Gust front “shelf cloud” (or “arcus”) on the leading edge of a derecho-producing convective system. The photo was taken on the evening of July 10, 2008 in Hampshire, Illinois as the storm neared the Chicago metropolitan area. The derecho had formed around noon in southern Minnesota. (Courtesy of Brittney Misialek via the SPC website)

Many people learned the meteorological term, “derecho” for the first time after a large bowing line of thunderstorms did massive straight-line wind damage in the Fort Wayne area and elsewhere on June 29 and July 1, 2012. Since then, some people equate the term with “really, really bad thunderstorm.” These folks want to know if there will be a derecho tonight, because they think that only a derecho would produce the kind of thunderstorm damage they saw last June. They’re wrong.

According to the National Weather Service Storm Prediction Center website, a derecho (pronounced similar to “deh-REY-cho” in English) is a “widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms.” By definition, if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho.

What some people probably don’t realize is that you don’t need a derecho to get wind gusts of 58 mph or greater and the same kind of damage we saw last June. A bowing line of strong thunderstorms can do such damage, even if the line is less than 240 miles long and therefore not technically a derecho.

So if you know someone who is worried about whether there will be a derecho, tell them to forget it. Also tell them to pay attention to any thunderstorm warnings that come out, because any storm that generates such a warning can cause the kind of damage we saw last June.

Last June’s storm demonstrated that people don’t pay enough attention to thunderstorm warnings. They don’t take action unless a tornado warning comes out. That’s foolish, and last June’s storms proved it.

Derecho or no derecho, we could have some really, really bad thunderstorms tonight. Make sure you’re able to receive any tornado or thunderstorm warning that comes out, regardless of the time of day, and don’t ignore any thunderstorm warnings, just because they aren’t tornado warnings!

The National Weather Service (NWS) Storm Prediction Center (SPC) has published its 9 a.m. EDT update to its Day One Convective Outlook, in which the SPC outlines the various levels of severe weather risk around the country between now and 8 a.m. EDT tomorrow. A brief summary of the risk levels for northern Indiana and surrounding areas appears below. For the sake of SKYWARN storm spotters, I will refer to the quadrants set up by the IMO SKYWARN organization (see a map of those quadrants). For readers who are not part of IMO SKYWARN, Fort Wayne and Allen County, Indiana are part of quadrant two, which is also the quadrant in which I provide leadership among ham radio-equipped storm spotters.

Tornado probabilities

Probability of a tornado within 25 miles of a point. Hatched Area: 10% or greater probability of EF2 – EF5 tornadoes within 25 miles of a point. (More Info)

As you can see on the map at left, almost all of Indiana and all of the four IMO SKYWARN quadrants have a five percent probability of a tornado within 25 miles of any point (for a better understanding of the significance of the these seemingly low probability numbers, see my blog post of yesterday). The greatest probability of tornadoes today and tonight is in extreme northern Illinois and southern Wisconsin. There, the probability of a tornado of any size is 10 percent. Note that “hatched area,” which is basically the same as the 10 percent area. In that area, there’s also a 10 percent or greater probability of a tornado that’s at least an EF2 in strength (see a description of the enhanced Fujita scale). Notice that Chicago and its western suburbs appear to be in this area of greatest tornado risk.

Damaging wind probabilities

Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% or greater probability of wind gusts 65 knots or greater within 25 miles of a point. (More Info)

Damaging “straight-line” thunderstorm winds present a much greater risk in the IMO SKYWARN area today and tonight. All of quadrants two and three, the Indiana counties of quadrant four and the Indiana and Ohio counties of quadrant one have a 45 percent probability of damaging thunderstorm winds or wind gusts of 50 knots (57.5 mph) or higher within 25 miles of any point. The same risk covers most of the northern halves of Illinois and Indiana and most of the western half of Ohio. The remainder of Indiana has a 30 percent risk, as does most of the remainders of Illinois and Ohio and extreme southern Wisconsin and Michigan. In addition, there’s a 10 percent or greater probability of wind gusts of 65 knots (74.8 mph) or greater within 25 miles of any point in quadrants two and three, the Indiana counties of quadrant four and the Indiana and Ohio counties of quadrant one. See a blog post that explains what kind of damage to expect from these wind speeds.

