A classic summertime severe weather outbreak is likely to unfold over the next few days across the eastern half of the U.S., with each day seeing the potential for extensive straight-line wind damage, large hail, and some tornadoes. The storms could organize into a much-hyped feature known as a “derecho.”

The risk for dangerous thunderstorms will migrate southeastward each day through Tuesday, beginning in the Upper Midwest today, extending into the Lower Midwest and Ohio Valley by Monday. The severe storm risk shifts to the southeast states by Tuesday.

A large, well-defined upper-level ridge is in place over the Plains states, leading to a phenomenon known as the ring of fire. Locations under the ridge are very hot, while severe thunderstorms blow up in the extreme instability, wind shear, and moisture pooled-up along the outer edge of the ridge.

This is a classic setup for a dangerous phenomenon known as a derecho, which is a highly organized squall line that produces extensive wind damage over a 250-mile path. The complex is only classified a derecho if the length of its path of damage reaches that magic, arbitrary 250-mile marker; a storm that produces significant wind damage over a 230-mile path isn’t technically a derecho, but it’s dangerous just the same.

Just saying the term “derecho” in public is frowned upon in the weather world because of the amount of hype it can cause (and some technical reasons), but to be honest, saying “a derecho is possible” is a hell of a lot more effective in people taking the threat seriously than saying “Indiana could experience an intense mesoscale convective system on Monday night.” Responsible sources don’t throw around the term lightly, so take the threats seriously.

Beware Nighttime Storms

Severe thunderstorms are dangerous enough, but what’s most concerning is that the majority of the storms anticipated over the next couple of days will unfold overnight when people are tuned-out or asleep. Every year, people stumble to the nearest news camera and shout out “WE HAD NO WARNING” when their homes are demolished at 3:00 AM, and without fail, news organizations manage to spin “we had no warning” into a failure on the part of meteorologists instead of a failure on the part of residents in the path of the storms.

You need to be proactive about your own safety. Make sure you have a way to receive alerts both day and night, ensuring you can quickly act if dangerous storms threaten even if you’re dead asleep. Go to Walgreens or Walmart or Lowe’s or Home Depot and buy a weather radio that, when properly set-up, automatically activates a loud tone when a warning is issued for your county. The baseline models are $20-$30, but they’re well worth the cost. Email me if you can’t figure out how to set it up and I’ll gladly guide you through it.

Sunday, July 12

Risk Level: 4 out of 5, or a moderate risk.

Hazards: Powerful straight-line winds in excess of 70 MPH, hail the size of golf balls or larger, a couple of tornadoes.

Storm Mode: Supercells at first across Minnesota and Minneapolis—this is where the hail/tornado risk will come from—evolving into a powerful squall line that races east or southeast after nightfall.

Select Cities at Risk: Minneapolis, St. Paul, Rochester, Fargo, La Crosse, Madison, Davenport

Discussion: All of the ingredients are in place for a dangerous severe weather event this evening and overnight tonight in the Upper Midwest. Extreme instability, deep wind shear, a pool of very high dew points in the mid- to upper-70s, and a stationary front will all serve as the catalyst for what is likely going to be a pretty bad night for residents of parts of the Midwest.

The storms will likely start out as supercells in northern Minnesota and North Dakota, which will carry the greatest risk for hail larger than golf balls and a couple of tornadoes. As the evening wears on, however, the storms will coalesce into a mesoscale convective system (an organized squall line) and race east-southeastward along the stationary front at the surface. These lines of storms move quickly, so be prepared to take swift action tonight if you’re in or around the areas identified by the SPC at risk tonight.

Monday, July 13

Risk Level: 4 out of 5, or a moderate risk.

Hazards: Powerful straight-line winds, a few tornadoes, large hail.

Select Cities at Risk: Madison, Milwaukee, Chicago, Champaign, Terre Haute, Lafayette, Indianapolis, Cincinnati, Louisville.

Discussion: Monday has the potential to be the most dangerous of the three days. The risk on Monday will mirror today’s potential in the Upper Midwest, but the outcome is highly dependent on what happens on Monday morning. Any lingering convection or cloud cover will hinder the atmosphere’s ability to quickly recover and destabilize itself enough to support a robust second round of severe thunderstorms.

The Storm Prediction Center’s current thinking indicates that the worst storms will fire up in southern Wisconsin on Monday evening, organizing into a severe squall line as it shoots southeast through Chicago and makes its way through the areas under the moderate and enhanced risks.

Tuesday, July 14

Risk Level: 3 out of 5, or an enhanced risk.

Hazards: Straight-line wind damage, a couple of tornadoes, some large hail.

Select Cities at Risk: Chattanooga, Huntsville, Nashville, Knoxville, Asheville, northern suburbs of Atlanta

Discussion: I could really copy/paste the previous two discussions in here and, for the most part, it would fit.

The risk on Tuesday a little less certain since it’s three days out, but another organized batch of severe thunderstorms is possible in the southeast on Tuesday afternoon and night. As we likely will have seen on Sunday and Monday, the initial storm mode will start as supercells—possibly producing tornadoes and large hail—before organizing into a squall line that produces wind damage across the enhanced risk zone.

Things You Need to Do to Prepare

  • Buy a weather radio, set it up properly, and keep it where you can hear it.
  • Keep cell phones fully charged until the threat for severe weather diminishes. You don’t want to get caught in a power outage with a cell phone that only has a few minutes of battery juice left in it.
  • Secure loose outdoor objects—umbrellas, tables, chairs, grills—and bring smaller objects indoors. Lawn gnomes are creepy enough on the ground, let alone soaring through the neighbor’s sliding glass door.
  • Trim tree branches and limbs that could fall during a storm.
  • Make sure you have supplies to deal with any potential outages of power or water. Some things you should have include first aid supplies, batteries, drinking water, food you don’t have to cook, and water to flush the toilet. It’s not a bad idea to keep some cash on hand—if your whole town loses power, nobody will be able to process your debit or credit cards.
  • Gather emergency supplies into a central, safe location in your house for quick access if you sustain damage or lose power for an extended period of time.
  • Mentally prepare yourself for your local broadcast meteorologist to interrupt your favorite primetime show. You’ll get to watch it another day. The town getting wiped away three counties over may not have the same luxury, so let them get to shelter with his or her on-air assistance.

Also, remember that a severe thunderstorm/tornado watch means that conditions are favorable for the development of severe thunderstorms over the next couple of hours. A severe thunderstorm or tornado warning means that severe weather is imminent and you need to take immediate action to ensure your safety. Watches are issued for many counties or even entire states, often staying in effect for six hours or longer. Warnings cover a handful of communities and are issued based on the path of individual thunderstorms, lasting for 15 to 60 minutes at a time.

The Storm Prediction Center handles severe weather forecasts and issuing watches, while local National Weather Service offices issue warnings for individual thunderstorms.

[Top Image: Jerry Huddleston via Flickr | Model Image: Tropical Tidbits | Maps: author]

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