Eclipse temperature drop, Texas eyes “Harvey”

Free AC. Good sleeping weather. September preview. Trot out your favorite weather cliche. Minnesota enjoys fresh Canadian breezes this week. Yes, it's a 'windows open' forecast.

Temperatures fall during eclipse

We're still getting some great data and insight in from Monday's eclipse. At a time of day when temperatures typically rise toward daily maximums, temperatures dropped 2 to 5 degrees around Minnesota during peak eclipse. The Minnesota DNR Climate Working Group adds some detail.

Despite the less-than-ideal visual conditions, the eclipse had a notable effect on Minnesota's weather. On a typical day, the sunlight passes directly to the earth, but any clouds present will filter some of that sunlight. During the eclipse, however, Minnesota had 70-90% of the direct sunlight blocked by the moon, before that light even reached the clouds, resulting in far less than usual mid-day heating at the surface. As a result, locations with cloudy and clear skies alike saw temperatures drop between noon and 2 PM. At Rochester, skies were clear through the majority of the eclipse, and temperatures fell by five degrees in just two hours.

Still some smoke aloft

It was nice to see a bluer sky today. Still, there is some wildfire smoke drift over Minnesota courtesy of western USA and Canaidan fires.

Free AC

We continue to enjoy free AC this week in Minnesota. Northwest flow will keep daytime maximum temperatures in the 70s. Nights cool into the 50s south, with 40s and even a few pockets of 30s possible up north.

Good sleeping weather indeed.

Ideal human comfort

Northwest flow across Minnesota this time of year brings comfortable dew points in the 40s and 50s. That's ideal for most people. A minor Clipper rides southeast Wednesday night into Thursday morning with a few spotty light showers. NOAA's NAM 3 km model catches the northwest drift.

Temperatures still run in the lower 70s in the Twin Cities.

That's about 5-10 degrees cooler than average the next few days.

Warm Labor Day weekend?

This weekend looks cool and potentially wet. The upper air pattern seems to favor warmer temperatures across the Upper Midwest by Labor Day weekend.

High way back into the 80s looks likely with some humidity. A shot at 90 in southern Minnesota seems plausible the first week of September.

State Fair weather

I'm looking forward to hosting Thursday's live broadcast of MPR's Climate Cast at the Great Minnesota Get Together. This is a great chance for us to talk about all things climate science in person. I'll be taking questions from you at the MPR  booth near Judson and Nelson. I'll also ask some 'climate quiz' questions, and I may have a few prizes for those of you who participate. I hope you can join us between 3 and 6 pm Thursday.

What's the range in weather conditions at the Fair? The Minnesota DNR Climate Working Group put together this great summary of historical State Fair weather.

Temperature

There can be some spells of hot weather during the Minnesota State Fair. The hottest day in the history of the Minnesota State Fair was on September 10, 1931 with 104 degrees. The hottest average temperature for the duration of any State Fair back to 1885 is also 1931 with 92.6 degrees. Note that the Minnesota State Fair in 1931 ran eight days from September 5-12. The 2013 Fair was the third warmest on record with 88.2 degrees and also had the most 90-degree high temperatures on record with six days. 2012 had the sixth warmest average maximum temperature with 87.1 degrees. The coolest Minnesota State Fair was during the six-day run of the Fair from September 5-10 1898 with an average maximum temperature of 64.2 degrees. The coldest maximum temperature for the Fair is 52 degrees on September 7, 1911, and the coldest minimum temperature is 33 degrees on September 13, 1890. The coolest Fair morning in recent years was a chilly 36 degrees on September 1, 1974.

Precipitation

On average it rains about three to four days during the Fair's 12-day run. The wettest Fair was in 1977 with 9.48 inches, and the driest Fair was 2003 with only .02 inch of rain.

The largest rain event in the State Fair's history was August 30, 1977. At 8:20pm heavy rains hit the State Fairgrounds. The U of M St. Paul Campus climate observatory one mile north of the Fairgrounds reported 4.06 inches of rain. This caused some of the worst street flooding seen at the Fairgrounds. The bulk of the rain fell in a 3 1/2 hour period from 8:15pm to 11:45pm. The grandstand show was cancelled, and people had great difficulty trying to leave the Fair. The Twin Cities International Airport saw 7.28 inches from this event, second all-time only to the July 1987 "Superstorm" total of 10.00 inches. People driving on I-94 leaving the Fair found water "up to their hood ornaments" in low areas under bridges.

Severe Storms

The peak of severe weather in the Twin Cities happens in June, but severe storms can and have happened during the State Fair. As recently as 2007, just two weeks before the Fair was to start, a thunderstorm with high winds estimated to 67 mph hit the fairgrounds. Below is an excerpt from Storm Data on August 11, 2007.

A long and wide swath of damaging wind extended from just west of the Minnesota State Fairgrounds in Falcon Heights to the northeastern corner of the city of St Paul. Approximately 4000 trees were felled. Affected neighborhoods were St. Anthony, Como and Phalen. The worst damage was in the Como Park area, where many trees fell on houses, vehicles, sheds and garages. One tree crashed into a house and destroyed it. One man was injured when a window blew in on him. At the State Fair, part of the grandstand roof was torn off, and roofs were also torn off some exhibition buildings. Dozens of vendor booths were blown around, with many severely damaged. The damage was oriented from west to east. At one point shortly after the storm, Xcel Energy reported over 250,000 outages across the Minneapolis-St. Paul region.

Texas: Friday tropical threat

Most of the major models are still painting a tropical threat to Texas by Friday. What once was Harvey is emerging over the Caribbean once again and showing signs of life.

Conditions in the Gulf are favorable for development in the Gulf. The system appears likely to head toward the Texas coast by Friday. Here's the latest from NOAA's National Hurricane Center. They give the system a 90% chance of developing into a tropical cyclone. The hurricane force wind, storm surge, and heavy rainfall threat is growing for Texas.

Stay tuned.