A sultry Monday turns stormy this evening, overnight

Temperatures in the 90s and dew points climbing well into the 70s certainly qualifies as sultry in my book.

So far this summer has been pleasantly non-sultry. The Twin Cities have had just one 90-degree day prior to today, and the big moisture plumes from the south have not been reaching us for the most part.

Dew point is quite non-linear in terms of moisture content. That is, for every twenty degrees that the dew point temperature increases, the amount of moisture in the atmosphere doubles. So each degree that the dew point increases requires more additional moisture to get there than did the previous degree.

Dew points of 80 degrees or more are not common in Minnesota, but they do occur occasionally. The record dew point for the Twin Cities is 82 degrees set on July 19, 2011. That day already was extremely muggy before an afternoon thunderstorm came through. The rain evaporated from the ground and pushed the dew point to the record level at 3 p.m. and 4 p.m. Data indicate that it actually went above 82 between hours, but only hourly values traditionally are used for dew point records, so the record was posted as 82 out of operational fairness.

That same day, the all-time record dew point for Minnesota of 88 degrees was set in Moorhead.  Although the weather instruments were in a soybean field during a period of rapid crop growth, and therefore moisture transpiration from the plants, those conditions are typical of that area and the reading was considered valid.

Severe weather is ongoing in North Dakota and is likely in Minnesota this evening and overnight. The convective outlook has expanded the moderate risk area over northern Minnesota a bit.

The type of storm likely to cross the state is called a derecho. It is a broad, fast-moving, long-lived line of severe storms. They often produce long, wide paths of straight-line wind damage.

These storms can persist for quite a few hours and can cross several states. A famous derecho in these parts was the so-called I-94 Storm of July 19, 1983. This storm developed over North Dakota, turned toward the right and shot down Interstate 94 through Alexandria and the Twin Cities on its was to Chicago before collapsing in northern Indiana and southwestern Michigan.

Derechos develop as a line of thunderstorms. As the storms mature, rain falls and cools the air below, which supports the gust front and allows the storm to continue to grow and propagate. Often a rear-inflow jet (dashed brown arrow) tilts the storm backward. This allows the rain to fall farther back in the storm rather than down through, and therefore weakening, the updraft.

As a result of these and other processes involved, the gust front accelerates, which forces more warm, moist air upward, and the storms keep regenerating until they run out of fuel (moisture) or dynamic support.

Keep abreast of the weather coming from the west or northwest this evening and overnight, especially in the  northern half of Minnesota from the Red River to Bemidji, Brainerd and Mille Lacs east to Duluth. The National Weather Service issues warnings that can be picked up by a National Oceanic and Atmospheric Administration Weather Radio, computer or smart phone.

The southern end of the squall line is likely to reach the Twin Cities metro area around midnight tonight unless the storms slip by just to the north.

The National Weather Service has extended the Excessive Heat Warning for the metro area until 7 a.m. Tuesday as temperatures and dew points will remain elevated overnight. While the temperature will warm to about seasonal normals on Tuesday, it will be cooler than today and the dew point will slide downward during the day.