Cooler with a few minor rain chances; why aurora forecasting is a challenge

It’s a cooler day Tuesday behind Monday’s cold front with highs mainly in the 70s. We have a few minor rain chances ahead and the possibility of auroras in northern Minnesota.

Cooler Tuesday after a hot Monday

Monday brought our 14th day of 90-degree temperatures in the Twin Cities. We topped out at 93 degrees, our second hottest day this year. Many areas across southern Minnesota hit 90 or hotter ahead of a cool front.

That cool front touched off some severe thunderstorms that mainly produced large hail. Medford, about an hour’s drive south of the Twin Cities, had golf ball-sized hail while several other spots saw hail 1 inch to 1.5 inches in size.

The storms didn’t yield much rainfall for central Minnesota. Storms developed right on top of the south metro and then pushed southeast. Some localized spots saw up to 1 inch or more of rainfall in southeastern Minnesota.

Tuesday will be a cooler day behind that front with highs mainly in the 70s statewide. Dew points are also much drier and more comfortable, in the 40s, which is pretty dry for July.

A few rain chances

The next system is moving out of the Dakotas toward Minnesota and will increase the cloud cover Tuesday afternoon.

Southwestern Minnesota will see a few light showers by evening and those showers will spread across southern Minnesota late Tuesday night into Wednesday morning with some isolated showers north Wednesday afternoon.

Beyond that, a couple more minor disturbances will create some shower chances.

Unfortunately, most of the rainfall will not add up to much over the next several days.

Aurora chatter

You may have heard some rumblings of possible auroras coming up midweek. There was a coronal mass ejection, which is blast of energized particles from the sun that then heads toward earth, early Monday morning.

Usually, within about 72 hours this reaches the vicinity of earth, and if things align just right with our magnetic field and orbit, northern lights, or auroras occur. The more energy, the better the show and the farther south it’s visible.

We measure this by the KP index. For it to be visible in southern Minnesota we need a KP index of 5 to 7, a pretty energetic display. Far northern Minnesota can often see the lights with a KP index of 4. KP is derived from the German “planetarische Kennziffer” meaning planetary index.

Articles the last couple days overstated the aurora possibility for Wednesday and Thursday based on a 27-day forecast which looks more at the solar cycle rather than a real forecast of solar activity.

The reality is that auroras are not easy to forecast at all, and usually only within a 72-hour window, and still there are often surprises.

NASA and the National Oceanic and Atmospheric Administration have sophisticated models to predict where these coronal mass ejections will go and how they might react with earth.

NOAA models have this activity missing earth but NASA’s give us a glancing blow, which could mean some activity Wednesday night.

The official forecast from NOAA’s Space Weather Prediction Center calls for minor to moderate activity with a KP index nearing 4 Wednesday night.

This would make it visible in places like the Boundary Waters Canoe Area Wilderness.

So for most of Minnesota don’t count on seeing the lights, but it’s possible we could have a nice surprise but we’ll also need the weather to cooperate. Right now it looks like we’ll have scattered clouds but not totally overcast skies Wednesday night.