First metro frost coming 11 days later in past decade?

It’s another blissfully mild late September day in Minnesota.

Blue skies, mild afternoon temps and balmy breezes. Any day now the weather will turn for the colder, and we’ll wake up to frosty lawns and rooftops in the inner metro core right?

Frosty spruce. Image: Paul Huttner – MPR News


It turns out there’s a noticeable shift toward later fall ‘first frost’ dates in the metro in the past decade. I did some digging today looking back through the past decade at dates for the 1st fall season’s 32-degree temperature at Minneapolis-St. Paul International Airport.

These numbers caught my eye.

Oct. 7 — the average date of the first 32 degree temp at MSP Airport. (Data since 1948)

Oct. 18 — Actual average date for the season’s first 32 degree temp at MSP Airport since 2004

11 days later — shift in average first frost date at MSP Airport since 2004

10 years — last time MSP recorded an earlier than average frost (28 degrees on October 2nd, 2003)

13 years — last time MSP recorded frost in September ( 31 degrees on September 24, 2000)

No September frost at MSP in 13 years?

That’s remarkable considering the earliest 32-degree temperature on record is Sept. 3, 1964. The trend of noticeably later frosts is now a decade long. In any normal universe you would expect several years of ‘earlier than average’ first frost dates in the past 10 years.

To me the data clearly support a trend toward later first frost dates over the past decade. That fits well with the overall warming patterns observed in Minnesota.

Forecast 2013: Frost free for now

So when will we see the season’s first frost at MSP in 2013?

So far this month, frost/freeze conditions have been common in northeast Minnesota. Temps in the 30s and patchy frost has even nipped at the far north part of the Twin Cities metro area along Interstate 35.

Here’s a look at the coldest temps observed so far this month from the Minnesota Climate Working Group.

Image: Minnesota Climate Working Group

For now, the weather maps still show a mild bias through the first week of October. The Global Forecast System is hinting at a cool morning around October 5th. If we don’t dip to 32 that day, the overall chances of a frost in the inner metro core (I-494/I-694 loop) before the long term average of Oct. 7 look slim.

Image: NOAA GFS via IPS Meteostar

After that there are signs of a push of cooler Canadian air mass, and better chances for frost closer into the metro around Oct. 10 or 11.

GFS: Colder high pressure overhead on October 10th & 11th? Image: NOAA

The Weather Lab roses are safe for now.

Image: Paul Huttner MPR News

Stay tuned.


  • Gordon near Two Harbors

    It would be interesting to compare data from climate stations outside of the immediate urban core to see if this trend is occurring there too. I suspect that the lack of early fall sea ice in the arctic is delaying the development of colder air masses until later in the season.

  • Fred Garvin

    This is great news–a longer growing period in MN means greater diversity of crops and higher yields.

  • Fred Garvin

    “The trend of noticeably later frosts is now a decade long.”
    A 10 year “trend”? Are you SERIOUS?
    In which branch of enviromental science does 10 years of data support a “trend”?
    Only if one wishes to gin up a ALARMING CLIMATE WARMING/CHANGE NEWS HEADLINE!!!!

  • Starquest

    More GFS flogging without any disclaimer. At about 10 days is where climatology becomes more accurate than forecasts.

    The GFS says a low of 27 on 10/11? That would be 14 below the average low of 41. My money is on the temp being closer to 41 than 27.