Nocturnal thunderstorm, derecho severe weather threat this weekend

Welcome to the nocturnal thunderstorm season in Minnesota.

Mid- to late July is prime time for overnight storms that can turn severe in Minnesota and the Upper Midwest. Potent warm fronts ride north into Minnesota in July. That means plenty of warm, humid air near the surface. Longer nights help cool the atmosphere aloft.

That creates an unstable situation as steamy air near the ground rises quickly into the colder air aloft. The result is severe storms that can race across the Upper Midwest on July nights.

Volatile setup

The weather maps this weekend look favorable for strong to severe storms to develop and sustain their intensity through the nighttime hours. Intense heat and humidity push north behind a steamy warm front Friday night into Saturday morning. A pool of dew points in the 70s pushes north Friday night to fuel the developing storms.

A strong southerly low-level jet stream around 5,000 feet above ground will feed developing storms and sustain the low-level inflow as the upper winds push them eastward. This warm moist low-level inflow is like the storm’s oxygen supply.

Overnight threat

NOAA’s NAM 3 km resolution model depicts intense storms in the Dakotas around 7 p.m. Friday night racing east into Minnesota. Note the bow-shaped morphology of the squall lines pushing east overnight. This is usually a damaging wind signature on the storms leading edge.

In this model forecast scenario, they reach eastern Minnesota and the Twin Cities around 5 a.m. Saturday.

Severe risk zone

The highest severe risk favors western Minnesota late Friday night. But if the storms sustain intensity overnight, we could see strong to damaging wind storms blow through the populous Twin Cities in the early hours Saturday morning.

The potential is there for another strong to severe line of storms Saturday night into early Sunday as cold front slices into Saturday’s hot, unstable air mass.

Possible derecho

If the storms are well organized and sustained, they may produce damaging winds along the leading edge over long distances. In that event, we may have a derecho on our hands Friday night into Saturday morning.

Here’s much more on derechos from NOAA.

What is a derecho?
A derecho (pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") is a widespread, long-lived wind storm associated with a band of rapidly moving showers or thunderstorms variously known as a squall line, bow echo, or quasi-linear convective system. Although a derecho can produce destruction similar to that of a tornado, the damage typically occurs in one direction along a relatively straight swath. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage.

How strong are derecho winds?
By definition, a derecho must include wind gusts of at least 58 mph (50 knots or 93 km/h) or greater along most of its length. While derecho winds typically are less than 100 mph, gusts as high as 130 mph have been recorded --- equivalent to those with strong EF2 tornadoes.

What are the dimensions of a derecho?
A derecho wind damage swath must extend more than 250 miles (about 400 kilometers). This criterion is used to eliminate more common, shorter-lived, and generally less-organized wind-producing convective systems. Although no formally-recognized width criterion exists for a derecho, the nature of the storm systems responsible for their development dictates that most derechos span at least 50 miles (80 km) in width.

What is the origin of the term "derecho?"
The word "derecho" was coined by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper published in the American Meteorological Journal in 1888. "Derecho" is a Spanish word meaning "direct" or "straight ahead;" Hinrichs coined it to distinguish straight-line wind damage from that produced by tornadoes. While the term was used in the meteorological community for a short time during the late nineteenth century, it disappeared from use for nearly 100 years until resurrected by severe weather meteorologists in the mid-1980s.

Be ready for possible severe nocturnal thunderstorm overnight this weekend.