Frost advisory expanded in St. Louis County until 7 a.m. Thursday

Upper-20s temperatures settled over portions of St. Louis County overnight, prompting the National Weather Service in Duluth to expand a frost advisory until 7 a.m. Thursday and urge residents to protect sensitive vegetation before sunrise.

The danger is greatest for gardens, flower beds and any early-season planting left uncovered. The National Weather Service says frost typically forms when air temperatures fall to about 33 to 36 degrees or colder with light winds, though it is not guaranteed if the air is very dry or winds are around 8 mph or higher. A freeze begins at 32 degrees or lower and can damage unprotected plants if readings stay below freezing for several hours.

If temperatures reach 28 degrees or lower for at least an hour, the damage becomes more severe. The Weather Service classifies that as a hard freeze, a threshold that usually destroys most seasonal vegetation and can damage unprotected outdoor plumbing and sprinkler systems. That puts home gardeners, school gardens and community plots in St. Louis County on alert, especially where tender seedlings, hanging baskets or recently transplanted vegetables were left exposed.

The Minnesota Department of Natural Resources says last-spring and first-fall freeze dates matter to home gardeners, commercial horticulture, farmers and agribusiness. Its freeze and frost probability tables are based on the 1991 to 2020 period of record, a reminder that long-range forecasts cannot replace local climate history when growers decide when to plant, cover, or wait. In Duluth, National Weather Service climatology puts the mean first freeze around Sept. 30, underscoring that a May frost is unusual for the calendar but still part of the region’s normal shoulder-season pattern.

The cold snap came as another weather hazard was building. National Weather Service Duluth said dangerous fire weather was worsening later Thursday and Friday, with red flag warnings in effect, and forecasters also flagged a potentially anomalous severe-weather setup from Sunday into Monday with heavy rainfall and possible flash flooding. For St. Louis County households, the immediate task is to get plants, hoses and exposed irrigation protected before 7 a.m. Thursday, before the next round of weather risks arrives.