It's like a switch was flipped. October was off to a blazing start with record-setting heat. Now we are bracing for the first frost of the season. The switch came at the right time. On average, Lexington's first frost occurs on October 14th. The earliest frost occurred on September 23rd mostly recently in 2012. The latest the first frost has ever occurred was November 9th. Last year's first frost occurred on October 18th. This will be Lexington's first frost since April 1st.
There are a number of meteorological conditions that can lead to frost formation:
Clear skies lead to radiational cooling, allowing the greatest amount of heat to escape into the atmosphere.
Calm to light winds prevent stirring of the atmosphere. These conditions allow a thin layer of super-cooled temperatures to develop at the surface. These super-cooled temperatures can be up to 10 degrees cooler than 6 feet above the surface, where observations are typically taken.
Cool temperatures, with some moisture, to promote ice crystal development. Radiational cooling will help freezing temperatures to cool to the dew point. Once that is achieved, condensation occurs. Frost will then develop on exposed surfaces.
Topography plays a role in determining if and where frost develops. Cold air settles into valleys at night since it is heavier than warm air, therefore these areas are more prone to frost formation. Valleys also shelter the area from stronger winds, enhancing the potential for frost.
Soil moisture/temperature and stage of vegetation, it's "green-ness," are also factors.
A Frost Advisory goes into effect for most of the area tonight and will run through 9 AM Sunday. Areas of frost are expected with temperatures bottoming out in the mid 30s.
Make plans to protect your outdoor plants. Bring your pets inside for the night.
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