The 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 are expected to drop sharply with the arrival of fall. Cooler air moving in from Canada is causing this dip in temperatures.
Understanding NWS Alerts
Freeze warnings and frost advisories are issued by the NWS to alert residents of temperatures that could harm outdoor plants or animals caught outside. Typically, these alerts mark the end or beginning of the growing season, warning farmers and gardeners about potential crop damage.
This is the first freeze warning issued by NWS Marquette this season. The region usually experiences its first freeze in late September or early October, making this weather event slightly earlier than normal. Ben Warren, an NWS meteorologist, noted that typical overnight temperatures at this time are in the 40s.
The first day of fall is marked on Tuesday, September 22.
Freeze Warning vs. Frost Advisory
A freeze warning is more severe than a frost advisory. It occurs when temperatures are expected to fall below 32 degrees Fahrenheit for an extended period. A frost advisory is issued when frost may occur, even if thermometer readings say temperatures are above freezing. Ground temperatures can differ from air temperatures, causing frost formation even when air readings are higher.
Details on the Freeze Warning
The NWS Marquette office in Michigan issued the freeze warning effective at 2 a.m. Tuesday for Baraga, Iron, and Southern Houghton counties. Temperatures may drop to 30 degrees Fahrenheit. Affected cities include Kenton, Iron River, Sidnaw, and L’Anse.
Residents are advised to protect water pipes by wrapping, draining, or allowing them to drip slowly. Those with in-ground sprinkler systems should drain them and cover above-ground pipes to prevent freezing.
Details on the Frost Advisory
Frost advisories were issued by NWS offices in Duluth, Minnesota; Gaylord and Marquette, Michigan; Grand Forks, North Dakota (for its Minnesota forecast area); and Green Bay, Wisconsin. Temperatures will fall to 33 or 34 degrees, depending on the location.
“Frost could harm sensitive outdoor vegetation. Sensitive outdoor plants may be killed if left uncovered,” warned NWS Grand Forks.
Influence of El Niño on U.S. Winter Temperatures
A strengthening El Niño in the Pacific Ocean might impact U.S. winter weather this year. El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO) cycle. It has reached record-breaking strength already and is expected to strengthen into November or December, the NOAA said.
El Niño impacts are most evident during fall and winter, causing cooler, wetter conditions in the South and warmer, drier conditions in the North. It can affect the storm track, determining whether precipitation falls as snow or rain, and may lead to extreme cold spells.
Future Warnings
The frost advisories and freeze warning will expire on Tuesday as the sun rises and temperatures increase. However, similar warnings are likely to follow as fall progresses.
Despite fall’s arrival, NOAA’s Climate Prediction Center forecasts a high chance of above-average temperatures across most of the U.S. through October 5, based on its latest outlook.