Hot and smoky, weekend thunder threat. La Nina winter ahead?

Welcome to July. In September.

Minnesota's unseasonably warm September weather continues for another day. We're riding the northern tip of a July-like air mass, as colder air paints strange shades of blue into the northern Rockies.

Many spots in southern Minnesota logged our 11th 90-degree day of the year including the Twin Cities. We enjoy one more mid-summer-like day Friday, then the cold front arrives. Yes, Minnesota weather fans, we're easing into that season when the term cold front will get a lot of action in the next few months.

Air quality alert continues Friday

White is the new blue in Minnesota skies this year. The latest smoke plume from western fires wafts overhead again today.

Air quality alert Friday

The unseasonably warm weather and smoke are a good recipe for ground level ozone and particulates. That's why the Minnesota Pollution Control Agency has issued another air quality alert for Friday.

Ground level ozone

Wait, I though the ozone layer was a good thing? It is. In the upper atmosphere. But when pollutants at ground level react with sunlight, ground level ozone can be unhealthy for us.

Here's more detail on ozone from the Minnesota Pollution Control Agency.

Ozone can be good or bad, depending on where in the atmosphere it’s found. You’ve probably heard of the “ozone layer” in the upper atmosphere — up there, ozone is a good thing, as it protects us from harmful ultraviolet (UV) radiation from the sun. But down here, where we live and breathe, it’s called ground-level ozone. And having too much of it can be harmful to human health.

Ozone is a gas composed of three atoms of oxygen (O3). In the upper atmosphere it comes from natural and man-made sources. The ground-level type we’re concerned about is not emitted directly, but is created when oxides of nitrogen (NOx) and volatile organic compounds (VOC) cook in sunlight.

To help distinguish “bad” ozone from “good” ozone, remember: “Good up high, bad nearby.” Today’s Air Quality Index is in the orange zone, meaning the air is unhealthy for sensitive groups.

Ozone is most likely to reach today’s levels on hot sunny days in urban environments, but can still be high during colder months. It can also be transported long distances by wind, so even rural areas can experience high ozone levels. People most at risk from elevated ozone levels include those with asthma, children, the elderly, and anyone working or playing hard outdoors. To protect yourself, stay indoors with air conditioning if possible, and if you need to be outside, take it easy. If you experience respiratory symptoms, use your inhalers as directed and contact your health care provider. To learn more about ozone, visit https://www.pca.state.mn.us/air/ozone.

Doubly whammy: Wildfire smoke mixes down

Throw in another aggressive wildfire season out west and you add smoke particulates to our dirty air quality cocktail. Here's more on our increasing wildfire trends as climate warms from a woman who fought on the fire-line.

Thunder threat increases

A cold front advances into our warm humid air mass as we approach the weekend. Northern Minnesota sees the first waves of storms. Southern Minnesota and the Twin Cities looks ripe for two waves of thunderstorms. The first likely arrives Friday night after midnight. The second waves develop Saturday afternoon.

Here's the timing on storm waves according to NOAA's North American Mesoscale Forecast System 3 km resolution model.

Marginal severe risk

There is a low chance that a few storms could pack some damaging winds and hail Friday night and again Saturday. Keep the weather radio handy.

Temperature correction

Saturday's cool front brings a fresher but cooler air mass Sunday and Monday. We warm up again next week.

La Nina watch

The National Oceanic and Atmospheric Administration has pulled the trigger on a La Nina watch as waters cool in the tropical Pacific.

Seasonal jet stream trends in La Nina winters favor colder than average temps in the northern United States.

About 70 to 80-percent winters skew statistically toward colder than average temperatures overall In Minnesota and the Upper Midwest. There’s also a slight bias in La Nina winters toward higher than average snowfall.

Stay tuned.