Friday, September 21, 2018 7:00 AM

Yo-Yo-ing Towards Cooler Weather Today and Tomorrow

Severe Weather Hit South Central and East Central Minnesota, Among Other Places

It was a busy day for thunderstorms in Minnesota, northern Iowa, Wisconsin, and eastern South Dakota. The afternoon and early evening batch of thunderstorms produced widespread wind damage across south central and southeastern Minnesota with large trees down from east of Mankato through Waseca, Faribault (including planes being flipped at the Faribault Airport), Farmington to Red Wing. There was another cluster of wind damage near Fairmont. One tornado was confirmed near Grenada with other possible tornadoes near Elysian and Randolph. On top of that, there were huge rainfall totals between Wednesday night and Thursday. Parts of northern Iowa and the Sioux Falls area saw 3-7 inches of rain. Southwestern Minnesota got enough rain to flood some small streams and creeks. And, Cambridge (over 4 inches of rain) and parts of the Twin Cities saw some flooding, including near I-35W south of Minneapolis.

The thunderstorms that hit St. Cloud were relatively tamer, but there were 0.78 inch of rain recorded at the Airport and 0.89 inch since Tuesday.

The cause of the storms was the same old front with steamy air to the south (temperatures again climbing into the 90's in most of Iowa). Yesterday, however, the strong low pressure system pushing across the Dakotas (see Mid-Tropospheric water vapor loop from Colorado State slider satellite menu) helped to strength the circulation around the front, increasing the flow of sticky air above the cooler air at the ground.

Clouds, Wind, Sprinkles to Produce Chill Today

This morning, the deepest thunderstorms have now pushed into Wisconsin, Illinois, and Michigan (see NWS Upper Mississippi Valley radar loop) as the cold front has pushed through Minnesota for good (see NWS WPC Latest North American zoom-in surface map). Northwest winds are bringing in cooler air. Temperatures are still in the middle 50's in central Minnesota (see UCAR Minnesota surface chart), but are in the lower 40's in northwestern Minnesota and North Dakota. The cooler air has a lot of lingering low clouds (white areas on the Shortwave Albedo from Colorado State slider satellite menu) back across most of North Dakota, as the upper-air low pressure system (counterclockwise swirl) still has to push through. That will take most of today, so expect a lot of clouds and a few stray showers or sprinkles. However, temperatures won't go much of anywhere with highs likely staying in the middle to upper 50's. The chill will be reinforced by a strong northwest wind, blowing at 15-25 MPH with gusts over 30 MPH. We might be able to see some sunny breaks by late in the day.

Small Chance for Frost by Tomorrow Morning, More Pleasant Saturday

The October-ish weather will continue overnight. Skies will clear out around sunset, and the winds will ease off, allowing temperatures to fall back to near 40 or even the middle to upper 30's. I don't think there's a good chance for frost in central Minnesota tomorrow morning, but I can't completely rule it out. So, today would be a good day to cover any delicate plants. Tomorrow, we will have a lot more sunshine and temperatures will rebound to the more seasonable lower to middle 60's.

Back to the 70's Sunday, Next Rain Threat Monday?

The cool off will ease up by Sunday. A breeze and some middle clouds will keep temperatures in the upper 40's on Sunday morning and highs will rebound into the 70's. However, the next strong low pressure system is due to pull its cold front through Minnesota by late in the day on Monday. That will create another chance for widespread showers and perhaps some thunderstorms. The additional clouds may make it difficult for Monday's high to get out of the 60's.

Much Cooler Air Coming Late Next Week???

Slightly cooler air will work its way into Minnesota on Tuesday and Wednesday, but highs will likely be in the 60's. It is 6-8 days out, but the computer forecasts are showing a much stronger cold front coming through Minnesota on Thursday. If that happens, there would be the potential for much cooler air moving in next weekend. This would keep highs in the 50's and give central and southern Minnesota a chance for at least a frost and perhaps a hard freeze, if we get a clear and calm night. So, this weekend would be a good time to consider bringing those delicate plants inside. The average date for St. Cloud's first frost is September 26 with most years between September 19 and October 4. Since the last frost of the winter was April 28, among the earliest 20 percent of years, we've already had a prolonged growing season.

