Friday storm chances, early August heat wave?

I'm watching some changes in the atmosphere that suggest we may be entering a new phase of summer across Minnesota. There are hints the heart of summer and summer's peak heat could be ahead in the next few weeks. The maps continue to hint that a possible early August heat wave may be brewing.

I'm also keeping one eye on the potential for another round of strong-to-severe Friday storms across Minnesota.

First we enjoy a beautiful summer night and two more blissfully quiet summer days this week.

Summer Glory Index

Yes, it's been a delightful first half of summer for most Minnesotans. But all weather is local. This summer has not been so glorious recently to Brainerd Lakes and metro residents that lost trees and power for several days in the past two weeks.

With that necessary perspective, the Summer Glory Index is an interesting way to tally up this summer's cumulative glory courtesy of the Minnesota DNR Climate Working Group.

More details on the first half of summer of 2015 from the Minnesota DNR.

If the 2015 Summer has seemed unusually pleasant, that’s because it has been!

Here we are at summer’s half-way point, and we have seen no major heat waves, no consistently unbearable humidity levels, and we have not had much in the way of widespread excessive rainfall. Indeed, we appear to be in the midst of a relatively comfortable, inoffensive summer.

So, how unusual is this predominantly tolerable summer weather, and is there a way to quantify and rank it?

The all-new Summer Glory Index (SGI) can help us answer these questions. Any day from June through August can earn up to 40 Glory points for being within “ideal” temperature, dew point, and precipitation ranges, and can lose points for being too hot, cold, wet, or humid. A given year’s SGI is then the sum of all the points earned and lost, and tells us how frequently we’ve experienced glorious conditions. Like its counterpart, the Winter Misery Index, the SGI allows us to compare individual summers to the historical record.

The index is based on measurements at MSP, which has a long digital record of dew point going back to the early 20th century.

Through July 15, this summer has been the third most glorious on record. Only 1922 and 2008 were better. Our current high ranking should come as no surprise: to date, Minneapolis has only topped 90 degrees once, has not recorded a minimum temperature of 70 or greater, has only had one 6 PM dew point reading above 70 degrees, and has had just two calendar days with over an inch of rain.

Of course, there is no guarantee that the second half of summer will match the first half’s splendor. Nine of the top-10 summers up through July 15th slipped to between 19th and 69th for the second half of the season, and the summer of 1940, after a stellar first half, had one most miserable second halves on record. Stay tuned to find out how this currently-spectacular summer finishes.

Two more blissful days

We've earned the respite. The break from storms and tropical humidity has given AC units a much needed rest. You'll notice heat and humidity gradually climbing again over the next 48 hours as a warm front works north through Minnesota ahead of the next low pressure system crawling out of the Rockies by Friday.

Wednesday and Thursday look generally summery and storm-free, with the exception of a few isolated thunderstorms that could bubble up along the advancing warm front. By Friday, the inbound low begins to work on an increasingly humid and unstable air mass. Dew points push toward the stifling 70 degree mark once again. Scattered storms brew across Minnesota again by Friday.

Early indications support the notion of more strong to severe storms Friday. Timing is still unclear, but another round of strong Friday night storms is possible. NOAA's Day 4 convective outlook paints a 15 percent chance for severe storms across eastern Minnesota including the Twin Cities Friday.

North Shore up-welling

Lake Superior water temperatures have warmed nicely in recent weeks. My MPR News colleague Cathy Wurzer reports near-shore temperatures in the mid 60s near Two Harbors last weekend. Monday's strong westerly winds changed that in a hurry. The strong offshore surface winds pushed that warmer water out to sea and along the Wisconsin shore. In its place, much colder water from depth rises up to replace the departing surface water. This process is called upwelling.

The result is water temperatures plunging into the frigid 40s along the North Shore.

Early August heat wave ahead?

The relatively chilly water of Lake Superior may be increasingly tempting in the coming weeks.

NOAA's GFS model has hinted at a possible early August heat wave now for a few days. It's still too early to spout a high level of confidence on this one nearly two weeks out, but the chances for the hottest stretch of weather this summer are increasing for early August. Keep in mind these long-range forecasts show more skill with temperature trends than day-to-day precipitation or variation.

Stay tuned and keep the AC button handy.