Saturday, April 26, 2025  3:00 AM 

Prepared by Bob Weisman, SCSU Meteorologist and Professor Emeritus

Late Night Storms Tonight and Tomorrow Night, Monday PM Storms With Severe Threat Possible

Both Regimes for Showers and Thunderstorms in the Forecast

As we get into the thunderstorm season, let's talk a bit about the different regimes for thunderstorms. All thunderstorms develop when there is a large temperature difference between either the ground or just above the ground and the middle atmosphere. That temperature difference is usually about 80 degrees. The typical cause of this temperature difference we learn about in middle school or in a Skywarn class is from heating during the day. Severe weather forecasters will typically mention how important getting sunshine is to develop these storms that develop during the late afternoon or early evening. These storms are typically the ones that create the overwhelming majority of the storms producing large hail and the wider, wedge-shaped tornadoes, which both are the most common in the late afternoon or early evening. While a slow-moving system can trigger these storms, they are typically seen in Minnesota when a cold front is due to pass. That included the Wadena and western Minnesota tornadoes in 2010.

However, there is a large portion of the country that sees middle of the night through early morning thunderstorms, "the things that go bump in the night." In these cases, there aren't very many thunderstorms in the warm sector of a storm near the cold front. They are blocked by a plume of very warm air at 5,000 to 10,000 feet above the ground, so even daytime heating can't generate a big enough temperature difference. So, the very warm and humid air at the ground streams northward until it hits the warm front. When this steamy air moves that far northward, it gets out from under the plume of warm air, allowing the needed temperature difference between the warm air, now above the ground, and the middle atmosphere. These storms tend to peak late at night because the air flow in the warm air after it is lifted can be faster, since it is no longer scraping against the ground, where it is slowed by friction. The Plains states, especially the Dakotas, western and central Nebraska and Kansas, tend to get more thunderstorms overnight than during the afternoon. In Minnesota, there is a split with western Minnesota often seeing the leftovers of the overnight storms, while late day showers and thunderstorms happen more often in eastern Minnesota. Still, we do get some of these nighttime and early morning storms. The July 4, 1999 Boundary Waters blowdown storm, which toppled an estimated two and a half million trees in the BWCA. The storm first became severe at 6 AM with a 90 MPH wind gust in Fargo. The storm did most of its BWCA damage during the late morning and the noon hour. The primary severe weather from this system was straight line damaging winds, but these overnight storms can also produce some damaging hail and also can create flooding rainfall.

Overnight-Early AM Storms Tomorrow and Monday

Minnesota will have a chance for both storm regimes during our shower and thunderstorm chances. There will be a good chance for the overnight and morning storms both late tonight into tomorrow morning and again late Sunday night into Monday morning. The warm plume of air aloft I mentioned in yesterday's forecast is forecast to be weaker on Monday morning, so there will be at least a 50-50 shot for these storms. As the higher moisture air streams northward from the Texas Panhandle and Oklahoma (see yellow and brown areas on the UCAR hourly dewpoint map), the Sunday night-Monday morning storms will have a better moisture supply, so I see heavier rainfall (compare Days 1 and 2 on the NWS WPC quantitative precipitation forecast) and some possibility for damaging hail from those early Monday storms.

Maybe Late Day Storms Monday

The cold front from this system, now approaching the California coast (see Pacific mid-tropospheric water vapor loop from Colorado State satellite slider), will be pulled through Minnesota during Monday evening. That will be the day with the best chance for late afternoon and evening thunderstorms. The other elements, strong change in wind in the lowest 5,000 feet, summer-like dew points in the 60's, and the potential for hot afternoon temperatures well into the 70's and even the 80's, for producing severe weather have a chance of happening. The most uncertain part is how much sunshine we and southern Minnesota get during Monday afternoon. The better chance for Monday morning storms has the potential to leave a lot of lingering clouds around northern and central Minnesota, so we may have trouble getting the warm front to go to our north on Monday afternoon. That could keep us out of the prime heating position. There are still hints that the late day storms may develop in southern and east central Minnesota to our east (now being forecast as category 3 of 5, including a tornado risk, by the NWS Storm Prediction Center), so the chance for St. Cloud severe weather is far from guaranteed. If the storms are to our east, we will have a good chance of seeing highs approach or top 80, but the chance for afternoon storms will be smaller. Still, you'll need to keep track of any National Weather Service watches and warnings on Monday, especially during the afternoon and evening hours. Most likely, the Monday afternoon and evening picture won't be clear until sometime in the middle of Monday morning

Even if we miss the Monday afternoon storms, there will be the chance for some light rain as the upper air low goes by Monday night. There will be more of that rain in northern Minnesota with a chance for a few late night snow showers. This is why northern Minnesota overall is forecast to see heavier precipitation than southern Minnesota on Monday (see Day 3 of the NWS WPC quantitative precipitation forecast). That's where it's needed the most.

