Derechos

Every now and then a particular synoptic setup occurs in which a large complex of thunderstorms tracks hundreds of miles and causes widespread wind damage.  When these bow echoes remain fairly organized and severe over a certain distance and length of time, they are often referred to as Derechos; a Spanish word which means “direct” or “straight ahead”. These large bow echoes can often time be seen in spring, summer, or even fall, depending upon the condition of the atmosphere.

Most often, derechos occur in the months of May, June, and July, anywhere from the high plains to the east coast, but can also occur anytime of the year and almost anywhere in the country. It is important to understand that derechos can produce severe wind damage and can resemble damage caused by a tornado, but are caused instead from straight line winds.

Derechos are produced when a segment or cluster of thunderstorms generate a large, linear structure of downdrafts or downbursts. Within these family of downbursts lie downburst clusters, and within each downburst cluster you will find what is known as microbursts. These segments of heavy straight line winds and microbursts can generate winds as high as 128mph, which happened on May 31, 1998 in eastern Wisconsin.

Because each section of the derecho has different mesoscale dynamics, wind damage can be sporadic, but overall, a geographic relationship can be easily spotted when looking at wind damage reports on a map. Let’s take a look at an example radar animation of a derecho event below, which took place on June 29, 2012:

The most recent event nearby Kentucky that would qualify for being designated a derecho took place on June 12, 2013. A similar time of year as the event displayed above and a similar synoptic setup as well. Each case was classified as a “Progressive derecho“; which is a result of storms that initiate along a stationary boundary with upper level winds running parallel to the front, and occurs most often in the summer when cyclogenesis is more rare with the jet stream retreating to the north.

There are also “Serial derechos” that occur most often in the spring and fall when mid-latitude cyclones are more common. These derechos are often born from multiple bow echoes within a large quasi-linear convective segment, (QLCS), and can span hundreds of miles and produce damage hundreds of miles long. Upper level winds are usually much more sheered in these cases and as a result, usually start out in more tornadic setup initially but evolve into a group as the storms merge and continue through their life cycles.

Occasionally, a “Hybrid derecho” will take place, which is a system that displays characteristics of both, and occurred on May 31, 1998 event mentioned earlier. Here’s a look at each kind and their associated mesoscale setup:

Each derecho has it’s own unique characteristics and dynamics, and resultantly present a challenge to forecast, just as we see in forecasting tornadoes. However, as time goes on, the SPC and the National Weather Service become more effective in forecasting such events, and with the help of doppler radar and satellite data, watches and warnings are very effective in reducing loss of property and lives.

The best way to prepare for a derecho is to be weather aware; always check your weather forecast from your local NWS office and know what the threats are for that day. It is impossible to know and forecast exactly when and where one may strike, but knowing broad areas that are favorable for the formation of thunderstorms and being prepared are key to being proactive.

I can write on and on about these dynamic and interesting complexes of thunderstorms. However, getting the initial information out there is most important. We are in the time of year when derechos are most common and live in an area here in Kentucky that often see them.

Here’s graphics on the June 12, 2013 derecho event mentioned earlier that struck just north of us here in KY (note, this was also a 2-day severe weather event):

 

The Storm Prediction Center and the NOAA Jetstream websites listed below are a great resource for building knowledge on this subject and I highly encourage checking them out. They helped a great deal in writing this short post. Thanks for reading, check us out on Twitter for more interesting weather information at  @WxOrNotBG.

SPC Derecho Site

NOAA Jetstream Site

NWS Northern Indiana Derecho page