Spotty Rains Continue

Spotty Rains Continue:

Rains over the second week of September were welcome, but highly spotty around the state, with north central and northeastern areas receiving the most. A number of northern climate stations reported over 1 inch of rainfall, while many southern counties reported less than a quarter inch of rainfall.

A freakish, and thankfully relatively small strong thunderstorm passed across areas of Duluth, MN on Monday evening (September 11) dropping from 1 to 3 inches of rainfall in a very short period of time. The National Weather Service had to issue flash flood warning around 10:00 pm as torrents of water were pouring off the slopping hillsides of Duluth. I35 expressway through Duluth was closed for a time due to flooding. Some climate observers north and west of Duluth reported 3 to 4 inches of rainfall, and one observer reported over 6 inches of rain.

The spotty rains of last week did not alleviate the spread of drought across the state. According to the U.S. Drought Monitor the areas of Minnesota encompassed by Moderate Drought, Severe Drought, and Extreme Drought all expanded in size over the past week. On a more optimistic note, the NOAA Climate Prediction Center Outlooks for the second half of September suggest a wetter than normal period for most of the state.

A Change in the Wind:

Fall brings a change in prevailing winds across the state. The dominant wind directions in Minnesota are northwesterly, and south to southeasterly, except for Duluth and north shore areas which see a significant frequency of easterly winds during the summer months (cooler and heavier air moving off Lake Superior towards the warmer land). September and October are transition months, when the southerly wind components diminish in frequency and the northerly components increase in frequency until they become fully dominant for the November through March period.

An observable transition has been occurring already this month and will continue. As a result, sharp contrasts in overnight minimums are in evidence over the first two weeks of the month. With prevailing southerly winds during the first week of September, overnight minimum temperatures ranged from the mid 60s to low 70s F in many areas, while during the second week of September with prevailing northerly winds overnight minimum temperatures sunk into the 30s and 40s F, with cooler and drier air from higher latitude. As these wind directions oscillate in the autumn season, they tend to remind us of the balmy summer days of summer one day and then suddenly change and raise our anxiety about the impending season of long nights, short days, and “layered clothing.” Speaking of which, the autumn season is a great time to show off those fashionable sweaters that have been closeted for months.

Weekly Weather Potpourri:

The Minnesota Climate Adaptation Partnership (MCAP) will host a public webinar on September 20 from noon to 1pm. The topic will be “Climate Adaptation Menus for Maple Syrup” and the presenter, Scott Hershberger of the University of Wisconsin-Madison, will focus on adapting practices in maple syrup production to climate change across the western Great Lakes area. To register for this webinar please go to the MCAP web site.

With over 16 inches of rainfall reported from last weekend’s storm over Libya in north Africa there were two dam failures near the city of Derna. Widespread flash flooding caused the deaths of over 11,000 people and displacement of over 40,000 more. The Weather Underground web site reports more details on this catastrophic storm.

This week’s AGU-EOS Bulletin contains an interesting article which describes a study of the world’s hottest and coldest cities. Using a thermal comfort index (combination of temperature and humidity) researchers found that the ten hottest cities are in Bahrain, Pakistan, Qatar, and Saudi Arabia. The ten coldest cities were located in just two countries, China and Russia. On average Yakutsk, Russia is the coldest city with an average annual temperature of just under 18°F and an extreme low of -84°F.

MPR listener question:

As we have seen the drought situation across our state worsen so far this month, some of us were wondering if the Minnesota 2023 drought could be mitigated even if we had that wettest October month in history. If that even possible?

Answer:

The wettest October in state history was in 2010 when the statewide average rainfall was 6.41 inches, over twice normal. Such levels of rainfall would undoubtedly help the drought situation in our state, but not entirely alleviate it. Many areas of the state are from 7 to 12 inches below normal for precipitation so far this year. That is a huge deficit to make up in one month.

Twin Cities Almanac for September 15th:

The average MSP high temperature for this date is 74 degrees F (plus or minus 9 degrees F standard deviation), while the average low is 55 degrees F (plus or minus 7 degrees F standard deviation).

MSP Local Records for September 15th:

MSP records for this date: highest daily maximum temperature of 98 degrees F in 1939; lowest daily maximum temperature of 46 degrees F in 1916; lowest daily minimum temperature of 36 degrees F in 2011; highest daily minimum temperature of 72 degrees F in 1897; record precipitation of 2.59 inches in 1992. A trace of snowfall fell on this date in 1916.

Average dew point for September 15th is 48°F; the maximum dew point on this date is 72°F in 1994; and the minimum dew point on this date is 19 degrees F in 2011.

All-time state records for September 15th:

The state record high temperature for this date is 100 degrees F at St Peter (Nicollet County) in 1939 and again at Tracy (Lyon County) in 1955. The state record low temperature for this date is 17 degrees F at Bigfork (Itasca County) in 1964. The state record precipitation for this date is 4.98 inches at Winona Dam (Winona County) in 2004. The state snowfall record is 0.2 inches at Warroad (Roseau County) in 1916.

Past Weather:

On September 15, 1916 with morning low temperatures in the 30s F, at least two dozen climate stations in Minnesota reported traces of snowfall, including the Twin Cities. Warroad reported a measurable 0.2 inches for the earliest ever start to the measurable snow season there.

The hottest September 15 in history was in 1939 when over 50 climate stations reported afternoon high temperatures of 90°F or greater. In many areas of the state the overnight minimum temperatures remained in the mid to upper 70s F.

The coldest September 15 in state history was in 1964 when nearly all climate stations reported morning low temperatures in the 20s and 30s F. Bigfork in Itasca County reported a state record low for the date of just 17°F.

Slow-moving strong thunderstorms over September 14-15 of 2004 brought one of the worst flash floods to ever hit southern Minnesota. Six or more inches of rainfall fell over portions of Blue Earth, Dodge, Faribault, Fillmore, Freeborn, Houston, Jackson, Martin, Mower, Olmsted, Steele, Waseca, and Winona Counties, covering an area of over 4000 square miles. Up to 13 inches was reported in Alden Township of Freeborn County. Every watershed in the area flooded, including the Root, Cedar, Zumbro, and Upper Iowa Rivers of southern Minnesota. Over $6 million in property damage was reported along with $22 million in crop damage. Many roads were impassable, and hundreds of home basements flooded.

Outlook:

Sunny and pleasant autumn weather over the weekend and through Tuesday of next week. Warming temperatures next week to above normal values with increasing chances for showers by Wednesday and Thursday.