Minnesota ‘derecho alley?’ Stormy pattern continues
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Severe Sunday
Sunday's wild severe storm event looked like an alien mushroom cloud over Minnesota from 23,000 miles up in space. The National Oceanic and Atmospheric Administration's GOES 16 captured the massive storm top as it rolled east.
Bow echo
Here's the radar shot of the classic bow echo racing eastward Sunday morning. When you see this bowed-shaped bulge on your favorite radar app, expect damaging winds and hail.
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Probable derecho event
Sunday's damaging wind event will likely qualify as a derecho. Wind damage reports poured into NOAA's Storm Prediction Center from South Dakota to Michigan.
The damage swath ran 500 to 800 miles, well above the 250-mile derecho criteria. How do you say derecho? Here's more on derechos from NOAA.
Definition of a derecho
A derecho (pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") is a widespread, long-lived wind storm. Derechos are associated with bands of rapidly moving showers or thunderstorms variously known as bow echoes, squall lines, or quasi-linear convective systems.
"Derecho" is a Spanish word meaning "direct" or "straight ahead." (Click here to hear a pronounciation of the word "derecho"). In contrast, the word "tornado" is thought by some, including Hinrichs, to have been derived from the Spanish word "tornar," which means "to turn." Because "derecho" is of Spanish origin, the plural form is spelled "derechos;" i.e., the letter "e" is not added after the letter "o."
Although a derecho can produce destruction similar to that of a tornado, the damage typically occurs in one direction along a relatively straight path. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage. By definition, if the swath of wind damage extends for more than 250 miles (about 400 kilometers), includes wind gusts of at least 58 mph (93 km/h) along most of its length, and also includes several, well-separated 75 mph (121 km/h) or greater gusts, then the event may be classified as a derecho. Recently, an updated definition of "derecho" was proposed.
The primary aim of this work is to make the definition more physically-based, i.e. focused on the meteorological processes believed responsible for the production of organized, damaging surface winds. The proposal requires the presence of certain radar-observed storm structural features such as bow echoes and rear-inflow jets (discussed below in Derecho-producing storms and Derecho development). The length requirement in the proposed definition was increased to 650 km (400 miles), while the requirement for specific observed wind gusts was dropped.
Anatomy of a derecho
Derechos are self-sustaining once they get established. The continuous supply of rain-cooled air (cold pool) rushing to the ground from aloft keeps the momentum moving forward. Warm humid air feeds the front of the storm. Jet stream winds aloft create updrafts and drive the storm forward.
Minnesota: Derecho alley?
A look at climatology of derechos shows Minnesota may be in what we could call "derecho alley." Some studies show Minnesota is in a frequency maximum for moderate to high intensity derecho events. It makes meteorological sense. We spend our summers in Minnesota riding the northern edge of intense summer heat and humidity, with frequent cold air intrusions from Canada.
The derecho frequency maximum runs along the Interstate 94 corridor from near the Twin Cities to Chicago and Indy.
The climatology of MH derecho occurrence in the United States is shown in Figure 2. The highest annual frequencies of occurrence appear along the "Corn Belt," from Minnesota and Iowa into western Pennsylvania, and in the south central states, from eastern parts of the southern Plains into the lower Mississippi Valley.
However, the frequencies vary by season. During the warm season (May through August), MH derecho events are most frequent in the western part of the Corn Belt (Fig. 3). During the remainder of the year (September through April), the maximum frequencies shift south into the lower Mississippi Valley (Fig. 4).
Stormy pattern continues
Storms favor the Interstate 94 corridor today. There is a slight risk for severe storms in southern Minnesota today and a marginal (lower) risk in the Twin Cities.
A better chance for spotty coverage scattered storms approaches the Twin Cities late tonight. Here's NOAA's High-Resolution Rapid Refresh model through Monday night.
Tuesday: Enhanced severe risk
The highest risk for severe storms this week looks to be Tuesday night. NOAA's SPC paints a higher "enhanced risk" over western Minnesota and a slight risk for the Twin Cities.
Tuesday night bow echo?
We're not out of the severe woods just yet. NOAA's North American Mesoscale Forecast System model is picking up on the idea of another bow echo racing across Minnesota approaching the Twin Cities toward midnight Tuesday night.
Super soakers
Patchy rainfall is common in summer convective storms. Look for more localized soaking rains this week. Here's NOAA's NAM model rainfall output.
Feels like summer
We stay warm and sticky through Wednesday. Cooler breezes arrives starting Thursday into next week.
Keep the weather radio handy through midweek.