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If you’re a weather enthusiast, you’ve probably heard chatter about the potential for some kind of a major severe weather outbreak in the central U.S. next week. We need to start any discussion of a forecast almost a week away with what the models are good at and what they’re not.

Computer models are pretty darn accurate seven days out with general patterns, trends and storm systems, but less so with the exact track of a narrow snowfall or specific temperatures. And everything is contingent on timing. 

A hurricane forecast 72 hours out is more accurate now than a 24-hour forecast was in the early 1980s. The window of accuracy has definitely increased, but specifics are still murky past several days. 

Keep in mind that what separates severe thunderstorms from an average thunderstorm is organization. Organizing a storm into a supercell or a derecho requires several components to come together perfectly and at the right time. More on that below.

Historic events are defined by standard deviations

One way to look at the potential severity or historic nature of an event/occurrence is by looking through the lens of deviation, or how far from "normal" the forecast conditions are. 

For example, the average high temperature in the Twin Cities on April 6, using 1991-2020 climate data, is about 52 degrees. However, the standard deviation (basically the normal range) is plus or minus about 12 degrees. That means anything between 40 and 64 is within the normal range. 

From that we can calculate probabilities. Sixty-eight percent of the time the high temperature on April 6 in the Twin Cities will be between 40 and 64 degrees, but 32% of the time it could be colder or warmer and we can measure that by a number of deviations. 

This exercise creates a normalized or standardized anomaly. Instead of saying the high was 12 degrees above normal (64 degrees when the normal high is 52), we can say it was 1 standard deviation above normal, which is pretty well within an expected range. 

Two standard deviations is getting crazier, like a 75-degree high when the normal is 52. It's unusual but it happens. Three standard deviations (86 or 87 degrees when the normal is 52) is rare, like once-in-a-generation rare. 

You need to understand deviations in order to understand the context for when a storm system qualifies as historic. 

Which leads us to the hype about next week. 

4 measurements fueling hype for next week

Keep in mind that these are projections for a week from now (April 13), so you cannot fully trust the numbers yet. They serve as indicators of potential, not hard facts and we've seen time and time again hype about storm systems a week away turn into a major bust. 

Knowing that, let's use four measurements from the April 6 (early morning) European weather model to see what it's forecasting for next Wednesday, April 13. 

  1. How low the pressure aloft could be (a big pattern view)
  2. How low the pressure at the surface could be (a developing storm system measurement)
  3. How much moisture could be around (essential for severe storms)
  4. How warm temperatures could get (the heat or energy available)

Temperatures are forecast to be about 10 to 20 degrees above normal over southeast Minnesota, eastern Iowa, southern Wisconsin and northern Illinois. These translate to 1 to 2 standard deviations above normal, which is unusually warm but not unheard of.

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The amount of moisture, or precipitable water (total water in the atmospheric column available to produce clouds and rainfall) is forecast at 3 to 4 standard deviations. That's rare moisture that far north, this early in the season. It's something on the order of a 4% probability. 

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The combo of above normal warmth and above normal moisture is why the model is forecasting above normal instability potential, or C.A.P.E. (convective available potential energy = i.e. thunderstorm energy).

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The surface pressure is forecast to be unusually low, about 2 to 3 standard deviations lower than normal for the date. Basically, a 4% to 27% probability (a once-every-23-years occurrence, to once every four years).

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The model is also forecasting a very deep upper-level low (very cold aloft, which creates instability), something on the order of 3 to 4 standard deviations below normal. Again, that would be very rare for the date.

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When we look at all of those rare or unusual ingredients (for the date) coming together, we can guess that something pretty rare or unusual (for the date and location that far north) could result. 

Of course, we need all of those things to come together with our typical severe weather ingredients: shear, conditional instability, and timing. We also need to remember that this is the model’s forecast of those conditions, and the model could easily change its forecast. 

Next week's potential for severe weather isn’t something that the average person should be on high alert for, but in our modern, social media-driven world, it’s possible you’re seeing it discussed and it might be causing anxiety or even panic. 

For now, it is something for meteorologists to keep an eye on. As usual, stay tuned. 

How could next week's system affect Minnesota?

Per usual this time of year, Minnesota lives along the battle zone of cold and warm air, so the question will continue to be how far north severe weather could occur. 

Below is a sampling of model scenarios, including where each model is projecting the warm front to be. The American (GFS) and Canadian (GEM) models have the warm front further north. Warm fronts are often involved in the best tornado scenarios in Minnesota, compared to cold fronts and dry lines farther south in the Plains. 

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The warm front can be thought of as the northernmost extent of possible severe weather. It's also worth noting that at this early point, the Storm Prediction Center (SPC) is focused farther south, and understandably so. 

Meanwhile, on the northern side of the storm, there could be an area of heavy snow. 

Here's the Day 7 severe weather outlook area being monitored by the SPC. 

day7prob

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