Forecasting These Storms Is More Difficult Due to Missing NWS Upper Air

I can't leave this subject without ranting about the National Weather Service budget cuts, which have reduced the upper air network in the Central Plains (see absent observations in Denver, the Black Hills, and western Nebraska on the 7 PM upper air map from the University of Wyoming). This affects the forecasting of both types of storms. The missing upper air stations especially define whether or not there is a plume of warm air aloft strong enough to keep the cold front storms from developing until late afternoon. Having fewer storms develop allows each system to have more energy, which is typical of the supercell storms that produce most of the very large hail and wedge tornadoes. While some of these stations report at 7 AM, the computer forecast "remembers" the forecasts for the previous 12 hours in each run, so gives equal weighting to the evening observations with missing upper air stations as the morning observations.

This outage is especially acute for the overnight storms, since the region overlaps with the prime overnight thunderstorm area. Since the flow of low-level humid air often moves through the western High Plains, it's harder to track the fuel supply. Also, the missing observations include the state of the plume of hot air aloft and where the hot air air aloft cools enough to allow warm air lifted by the warm front to develop these strong thunderstorm complexes.

Beyond Monday, A Light Rain Threat Wednesday Night

Beyond Monday, we will have a better chance for sunshine on Tuesday with much drier and also cooler air. Highs will be back in the 50's. On Wednesday, we will get back into southeast flow and see warming back into the 60's. The next fast-moving storm system will push into Manitoba by Thursday morning, so another cold front will be due to pass on Wednesday evening. However, the warm air ahead of this system will be cooler and much drier than Monday's storm, so there will only be a shot at some light rain showers. There may also be some light rain showers the second half of Thursday with a pocket of cool air aloft. Warmer weather will return on Friday and Saturday, but it's too early to be sure if the warmth gets beyond the 60's.

Detailed St. Cloud, MN, and Vicinity Forecast

Confidence Level: "The Bugs Are Waiting for Me to Mow the Lawn"

Saturday 4/26/2025: Sunny in the morning, sunshine through high clouds during the afternoon, breezy, and a bit warmer. High: between 60 and 65. Winds: SE 5-15 MPH. Chance of measurable rainfall: 10%.

Saturday Night: Clouding up, breezy, and more humid. A chance for a late night shower or thunderstorm. Milder. Low: between 42 and 47. Winds: SE 8-15 MPH. Chance of measurable rainfall: 50%.

Sunday 4/27/2025: A chance for morning showers and thunderstorms. Some sunny breaks possible during the afternoon. Breezy and more humid during the afternoon. A slight chance for a late day shower or thunderstorm. High: between 58 and 63. Winds: SE 10-20 MPH. Chance of measurable rainfall: 50% in the morning, 20% in the afternoon.

Confidence Level: "Extra Bugs Will Come From Every Home on the Block to Hover Around My Head"

Sunday Night: Partly clear, breezy, much warmer, and noticeably humid with a better chance for late night showers and thunderstorms. Some storms could produce large hail. Low: between 52 and 57. Winds: S-SE 8-15 MPH. Chance of measurable rainfall: 60%.

Confidence Level: "Enough Bugs Will Fly into My Mouth to Throw Off My Diet"

Monday 4/28/2025: Mixed clouds and sun, very warm, and uncomfortably humid with a chance of both early morning and late day showers and thunderstorms. Late day storms could produce severe weather. High: between 75 and 80. Winds: SE 15-25 MPH with gusts over 35 MPH morning and midday, shifting to SW 10-25 MPH during the afternoon. Chance of measurable rainfall: 50% during the morning, 40% late in the day.

Monday Night: A chance for evening showers and thunderstorms with severe storms possible, then cloudy, windy, and turning much cooler. Maybe a late night sprinkle. Low: between 37 and 42. Winds: NW 15-25 MPH with higher gusts. Chance of measurable rainfall: 50%.

Tuesday 4/29/2025: Sunny, windy through the morning, and much drier with merely seasonable warmth. High: between 54 and 58. Winds: NW 10-25 MPH with higher gusts during the morning, NE 5-10 during the afternoon. Chance of measurable rainfall: 10%.

Tuesday Night: Clear, light winds, and cooler. Perhaps some fog. Low: between 40 and 45. Winds: SE 5 MPH evening, 5-10 MPH late at night. Chance of measurable rainfall: 10%.

Wednesday 4/30/2025: Sunny in the morning, mixed sun and clouds during the afternoon, breezy, and warmer, but continued dry. High: between 62 and 67. Winds: SE 10-20 MPH. Chance of measurable rainfall: 10%.

Extended: Warmer Wednesday with next chance for showers and thunderstorms late in the day??? Few showers Thursday and a bit cooler??

Forecast Confidence (10 - "The Rabbits Will Thump Even Though I Fed Them"; 0 - "The Rabbits Will Offer To Groom My Bald Spot"): 8 Saturday, 6 Saturday night and Sunday, 5 Sunday night, 2 Monday, 3 Monday night, 4 Tuesday through Wednesday.

Yesterday's High: 62°F; Overnight Low (through 2 AM Saturday): 37°F
St. Cloud Airport 24-Hour Melted Precipitation (through 2 AM Saturday): Trace; SCSU 24-Hour Precipitation (through 2 AM Saturday): TBA

Next Update: Sunday, April 27, 2025 6 AM

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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