Flash flood potential, excessive heat warning

This is turning out to be the "Severe Summer of 2016" across Minnesota.

Excessive heat and moisture surge north across Minnesota the next 72 hours. The inbound air mass is loaded with tropical energy capable of producing flash flood-triggering heavy downpours as the warm front oozes north tonight.

Thunderstorms Tuesday night and early Wednesday will likely deliver torrential rainfall rates of 2 to 3 inches per hour where they set up. That's enough to cause flash flooding in a hurry, especially late tonight into Wednesday.

Then, jungle-like heat and humidity surge into Minnesota Wednesday through Friday. Temperatures in the 90s Wednesday through Friday peak near 100 degrees in Minnesota Thursday.

What next? Locusts?

Flash flood watch tonight and Wednesday morning

Let's start with the heavy rain threat. A flash flood watch has been issued for late tonight.

Thunderstorms tonight and early Wednesday will likely deliver torrential rainfall rates of 2 to 3 inches per hour where they set up. That's enough to cause flash flooding in a hurry, especially late tonight into Wednesday.

Spotty thunderstorms rumble across southern Minnesota today. Summer's strongest warm front so far is capable of triggering an outbreak of what we meteorologists refer to as "nocturnal thunderstorms" overnight and early Wednesday.

The National Oceanic and Atmospheric Administration's NAM 4 km resolution model captures the essence of spotty thunderstorms today, then a wave of active heavy cells developing overnight into early tomorrow morning.

The sweet spot for heavy rainfall producing thunderstorms late tonight runs ahead of the advancing warm front, from the Twin Cities south to the Interstate 90 corridor and along the Mississippi River toward La Crosse, Wis.

https://twitter.com/NWSTwinCities/status/755356364936019968

Nocturnal thunderstorms

We're now entering the peak of the so-called nocturnal thunderstorm season in late July and August. Warm fronts bubble north with moisture and heat near the ground. Increasingly longer nights allow the air aloft to cool. That creates an unstable air mass, capable of spawning intense thunderstorms overnight.

I wrote this about nocturnal thunderstorms in July of 2012.

Nocturnal storms like to form and multiply at night and in the wee hours of the morning. Heat trapped near the surface this time of year and an active low level jet stream racing about 5,000 feet above us combined with an increasingly cooler upper atmosphere due to longer nights can create a warm bubble of air that likes to rise.

That’s what we call an “unstable situation” in meteorology.

Here’s an interesting excerpt on nocturnal thunderstorms form a great overall paper on thunderstorm development.

Low level jet streams

• Nocturnal (non-frontal) thunderstorms in the Midwest are typically associated with a low level jetstream oriented N-S or NE-SW.

• Boundary layer low-level jets are frequently observed at night along the western portions of the Great Plains with maximum winds often exceeding 25 m/s within 500m of the ground. A strong west to east horizontal pressure gradient along the lee of the Rocky Mountains with a sustained flow of air northward from the Gulf of Mexico generally occurs with this phenomena. While the presence of a low-level jet is not required for thunderstorms to occur, the downstream convergence associated with the jet core explains the increased potential for nocturnal thunderstorm development. This low-level jetstream has been described as an atmospheric analog to the Gulf Stream current and its diurnal variation is not well understood.

Throw in a slow moving front on the edge of the heat dome to the south and you have the perfect recipe for nighttime storms to fire around the “ring of fire.”

Heat wave kicks in

Temperatures push 90 degrees this afternoon in the Twin Cities and southern Minnesota. The core of the intense heat arrives Wednesday, peaks Thursday and lingers Friday. The Twin Cities National Weather Service has issued an excessive heat warning from Wednesday through Friday.

INCLUDING THE CITIES OF...MORRIS...GLENWOOD...ST CLOUD...ELK RIVER...MADISON...BENSON...MONTEVIDEO...WILLMAR...LITCHFIELD...MONTICELLO...MINNEAPOLIS...BLAINE...ST PAUL...STILLWATER...GRANITE FALLS...OLIVIA...HUTCHINSON...GAYLORD...

CHASKA...SHAKOPEE...HASTINGS...REDWOOD FALLS...NEW ULM...ST PETER...LE SUEUR...ST JAMES...MANKATO

400 AM CDT TUE JUL 19 2016

...EXCESSIVE HEAT WARNING IN EFFECT FROM 1 PM WEDNESDAY TO 7 PM CDT FRIDAY...

THE NATIONAL WEATHER SERVICE IN TWIN CITIES/CHANHASSEN HAS ISSUED AN EXCESSIVE HEAT WARNING...WHICH IS IN EFFECT FROM 1 PM WEDNESDAY TO 7 PM CDT FRIDAY. THE EXCESSIVE HEAT WATCH IS NO LONGER IN EFFECT.

