Frost Dates for 55802 — Duluth, Minnesota

Half of springs at Duluth Harbor Stn, MN see their last 32°F night by May 14; only one in ten is past it by April 24, and one in ten still has one after June 1. Planting tender crops after May 22 leaves a 30% risk of a frost on them, after May 27 20%, and after June 1 10%. The 18 days between the even-odds date and the one-in-ten date is wider than at 92% of the 142 Minnesota stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 12 in half of years, as early as September 25 one year in ten, and holds off past October 26 one year in ten. A hard freeze of 28°F follows 13 days later, around October 25; in spring the one-in-ten date for 28°F is May 18, 14 days before the 32°F one. The freeze-free season lasts 149 days in a typical year, shorter than at 57% of the 142 Minnesota stations; it reaches 175 days one year in ten and falls short of 124 one year in ten. On average 166 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 17 days longer, to about May 31, and return around September 30. Severe cold of 16°F or below comes every winter, usually between November 17 and April 3.

August is the warmest month at Duluth Harbor Stn, MN, averaging 74°F by day and 60°F at night; January is the coldest at 21°F and 4°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 76% of years and June still brings one in 12% of years.

Warmth for growing adds up to 1,664 growing degree days a year (base 50°F), fewer than at 94% of the 142 Minnesota stations. 19% of them (324) have built up by the end of June, and August adds the most, 526.

The nearest other station, Superior, WI, 3 miles south of Duluth, has a last frost 8 days earlier (May 6), a first frost 3 days earlier (October 9), a season 5 days longer.

The first measurable snow (0.1 inch or more) in Duluth usually arrives around November 16, and the last of the season around April 9. Those are the middle dates across the winters of 1991–2020 at Superior, WI, 3.4 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 35 days after the last snow.

ZIP 55802 is under a mile from Duluth Harbor Stn, MN, which stands at 610 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 5 other ZIP codes share the station; its figures are presented in full for 55804 (Duluth). The 2023 USDA Plant Hardiness Zone Map lists ZIP 55802 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°C) over 1991–2020; a zone describes winter lows, not frost dates.