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A Flush of September Warmth and High Dew Points

A Flush of September Warmth and High Dew Points:

The trend toward above normal temperatures this month, which began on the 10th, continues in a big way through this weekend. Most observers now report September mean temperatures that are 2 to 4 degrees F warmer than normal, but have yet to see real extremes. This will change for Friday through Sunday as several areas of the state will see daytime highs soar into the 80s and 90s F, with nighttime lows in the 60s and 70s F. As a result, it is likely that some new daily temperature records will be set in terms of both warm daytime highs and warm nighttime lows. So far the highest temperature measured this month was 93 degrees F on the 14th at Browns Valley (Traverse County). This is likely to be surpassed.

According to the Minnesota State Climatology Office it is especially unusual to see a series of nighttime low temperatures in the 70s F so late in the month of September. In fact for the Twin Cities climate record consecutive nightly lows in the 70F during the month of September is rare indeed, having happened only in 1897, 1927, 1939, 1948, and 1955. Should nighttime lows remain in the 70s F both Friday and Saturday (Sept 22-23) it will be the latest ever such occurrence.

In addition, a significant feature of this September warm spell is the high dew points. A new daily high dew point record was set in the Twin Cities on September 21 (Thursday) this week with a reading of 70 degrees F at MSP Airport. As a result a Heat Advisory was released for many portions of Minnesota on Friday, September 22nd. Many new daily high dew point records are likely to be set on Friday and Saturday too, as dew points spike into the 70s F. Already a new record high dew point was set at 8am on Friday morning at MSP with a reading of 71 degrees F, while the record for Saturday, September 23 is 73 degrees F from 1945 and this may be threatened this weekend.

Weekly Weather Potpourri:

This week NOAA's Tom Di Liberto provides an analysis of the impacts of Hurricane Harvey on the Houston, TX area. It is an interesting article about the meteorology of the storm, but also the societal impacts.

The United Kingdom Met Office is celebrating 100 years of numerical weather forecasting this week. They trace the beginnings of their numerical weather forecasting back to the WWI era when Lewis Fry Richardson began to use spatial grids of meteorological measurements to calculate changes in forecast parameters such as temperature and moisture. What took him weeks to do back then, can be done by computers in less than one second today.

Earlier this week NASA reported that the extent of end-of-summer Arctic Sea Ice was the 8th lowest in the satellite record period going back to 1978. Arctic sea ice reached its annual minimum areal extent on September 13th, and is now starting to build again with the approach of the Northern Hemisphere winter season.

MPR listener question:

With this expected Heat Spell, what is the warmest overnight low temperature during September ever observed in Minnesota?


In the Twin Cities climate record the warmest overnight low in the month of September was 77 degrees F on September 5, 1912. On a statewide basis the warmest September night was September 11, 1931 when the overnight low was 85 degrees F at Bird Island (Renville County). BTW that day was probably the warmest September day in Minnesota history with over 100 communities reporting afternoon highs in the 90s F and many places in western Minnesota well over 100 degrees F.

Twin Cities Almanac for September 22nd:

The average MSP high temperature for this date is 69 degrees F (plus or minus 10 degrees F standard deviation), while the average low is 49 degrees F (plus or minus 8 degrees F standard deviation).

MSP Local Records for September 22nd:

MSP records for this date include: highest daily maximum temperature of 95 degrees F in 1936; lowest daily maximum temperature of 49 degrees F in 1913 and 1983; lowest daily minimum temperature of 26 degrees F in 1974, 2007, and 2011; highest daily minimum temperature of 71 degrees F in 1937; record precipitation of 2.80 inches in 1895. No snow has fallen on this date in the Twin Cities.

Average dew point for September 22nd is 45°F; the maximum dew point on this date is 70°F in 1903; and the minimum dew point on this date is 20°F in 1974.

All-time state records for September 22nd:

The all-time state high temperature for today's date is 101 degrees F at Ada (Norman County) in 1936; the all-time state low for today's date is 10 degrees F at Thorhult (Beltrami County) in 1974. The all-time state record precipitation for this date is 4.84 inches at Cambridge (Isanti County) in 1968. Record snowfall is 2.0 inches at Baudette (Lake of the Woods County) in 1995.

Past Weather Features:

Strong thunderstorms brought three to six inches of rain to southern and central Minnesota over September 21-22, 1895. There were also many reports of hail. Fortunately crops had matured early that year and much of the harvest had already been completed. Widespread frost followed the thunderstorms.

By far the warmest September 22 in state history was in 1931 when over 40 Minnesota communities reported daytime highs in the 90s F. At seven locations the temperature reached 100 degrees F or greater.

The coldest September 22nd in state history was in 1974 when frost occurred in every corner of the state. Over two dozen climate stations in northern and western Minnesota reported morning lows in the teens F, and at Thorhult (Beltrami County) it was a nippy 10 degrees F. The high at Brainerd only rose to 41 degrees F.

An early autumn snow storm brought measurable amounts to western and northern parts of the state over September 22, 1995. Traces of snow were reported as far south as Luverne (Rock County), while in the north an inch or two was measured with temperatures in the upper 20s and low 30s F.

Strong and persistent thunderstorms brought record-setting rains to portions of southern Minnesota over September 22-23, 2010. Most southern Minnesota counties received over 6 inches of rain and parts of Faribault County received over 11 inches. Numerous roads were closed to flash flooding, and a week later the Mississippi River rose above flood stage.


Warm and humid over the weekend with chances for showers and thunderstorms. Continued chance for showers through early next week, but temperatures will decline near seasonal averages. Drier toward the end of next week.

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