Forest Lake ‘gustnado?’ Summery weekend

Severe storms blew through the north metro in the 2 a.m. hour early Wednesday morning. The storms generated a wake of wind damage reports.

National Weather Service survey teams assessed storm damage in the north metro Wednesday. What they found is evidence of a tornado. The EF-1 twister was on the ground for more than eight miles, packing peak winds of 90 mph.

The twister downed trees and heavily damaged this garage near Columbus.

Possible gustnado?

As good as Doppler radar is these days, smaller tornadoes can be tough to pick out. The radar loop from this storm shows an advancing gust front, with a spin up along the leading edge. This is likely what meteorologists call a "gustnado."

Gustnadoes develop along the leading edge or in wake flows of the gust front. They are usually short lived but can still do considerable damage. They are different from classic tornadoes that typically form in the rotating "mesocyclone" region of tornadic supercells.

Here's the definition from the AMS Glossary of Meteorology.

gustnado

Colloquial expression for a short-lived, shallow, generally weak, vertically oriented vortex found along a gust front. Gustnadoes are usually visualized by a rotating dust or debris cloud.

The Wiki description of a gustnado elaborates further.

A gustnado is a short-lived, shallow surface-based vortex which forms within the downburst emanating from a thunderstorm. The name is a portmanteau of "gust fronttornado", as gustnadoes form due to non-tornadic straight-line wind features in the downdraft (outflow), specifically within the gust front of strong thunderstorms. Gustnadoes tend to be noticed when the vortices loft sufficient debris or form condensation cloud to be visible although it is the wind that makes the gustnado, similarly to tornadoes. As these eddies very rarely connect from the surface to the cloud base, they are very rarely considered as tornadoes. The gustnado has little in common with tornadoes structurally or dynamically in regard to vertical development, intensity, longevity, or formative process --as classic tornadoes are associated with mesocyclones within the inflow (updraft) of the storm, not the outflow.[2][3]

In spite of the different physical processes between tornadoes and gustnadoes, gustnado damage effectively looks the same as tornado damage. It's simply storm semantics when the trees in your back yard look like this.

Summery weekend

Summer weather returns this weekend. Saturday feels warm and humid. A weak cool front slides south Saturday evening, and Sunday will feel cooler and more comfortable.

Low storm risk Saturday

The cool front may trigger a few spotty storms as it advances south late Saturday. The best chance for development is south and east of the Twin Cities Saturday evening.  The National Oceanic and Atmospheric Administration's NAM 3 km resolution model depicts storms firing near Rochester, Minn., and moving southeast.

There is a marginal to slight risk of severe storms for Wisconsin and southeast Minnesota Saturday as highs push 90 degrees. Sunday looks pleasantly cooler and less humid. Warmth and humidity return north Monday and Tuesday.

Keep one eye on the Doppler late Saturday afternoon.