Derecho, severe weather pattern now over the heart of the Great Lakes, Michigan

Over the next few weeks Michigan will be positioned in the severe thunderstorm zone of the current upper-air pattern. Here’s what that means for us here in Michigan.

First, before we look at the expected weather pattern let’s step back and just look at the time of year. We are in July which is the hottest time of the year with the most pent up energy in the atmosphere. This energy turns into thunderstorms. If we look back through the decades much of Michigan’s severe thunderstorm activity has occurred in June and July.

We are in the heart of what typically is Michigan’s severe thunderstorm season. We also know that Michigan’s weather can be much different from normal. We can have all of our severe weather in spring and fall, and have a quiet summer.

This year it appears as though we will be in what I would call a “textbook severe weather pattern” for the Great Lakes region. The textbook pattern features a hot dome of air just to our south in the Ohio Valley and west to the Midwest. On the north side of the hot dome is usually a faster band of upper-level winds. This is what we call the jetstream. I always tell you the jetstream is the engine for severe thunderstorms. The faster the winds are blowing aloft the stronger and more severe thunderstorms can be under the jetstream.

Here’s an animation of the upper-air forecast over the next two weeks. This is the forecast for around 15,000 feet up in the atmosphere. It’s the steering current for severe thunderstorm complexes. You see the pronounced hot dome to our south and west. You also see the thin black lines sort of tightly packed together. The upper wind blows along these black lines. Remember the closer the black lines the stronger the wind aloft. The other very important thing to see is these small “U” shaped indentations in the upper-air flow. Each U shaped bend in the flow is a storm system.

We meteorologists look for what we call “jetstreaks” in the jetstream. A jetstreak is an elliptical shaped patch of faster winds. You’ll notice below in the jetstream forecast there will be three or four of these jetstreaks blowing across the Great Lakes region in the next two weeks.

What does this all mean for our day-to-day weather here in Michigan? It means a few things. First it means we will have mostly dry weather with an organized clump of thunderstorms every two or three days. These clumps of storms move fast in this type of weather pattern. Each clump of storms may only storm on your house for an hour or less. If we do simple math it means at your house you may only have four or five hours of rain in the next two weeks with each stormy period coming in one-hour periods.

The two wind forecasts are somewhat abstract. Below is a forecast you can definitely understand- the six hour rain amounts forecast. It gives us a great picture of the clumps of storms coming at us from the west.

Any one of these storm clumps could produce severe thunderstorms. We will know a day in advance if the wind driving the next storm batch is anything extraordinary. If we see 100 mph winds aloft in one of these disturbances you will see a moderate to high risk of severe thunderstorms.

This pattern tends to yield storms starting in the Dakotas and moving east into Michigan within a day. We often see the clump of storms organize into a severe squall line of storms as they move across Minnesota, Wisconsin and Michigan. If it’s a long lasting line of severe storms we call it a derecho. The National Weather Service officially defines a derecho as, “If the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph or greater along most of its length, then the event may be classified as a derecho.”

Our next chance of a line of severe thunderstorms with the next disturbance actually comes tomorrow, July 19. That will be the next clump of thunderstorms I track with you. Keep updated on Saturday’s severe weather chance at mlive.com/weather.

Right now I see three of these storm systems to affect Michigan: tomorrow, next Wednesday and next Sunday. With the fast speed of these systems that timing could change by a day, but you get the idea.

Enjoy solid summer weather in the 95 percent dry times and be ready to take cover during the few hours when lines of thunderstorms that blow through Michigan.