Below is a great summary on the historic “Derecho” last month courtesy of NWS, Blacksburg, VA
Historic “Derecho” Severe Wind Event
Storms across the Midwest and Mid-Atlantic
on June 29, 2012
Often when it is very warm and humid in the summertime, thunderstorms will develop with little or no mechanism to lift the air mass, other than the mountain ridges or very weak boundaries where light wind may be converging together. On June 29, 2012, the air mass over the eastern third of the Unites States was not only hot at the surface, breaking records with many temperatures reaching well over 100 deg F, but also unusually hot above the surface, such that no storms (not even cumulus clouds) could form unless given a big kick from below. A big kick is exactly what occurred, beginning with a small cluster of thunderstorms in the vicinity of a frontal boundary way up over northern Illinois and Indiana in the early afternoon (see Figure 1). Once this cluster of storms developed some low-level outflow, it was then able to continually generate new lift along the leading edge, and the storm system grew and became more powerful (see Figure 4), fueled by extremely hot air ahead of it at the surface to allow strong updrafts, as well as mid level dry air and northwest winds to help strengthen the downdrafts and keep the surface outflow accelerating. Without this initial kick from below, and then continued upward push like a snow plow from the growing outlow, storm would not have formed at all. The air mass out ahead of this system remained so hot into the evening hours, that it was able to continue plowing across the Appalachians mountains, and make it all the way to the Atlantic coast by around midnight. In it’s wake was widespread and at times significant damage, caused by winds as high as 80 to 100 mph. Millions of people from Ohio to New Jersey were without power due to fallen trees and power lines, and this during the season’s first heat wave. A satellite view of this storm system as it developed and raced to the east can be seen by visiting this link (you may need to be patient while it loads).
This kind of persistent and intense mesoscale thunderstorm complex, termed a “derecho” (spanish for direct or straight ahead), is not uncommon across the central and midwestern portions of the country, and on rare occasions will make their way toward and across the Appalachians. The last particularly intense derecho to cross the Appalachians was August 8-9, 2000, when actually two separate derechos managed to cross the mountains with wind damage reported in a majority of the counties within the Blacksburg county warning area (CWA) as well as upstream and downstream (see the maps at the bottom). There are many similarities from this unique “dual” derecho event from August 2000 and the one on June 29, 2012, with the exception that this most recent event produced even more extreme and widespread winds, and in some locations with little or no rainfall due to the fairly dry air mass.
For more information on derechos in general, including the specific definition and some historical examples, please visit the excellent Derecho FAQ page from the Storm Prediction Center in Norman Oklahoma.
June 29, 2012 Event Details
The following two maps show the position of the surface cool front across the Midwest and Ohio Valley, with the initially developing thunderstorm complex near Chicago (the infant that would quickly grow into the monster derecho), as well as an upper level map from the mid levels of the atmosphere and a vertical sounding from Blacksburg that evening showing northwest winds aloft and very warm air mass for momentum respectively.
Figure 1. Surface map with radar mosaic at 18Z (2pm EDT).
Figure 2. 500 mb map (~18kft aloft) showing some modest westerly winds behind the system.
Figure 3. Upper air sounding profile from Blacksburg at 00Z (8pm EDT) showing very warm and unstable, yet dry air through a deep layer, with deep northwest flow as well.
Below is a time-mosaic showing the radar depiction of this event as it developed in the Midwest and grew rapidly as it crossed the Appalachians and toward the East Coast, as well as the preliminary national map of wind reports in it’s wake.
Figure 4. Radar time-mosaic of derecho progressing from northern Indiana to the East Coast.
Figure 5. Preliminary severe reports from June 29. Wind reports in blue. Measured wind speeds of 65 kts of more denoted by black squares.
A closer look at the reports, including additional ones that have been added to our database with follow up phone calls since June 29th, can be seen below. We received a total of over 160 separate reports, but many of these reports included “several trees down” or “widespread tree damage” across an area. Thus, there are surely many more trees down and other damage that we have not heard about or never will. There are likely thousands of trees and large limbs down in many counties, with the exception of NW North Carolina and far SW Virginia where the winds were never especially strong, and likely hundreds of thousands of trees down across the region shown below. Not shown in this image are actual measured wind gusts from instruments. Some of the stronger ones recorded at regional airports are listed below in a table below. Finally, at the time of this writing the total number of fatalities and injuries across the entire path of the derecho has not been talleyed, but in the Blacksburg county warning area, we are aware of at least four fatalities (three indirect), and 15 injuries. Furthermore, a couple victims of heat-related illness may have resulted due to power being out in so many locations (millions lost power due to the storm, and many thousands remained without power for several days).
Figure 6. Wind reports collected in the few days following the derecho event.
Figure 7. Table of peak wind gusts reported at regional airports in the Blacksburg county warning area.
A closer view of the radar evolution as the storm crossed the Blacksburg county warning area (CWA) as seen from the Floyd County Doppler radar (KFCX), both in terms of the reflectivity (top loop) and Doppler velocity (bottom loop), is shown below. Notice that in the reflectivity image, there are very light returns along the leading edge of the outflow boundary. This is believed to be dust and debris and not precipitation as people reported no rain or only a few drops when this hit.
Figure 8. KFCX Doppler radar reflectivity (top) and velocity (bottom) loops. In the velocity image, green colors represent winds blowing toward the radar, red away from the radar, and purple colors indicate where the values could not be calculated properly, in part due to the fast movement.
Comparison to the August 2000 derechos
Below are some images from the event of Aug 8-9, 2000, where two distinct derechos moved through the region. While there were some similarities in the large scale pattern with the June 29 2012 event (such as large upper ridge to the west with NW flow), the first event on the Aug 8 came through much earlier in the day, and there was more moisture to work with. The radar evolution was quite similar however. This first derecho left a surface boundary across the region, helping to pool moist, unstable air right along and to the south of it during the overnight hours, and this helped the second to survive and track along and to the south of where the first one went, even through the middle of the night. Beneath the radar mosaic loop are some maps showing all the counties that had multiple reports of wind damage from these “dual” derechos (the maps are captioned with Figure 1 and 2, which is part of the image file).
Figure 9. Radar mosaic of two derecho wind storm events that crossed the Appalachian region back-to-back on Aug 8-9, 2000.
The thunderstorm complexes that are observed on radar and develop these linear characteristics are sometimes called squall lines, or bow echoes (if segments bow out moving faster than others), and are part of a larger family of thunderstorm complexes known as “mesoscale convective systems” (MCS). Most of them do produce some degree of severe weather (most often damaging winds), and there have been studies of these severe systems in the Appalachian region. If you are interested in some of this work, please see the following links:
Very few of these severe linear MCS storm systems are severe enough, widespread, and long-lived enough to meet the definition of “derecho” (again, see the link provided earlier for more specific information about derechos), but not only did the June 29, event meet this definition, it will go down in history as one of the most intense and devasting examples in recent history.