Warm, sticky weather will continue

Thunderstorms with heavy rain have been plodding eastward across the northern half of Wisconsin today. The effect of these "training" storms, following each other like train cars, coupled with the heavy rain leads to the possibility of flash flooding.

Sept 2 - ltng 2
Cloud-to-ground lightning indicating clusters of thunderstorms over Wisconsin and into Michigan this afternoon. Vaisala

The muggy air has engulfed just about all of Minnesota. Areas way up north that still enjoyed dew points in the 40s on Tuesday have readings that have climbed into the 60s and even a few low 70s this afternoon. Expect these conditions to continue into the weekend along with highs mostly in the 80s to some low 90s.

Warm nighttime temperatures

In addition to having toasty afternoons, this week is featuring exceptionally warm overnight low temperatures for September. The Twin Cities had low temperatures of 69 the past two mornings when the climatological average would be 58. Certainly, these elevated low temperatures are partly the result of a very warm air mass over the state. Some nocturnal clouds provide a bit of a blanket as well.

Political Coverage Powered by You

Your gift today creates a more connected Minnesota. MPR News is your trusted resource for election coverage, reporting and breaking news. With your support, MPR News brings accessible, courageous journalism and authentic conversation to everyone - free of paywalls and barriers. Your gift makes a difference.

But there is a third factor at work. Surface temperatures usually cool at night because the Earth radiates long wave infrared (IR) energy out to space. However, water vapor is not transparent to IR radiation. The water vapor absorbs some of the IR and then re-radiates it in all directions. While some of that energy does get out to space, some of it is radiated back to Earth where it slows the surface cooling process. Having large amounts of water vapor in the air, as we have this week, can cause much IR to be absorbed and significantly slow the nighttime temperature drop.

Conversely, very dry areas such as deserts with scant water vapor in the atmosphere to intercept the IR frequently cool very quickly after sunset. Campers in these dry, warm-in-the-daytime places often fail to anticipate the need for an evening sweatshirt.

Upcoming weather

Thursday looks like a quiet but warm and muggy day.

By Friday we should begin to see a change approach from the west. A developing weather system is likely to spread thunderstorms from the Dakotas into northwestern Minnesota. The Storm Prediction Center has posted a marginal risk area for severe thunderstorms for that corner of the state for Friday and Friday night.

September 2 - svr day 3
Severe storms might spread from North Dakota into northwestern Minnesota on Friday

The weekend pattern continues to be rather uncertain as far as who will get thunderstorms and when that will occur. As of now, it appears that there will be scattered thunderstorms over western, northern and maybe central Minnesota on Saturday. The chance for storms will increase for the metro area and southeastern Minnesota on Sunday, especially late in the day. Some storms could be strong and there is a good chance of heavy rain, but no severe weather risk areas have been issued so far.

A guaranteed forecast is that next week will be much cooler and less humid than this week. Many high temperatures will be in the 60s by Wednesday.