A Chilling Mid September

Frost and freezing temperatures have made an early appearance in the central and northeastern U.S. this year.  Last week's strong cold front that swept through the central U.S. brought the first freezing temperatures to the Midwest as far south as southern Wisconsin  Another sprawling cold air mass spilled south across much of the country east of the Rockies this weekend ending the growing season for many locations as far south as northern Missouri Sunday morning. Last night frost warnings were in effect for eastern Wisconsin, all of Illinois, Indiana, and Michigan (except areas close to the Great Lakes), and parts of Ohio, Pennsylvania, New York, Maine, West Virginia, Virginia, and North Carolina.

The areas in blue indicate the frost advisories in effect for Sunday night, September 23rd.

Low temperatures this morning reached the low to mid 30s south to the Ohio River, and frost was noted on the grass and rooftops in many areas.  On nights with strong radiational cooling, the vertical temperature gradient can be rather steep. The temperature at grass level can be degrees lower than the temperature at five feet, the approximate height of temperature sensors. If the temperature at sensor level is 33°F, the temperature right at the ground can be below freezing.  Even on such days, there may not be much damage to plants.  Damage to plants depends not only on whether or not it is below freezing, but how long the temperature is below freezing.

Minimum temperatures for the morning of September 24.

The colored dots on the map indicate locations where 32°F has occurred this fall.

The first occurrence of 32°F is about on schedule for the upper Midwest and Northern Plains, and anywhere from a week to two weeks early in the central U.S.  The temperatures in the low and mid 20s are a few weeks early.  The maps below show the extend of freezing weather so far this fall n the central U.S., and the median dates for the first occurrence of 32°F and 28°F.

This map shows the median date of the first occurrence of 32°F in the central U.S. Within a range, the earlier dates typically occur in the north and the later dates in the south. However, terrain and surrounding land use can have a significant influence on overnight low temperatures.

Same as the map above, except for 28°F.