National Weather Service (NWS) offices in Minnesota, Michigan, and Wisconsin have issued a frost advisory for Tuesday morning. An office in Michigan has also issued a freeze warning as temperatures drop this week with the start of fall. Cold air from Canada is causing the temperature decline.
Freeze warnings and frost advisories are alerts used by the NWS to inform residents about temperatures cold enough to damage outdoor plants or harm animals caught outside. These alerts typically occur at the beginning or end of the growing season to notify farmers and gardeners of potential weather damage to crops.
According to NWS meteorologist Ben Warren, this is the first freeze warning issued by the NWS Marquette office this season. The region usually experiences its first freeze in late September or early October, making this week’s cold weather somewhat premature. Warren noted, “Our normal overnight temperatures this time of year are usually in the 40s.” Fall officially begins on Tuesday, September 22.
Freeze Warning vs. Frost Advisory: Key Differences
A freeze warning is issued when temperatures are expected to fall below 32 degrees Fahrenheit for an extended period. It is more severe than a frost advisory. A frost advisory is given when frost might develop even if temperatures are above freezing. Ground temperatures can be colder than air temperatures where thermometers are located, which can cause frost formation even if the thermometer shows above-freezing readings.
Details on the Freeze Warning
The NWS office in Marquette, Michigan, issued the freeze warning, effective at 2 a.m. Tuesday for Baraga, Iron, and Southern Houghton counties. Temperatures are expected to drop as low as 30 degrees Fahrenheit in areas such as Kenton, Iron River, Sidnaw, and L’Anse.
The warning advises protecting plants and water pipes. “To protect water pipes from freezing, wrap them, drain them, or let them drip slowly,” the warning stated. Those with in-ground sprinkler systems should drain and cover above-ground pipes.
Details on the Frost Advisory
Frost advisories are more widespread. NWS offices in Duluth, Minnesota; Gaylord and Marquette, Michigan; Grand Forks, North Dakota, for its Minnesota forecast area; and Green Bay, Wisconsin, have issued them. Temperatures will reach 33 or 34 degrees, depending on the location. “Frost could harm sensitive outdoor vegetation. Sensitive outdoor plants may be killed if left uncovered,” NWS Grand Forks stated in the alert.
El Niño’s Impact on Winter Weather
A strengthening El Niño in the Pacific Ocean could affect winter weather in the U.S. this year. El Niño, part of the El Niño-Southern Oscillation (ENSO) cycle, is currently at record-breaking strength and is expected to continue growing into November or December, a NOAA spokesperson reported.
In the U.S., El Niño typically has the most significant impacts during fall and winter. These include cooler, wetter conditions in the South and warmer, drier conditions in the North. El Niño can also influence storm tracks, precipitation type, and extreme cold spells.
Looking Ahead
The frost advisories and freeze warning will end once the sun rises and temperatures increase on Tuesday. However, similar warnings are expected as fall weather continues. Despite the season, NOAA’s Climate Prediction Center predicts a high likelihood of above-average temperatures across most of the U.S. through October 5, according to its latest temperature outlook.