And so ends the growing season
Cool Canadian air has blown into Minnesota. Advancing clouds kept the sun from warming us on Monday. Some of the clouds had enough support aloft to drop light rain on parts of southern Minnesota. Pipestone and Jackson in the southwestern part of the state both reported light snow (!) and 37 degrees this afternoon.
Frosty temperatures are on their way
Skies will clear and the wind will become quite light tonight. A chilly air mass plus clear skies with little or no wind is the recipe for cold temperatures and frost. Heat will be radiated to space and the growing season will come to an end for most of Minnesota by Tuesday morning. Low temperatures should range from the mid 20s in the far north to the upper 30s in the southeast. The Twin Cities should dip to the mid 30s.
Frost advisories and freeze warnings
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Under clear, calm conditions, the surface cools at night and then cools the air in contact with it. Frost then can form when surfaces cool to 32 degrees but air temperatures remain a few degrees above freezing.
Frost advisories usually are issued at the end of the growing season when air temperatures are forecast to drop to 33 to 36 degrees and widespread frost is expected. Frost advisories have been issued for tonight and into Tuesday morning from the Worthington area through Mankato, Red Wing and the Twin Cities metro area to parts of west central Wisconsin including Eau Claire.
Farther north and west, freeze advisories have been posted for tonight and into Tuesday morning. Freeze advisories are issued when widespread temperatures of 32 degrees or colder are expected at the end of a growing season. Those temperatures can halt the season of even frost-hardy garden plants.
The big picture
Here is a look at the breadth of frost advisories and especially freeze warnings across the north central part of the country.
Tuesday-Thursday
Tuesday will be a cool day with lots of sun for most of us but also a chance of a few showers near Iowa.
Then we will turn warmer for Wednesday and Thursday.
Chances of rain late in the week?
At this point, some periods of rain seem likely for the Friday-Saturday period. Sunday looks drier, but there are no guarantees so far in advance.
Tropical update
While frost is our concern for tonight, at least we need not fear that our homes will be whacked and drowned by hurricanes. Harvey, Irma, Maria and then Nate certainly took their toll elsewhere.
And now we have another tropical storm. Tropical Storm Ophelia is spinning in place way out in the Atlantic Ocean and is unlikely to be a threat to the Caribbean area or North America.