* TEMPERATURES: HIGHS IN THE MID 90S AND LOWS IN THE MID 70S...TO POSSIBLY LOWER 80S IN THE TWIN CITIES METRO AREA.

* DEW POINTS: 75 TO 80 DEGREES...LOCALLY HIGHER.

* HEAT INDICES: WIDESPREAD 100 TO 110 DEGREES...LOCALLY HIGHER.

* IMPACTS: A HEIGHTENED RISK OF HEAT RELATED ILLNESS...ESPECIALLY FOR THOSE ACTIVE OUTDOORS OR FOR THOSE WITHOUT AIR CONDITIONING.

* TEMPERATURE...HIGHS IN THE MID TO UPPER 90S AND LOWS IN THE MID TO UPPER 70S...TO POSSIBLY LOWER 80S IN THE TWIN CITIES METRO AREA.

* IMPACTS...A HEIGHTENED RISK OF HEAT RELATED ILLNESS...ESPECIALLY FOR THOSE ACTIVE OUTDOORS OR FOR THOSE WITHOUT AIR CONDITIONING.

Thursday: Hottest day of 2016?

Thursday looks ghastly hot, I see a real chance of 100 degrees in the Twin Cities and across most of central and southern Minnesota. With air temperatures pushing 100 degrees, and dew points in the 70s, heat index or 'feels-like' temperatures in the Twin Cities could reach 110 to 113 degrees late Thursday afternoon.

Seriously dangerous, wilting heat indeed.

A wet heat

The real debilitating factor in this heat wave will be incredibly high dew points. Dew points hit the tropical to unbearable 75-plus degree threshold late Wednesday and Thursday, and a few locations have a shot at a jungle-like 80-degree dew point Thursday.

80-degree lows?

Temperatures may not dip much below 80 degrees Wednesday and Thursday night in many locations.

While working at WGN-TV I covered the deadly Chicago heat wave in 1995 that killed over 700 people.

One thing we found in the after action report I was part of with the City of Chicago, overnight lows above 80 degrees were a real factor in the increased death toll. People without AC just didn't have a chance to cool their bodies at night. The effects of heat are cumulative.

If you don't have AC, seriously consider getting to a cooling center like a library or other building. Take a cool shower if you can. Give your skin temperatures a chance to cool off. Know the signs of heat-related illness.

Heat Wave History for the Twin Cities:

• The most recent 100 degree day was on July 4 and July 6 of 2012.

• The record high temperature is 108 degrees, measured on July 14, 1936.

• The record dew point is 82 degrees, recorded on July 19, 2011.

• The record heat index is 117, which occurred on July 11, 1966.

Hot, but not even close to 1936

Some relief arrives this weekend as temperatures ease back into the 80s across Minnesota. This three to four day heat wave will be hot, but nothing like the intense and prolonged 1936 heat wave that produced 14 consecutive days above 90 and 6 days well above 100 degrees.

My parents tell me how they and hundreds of people slept by the shores of Lake Calhoun to try and escape the heat.

For some perspective, here's more on the 1936 heat event from the Minnesota Climatology Working Group.

Hitting 100 degrees is not a common occurrence for the Twin Cities metro area and happens in about one in five years. The most recent 100 degree readings occurred on July 4th and 6th of 2012.

The hottest temperatures in the Twin Cities occurred in July 1936, during the Dust Bowl era. An all-time record of 108 degrees was observed on July 14, 1936. A high of 106 was recorded for each day from July 10th through the 12th in July 1936.

In total an impressive 8 days in July 1936 were above 100 degrees, with another day in August also above 100, making for a record 9 days with temperatures at or above the century mark. July 1936 is also the hottest month ever recorded in the Twin Cities, with an average temperature at a very warm 81.4 degrees.

In addition to the heat, only 0.11 inches of precipitation fell, making this the driest July as well. The Dust Bowl Era produced many record hot temperatures in the Twin Cities, including the only 100 degree day observed in May.

80 degree dew point temperatures are extremely rare in the Twin Cities. The record maximum dew point temperature for the Twin Cities is 82 degrees, recorded on July 19, 2011. The old record was 81 degrees, on July 30, 1999.

The summer of 2011 was extraordinary humid, with dew points reaching or exceeding 75 degrees for 103 hours ending on August 29th. The 1981-2010 average for hours recording dew point temperatures above 75 degrees is only 18.

Stay cool this week. And check on your relatives, neighbors and friends who may not have access to AC.