Tropical heat now, flash flood threat Wednesday night

So, this is what Miami feels like?

Another tropical air mass has invaded Minnesota this summer. Climate watchers in Minnesota like Mark Seeley report a growing number of "high dew point episodes" as our climate warms.

A 1 degree Celsius warmer atmosphere can hold about 4 percent more water vapor. Throw in a late summer "corn sweat" and you can turbocharge dew points across Minnesota in a hurry.

The National Oceanic and Atmospheric Administration's NAM 4 km model captures the sea of high dew point air washing over Minnesota this week.

Fuel for heavy rains

As you walk outside and swim through the sticky air mass, think of all that moisture as a sponge that's been left to soak in a bucket of water for a few minutes. As you lift it out of the water, it's already dripping. Give it a good squeeze, and the water gushes out.

Our drippy air mass over Minnesota is about to get a good squeeze as the next low pressure system moves in from the Dakotas over the next 48 hours.

Heavy storms likely, precise timing uncertain

Timing summer convection is a little like predicting when your mostly dependable, but time-challenged friend will pick you up. You know he's coming, but not precisely when.

Looking at the maps, my best read on timing thunderstorm chances over the next 48 hours is to increase the threat overnight into the Dakotas, possible into western Minnesota Wednesday morning.

Heavy storms in the Twin Cities appear to be most likely overnight Wednesday night, into Thursday morning. A second wave of storms may fire as the cool front works through eastern Minnesota Thursday afternoon and evening.

NOAA's NAM 4 km resolution model captures the essence of multiple thunderstorms waves firing in the Dakotas and racing east over the next 48 hours.

The Twin Cities National Weather Service does a nice job of laying out the eastward progression of the storm risk.

Flash flood risk?

What ingredients have to come together for multi-inch, flash flooding rains? Dew points in the 70s? Check. A stalled frontal system? Yup. A slow moving low pressure system? Roger that.

The models continue to paint a multi-inch rainfall bull's eye over central Minnesota this week. I could easily see some 3- to 6-inch rainfall totals where storms decide to stall, especially Wednesday night and Thursday morning.

The highs risk zone for flash flooding looks to be across central Minnesota, but the Twin Cities could come into play for potentially flooding rains.

NOAA's three-day rainfall map stamps a 5-plus inches rainfall bull's-eye close to Brainerd, Minn.

These sort of "smoothed" forecast maps look reasonable in theory. But the reality of localized summer convective towers is they dump heavy rains on smaller areas.

Again, NOAA's NAM 4 km model captures the essence of more localized heavy rain zones. The takeaway? Many spots will likely pick up 2- to 5-inch rainfall totals this week.

Mega-Flood potential?

We've already had another 1,000-year flood event in Minnesota this summer. Rainfall of 7-plus inches spiked rivers and took out roads from the Brainerd Lakes area through Hinckley, Minn., into northwest Wisconsin.

The anatomy of these "super-rain" events that produce 7- to 10-inch-plus rainfall totals is rare, but they are increasing in frequency across Minnesota. The granddaddy of them all may be the July 23-24, 1987 superstorm. Over 11 inches of rain fell in the southwest Twin Cities, damaging some 7,000 homes to the tune of $25-million.

The rainfall map below is an eye test, but gives you the idea of how much of the Twin Cities picked up 6 inches-plus rainfall totals that night.

The 1987 flood event was likely a 1 in 1,000 to 1 in 2,000 year event. Enough rain fell during that week to fill Lake Calhoun 84 times.

I don't know if we'll see a superstorm type event this week, but the atmosphere is plenty juicy for extreme rainfall.