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SunnyDays

ERA5 almanac — 8 years of real data

Boundary Waters in May

Boundary Waters in May: 29% of days see measurable rain, 64°F average high, 10% of days historically clear (n=248, ERA5 2018–2025).

Boundary Waters in May is historically a poor month for sun: only 10% of days clear across 8 years (n=248, avg high 64°F). July is the clearest month for Boundary Waters historically. Historical average, not a forecast.

What you are reading: 8-year historical averages from the ERA5 reanalysis, not a forecast. A month that is historically 90% clear can still be overcast on your specific trip. For this week’s actual forecast, see the live Boundary Waters page. Both can be true at once: “historically strong in this month; this particular week is not.” We label which is which.

Early vs. Late May

Clear-sky percentage for Boundary Waters in May, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period % Clear days Sample (n) Avg high
Early May
1st–15th		 8% n=120 64°F
Late May
16th–end		 12% n=128 64°F
Full month average 10% n=248 64°F

Sunshine & warmth in May

A second measure using actual sunshine hours (ERA5 sunshine_duration), not the daily weather code. A Lowcountry day with 12 hours of sun and a 25-minute afternoon storm is counted below as a warm, sunny day — because it was. How we define this.

Sunshine-based metrics for Boundary Waters in May. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period Warm & sunny (≥72°F, >6 h sun) Sample (n)
Early May
1st–15th		 11% n=120
Late May
16th–end		 34% n=128
Full month average 23% n=248
12.6 h median sunshine per day 75% of days exceed 8 h sunshine 29% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 10%. Sunshine-based “warm & sunny”: 23%. The 13-point gap is from brief afternoon convective storms that stamp the whole day as “rain” in the WMO code but leave the bulk of the sunshine untouched.

Bottom line

Boundary Waters is a poor choice for sun in May: just 10% of days clear historically (n=248). July is the historically strongest month for sun here. The 8-year record is consistent — this is not a fluke bad stretch.

→ See all destinations in July — historically the best month for sun in Boundary Waters.

Every month in Boundary Waters

Compare across all months with data. Numbers are historical clear-sky %.

Historical clear-sky % for Boundary Waters, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 0% n=248 All destinations →
April 0% n=240 All destinations →
May ← this page 10% n=248 All destinations →
June 26% n=240 All destinations →
July 31% n=248 All destinations →
August 30% n=248 All destinations →
September 20% n=240 All destinations →
October 2% n=248 All destinations →
November 0% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Boundary Waters in May?

Sometimes. Boundary Waters sees measurable rain on 29% of days in May historically (n=248 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Boundary Waters good for sun in May?

Not usually. Boundary Waters is historically overcast in May: only 10% of days are clear (avg high 64°F, n=248 days, ERA5 2018–2025). July is historically far better for sun here.

All percentages are historical averages from the ERA5 reanalysis via Open-Meteo, 8 full years (2018–2025). “Clear” means avg high ≥ 72 °F, precipitation < 1 mm, and a non-precipitation weather code. This is history, not a forecast. The live 7-day outlook for any destination is on its own page and may differ from the long-run average. Data from Open-Meteo.