On the other hand, September 21 of 1995 is St. Cloud's earliest day for measurable snow. There was 0.2 inch on that day and the temperature never got warmer than 43 degrees. We're not going to get that cold.

Equinox Saturday, But Fall Has Been Under Way

Saturday also marks the date of the September equinox, the day that all points on the earth see 12 hours of daylight and 12 hours of darkness. Since meteorological fall are the three complete months between the summer and winter, fall began on September 1. So far, it hasn't felt that fall-ish (September temperature 5.4 degrees above average). And, yesterday didn't even pull that average down much, since the low temperature was so mild. However, we are going to see a fall towards normal as the seasonably cool weather becomes more frequent through next week.

Confidence Level: "The Bulb on That Hard-To-Reach Light in the Foyer Will Burn Out"

Friday 9/21/2018: Mostly cloudy, breezy, and cooler. Periods of light rain or sprinkles. Maybe a little late day sun. High: between 55 and 60. Winds: NW 10-25 MPH. Chance of measurable rainfall: 50%.

Friday Night: Clearing with diminishing wind and chilly. Low: between 37 and 42. Winds: light NW evening, SW 5-10 MPH late. Chance of measurable rainfall: 10%.

Saturday 9/22/2018: Sunny in the morning, partly sunny in the afternoon, breezy, and not quite as cool. High: between 60 and 66. Winds: SW 10-20 MPH. Chance of measurable rainfall: 10%.

Confidence Level: "Every Time I Try to Put in a New Bulb, the Light Fixture Will Try to Detach From the Ceiling"

Saturday Night: Partly clear, breezy, and not as cool. Low: between 45 and 50. Winds: SE 10-20 MPH. Chance of measurable rainfall: 10%.

Sunday 9/23/2018: Sunny, breezy, and seasonably warm. High: between 70 and 75. Winds: SE 15-25 MPH. Chance of measurable rainfall: 10%.

Sunday Night: Partly clear, breezy, and noticeably humid. Low: between 55 and 60. Winds: S 15-25 MPH. Chance of measurable rainfall: 10%.

Confidence Level: "The Only Way I'll Be Able to Change the Bulb and Hold the Fixture in Place Will Be to Yell At It"

Monday 9/24/2018: Clouding up with a good chance of showers and thunderstorms. High: between 60 and 68. Winds: SW 10-20 MPH, becoming NW late. Chance of measurable rainfall: 50%.

Monday Night: Partial clearing, breezy, and cooler. Low: between 43 and 50. Winds: W-NW 10-20 MPH. Chance of measurable rainfall: 20%.

Tuesday 9/25/2018: Morning sun, afternoon clouds with a chance of a shower or thunderstorm, breezy, and seasonably cool. High: between 60 and 68. Winds: W 10-20 MPH. Chance of measurable rainfall: 40%.

Extended: More persistently cool late next week?? Frost potential??

Forecast Confidence (10 - "The Rabbits Will Thump, Objecting to Me Yelling at the Light Bulb"; 0 - "I Will Be Hosting a DIY Show on Home Repairs"): 6 Monday through Wednesday, 5 Wednesday night and Thursday, 3 Thursday night and Friday, 2 Saturday.

Yesterday's High: 61°F; Overnight Low (through 7 AM Friday): 50°F
St. Cloud Airport 24 Hour Precipitation (through 7 AM Friday): 0.79 inch (0.90 inch since Tuesday); SCSU Precipitation (through 7 AM Friday): 0.95 inch (1.09 inch since Tuesday)

Next Update: Monday, September 24, 2018 8 AM

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Let me know what you think about this forecast and discussion by emailing SCSU meteorology professor Bob Weisman. Please note that I make the forecast, not the weather!

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