Hail probabilities

Probability of one inch diameter hail or larger within 25 miles of a point. Hatched Area: 10% or greater probability of two inch diameter hail or larger within 25 miles of a point. (More Info)

Hail probabilities are also very high. All IMO SKYWARN quadrants, plus most of Illinois, Indiana and Ohio, have a 30 percent probability of hail of one inch diameter or larger. In addition, all of quadrants two, three and four and most of quadrant one have a 10 percent or greater probability of hail of two inches diameter or larger. I hope the damage that an ice ball of that size can do is fairly obvious.

Flash flooding

The SPC convective outlook does not present a risk of flash flooding, but that risk certainly exists. See a multi-media briefing from the Northern Indiana weather forecast office, which includes a mention of three inches of rainfall or more. Urban areas that are prone to flooding, including those that saw flooding June 1, could be at risk for new flooding, if heavy rain again overloads municipal drainage systems.

Timing

The 9 a.m. SPC convective outlook indicates that most of the severe weather in northern Indiana and western Ohio will probably happen later tonight. That said, there is a chance that some isolated thunderstorms, including possible supercells, could pop up in the middle to late afternoon hours.

Where to find updates

Make sure your weather alert radio is working. It will sound off if any watches or warnings are issued for your area. If you don’t have a weather alert radio, buy one today! In the interim, keep a television (tuned to a local station) or radio (tuned to a local station that has a live DJ) on and within earshot during the day.

The next SPC Day 1 Convective Outlook is due out by 1:30 p.m. EDT. Also, watch for updates to the NWS Northern Indiana weather forecast office’s Hazardous Weather Outlook, which come out as necessary. Before the SPC issues a tornado or severe thunderstorm watch, it usually issues a Mesoscale Discussion. The SPC Mesoscale Discussion page displays these products as they’re issued.

I’ll update this blog as things develop, if I have time. But I must also get work done in my regular job today!

Stay safe!

Update: See my summary of the 9 a.m. convective outlook.

The northern half of Indiana, along with parts of Illinois, Michigan and Ohio, continued to be under a moderate risk of severe weather today and tonight when the Storm Prediction Center (SPC) issued its Day 1 Convective Outlook at 2:16 a.m. EDT. An updated outlook is due by 9 a.m. EDT, so I’ll hold off on summarizing the convective outlook until the updated one comes out.

In the interim, here are some useful resources:

The SPC issued a Public Weather Outlook at 5:02 a.m. This describes the severe weather thread in less technical terms than does the convective outlook.

The SPC also created a multi-media briefing at 5:16 a.m. EDT.

The Northern Indiana National Weather Service (NWS) weather forecast office (WFO) issued a Hazardous Weather Outlook at 4:55 a.m. EDT. It indicates that SKYWARN storm spotter activation will likely be needed this afternoon and tonight.

The Northern Indiana WFO published its own multi-media briefing at 5 a.m. It appears below.

The Northern Indiana WFO is now accepting questions about today’s weather via Twitter. If you have a Twitter account, send questions to @nwsiwx. The WFO will begin providing answers via Twitter at about 10 a.m. EDT. If you don’t have a Twitter account, you can still see the answers. See my blog post from yesterday to learn how. 

Storage building damaged by straight-line wind in 2011. NWS photo.

If you’ve been watching forecasts, you probably noticed that there’s a chance for straight-line winds tomorrow (Wed. June 12) of 75 mph or higher. But do you know what such winds can do? The following information comes from the Beaufort Wind Scale, which storm spotters often use to estimate wind speeds:

So, if we get the 75 mph to 80 mph straight-line winds that are possible, we should could easily see considerable and widespread damage to structures.