look at the low temperatures Friday night into Saturday morning -

There may be some frost Saturday morning, depending on how fast the clouds clear. Frost will be possible again Sunday morning with similar temperatures -

Enjoy the next few days with seasonal temperatures because cooler air and frosty conditions are on the way!

RK