Hi it's Rebecca again, After the destructive derecho last Friday; I thought I would do a write up about it. This post will explain what a derecho is as well as talk about the timeline of the super derecho on the 29th.What is a derecho?
A derecho is a widespread and long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms that assume a curved or bowed shape. The bow-shaped storms are called bow echoes. Bow echoes typically arise when a storm's rain-cooled outflow winds are strong, and move preferentially in one direction. Although tornadoes can be produced by a derecho most of the damage is from straight line winds. Normally 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 is called a derecho. The most severe derechos are given the adjective “super.” Derechos fall under the heading of Mesoscale Convective System ( MCS). A MCS is a complex of thunderstorms which becomes organized on a scale larger than storm scale and smaller than synoptic scale.
In meteorology there are four basic weather scales.The largest scale is synoptic-scale (also known as large scale or cyclonic scale) is a horizontal length scale of the order of about 600 miles or more. This is the high and low pressure systems you hear mentioned on TV weather reports (e.g. extratropical cyclones).
Meso-scale meteorology is the study of weather systems smaller than synoptic scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 5 miles to few hundred miles or so. Examples of meso-scale weather systems are sea breezes, squal lines, and mesoscale convective complexes.
Storm-scale is a scale of sizes of weather systems on the order of individual thunderstorms.
Miso-scale is the scale of meteorological phenomena that ranges in size from a few hundred feet to about 3 miles. It includes rotation within a thunderstorm.
Well I don't know about you but to me that's a lot of gobblygook. Therefore I will give this definition: A derecho is basically a large cold pool of air that gets dragged down from the upper atmosphere. When the cold air hits the warm moist air mass, storms explode and they self propagate. They will keep going as long as the air mass being pushed into has enough warm moist air for it to feed on. Once on the move they produce incredible straight line wind damage over hundreds of miles.
The Set Up:
The set-up was classic for a derecho. We had extreme instability and extreme heat over the entire region. Also there were other conditions present at are necessary for the development of a derecho. The first is a strong jet streak overhead (See image 1). Having a midlevel SE flow ( see fig 2). It is also an aid to development. Third there was a boundary zone that was separating a dry and cool air mass to the north from the humid and hot air mass to the south.
fig 1 shows where the 250 mb jet streak was located.
fig 2 shows the 500 mb level (18,000) feet. the lines (isotachs) show supporting winds (the yellow arrows show the direction). when you see isotachs lined up like this it means there is a very strong high pressure system over the SE U.S. A setup like this aids in the establishment of a derecho. On the 29th there was a moderate to strong vertical wind shear above ground in the lowest 2.5 km. The upper flow was also northwesterly. This type of setup allows small impulses to ride along the northwest flow at the edge of the upper-level heat dome to the south. . The winds at the 500 mb level also help stear weather systems like the super derecho.
All of these conditions were more than met across the Midwest and Mid-Atlantic on the 29th as there was extreme instability in place 6000 MLCAPE across Kentucky and Ohio with 4500s into West Virginia. The heat dome over the region had been responsible for hundreds of record highs being broken during the days preceding the derecho. On the 29th the surface air and dewpoint temps were at record high for June; many place were in the mid 90's into low 100's. with dewpoint in the mid 60 to low 70's. there was a strong upper level jet streak just to the north of this region and extreme heat enveloped the entire region along with a boundary zone set up just to the north.
A blog post I did that covers a bit more on derechos can be found here.
A blog post that talks about the "Ring Of Fire" by my good friend Andy Gregorio can be found here.
The June 29th Super Derecho.
The derecho on the 29th began as a cluster of storms that developed in Eastern Iowa. As it marched east into Northern Illinois and Indiana. The cluster erupted during Friday afternoon near Chicago, IL and then rapidly grew in intensity and coverage as they raced southeastward. The MCS developed into a bow echo in Indiana as it continued to intensify. Over Ohio it matured into a derecho . The derecho would continue expand and envelop larger and larger areas as it headed into the Mid-Atlantic. The derecho reached the coast at around midnight. The path of extreme damage was about 650 miles long in just 11 hours. (see fig 3 and 4) Winds of 70 to 80 mph were common along the damage path, with some seeing wind gusts that ranged between 90 to 100 mph. This event was very widespread.Of the over 1200 damage reports Over 800 were wind damage reports that will cost several million dollars (see fig 5). A thing that surprised me was that a derecho of this intensity only produced two confirmed tornadoes. There were millions of people who lost power. Unfortunately the storm caused 23 fatalities. As power crews race to restore power to around a million people, this number could climb farther if people circum to the heat of the ongoing heat wave.
Showing the progression of the derecho
Another view of the damage path
SPC damage reports
Timeline of June 29th 2012 super derecho:
At 8:00 a.m. EDT
A few thunderstorms start to breakout in Eastern Iowa.
At 8:30 a.m. EDT.
The storms are intensifying as they cross into western Illinois.
At 8:51 a.m. EDT.
The NWS starts to issue severe thunderstorm warnings for Northwestern Illinois.
At 11:35 a.m. EDT
The cluster of thunderstorms starts to form a bow echo west of Chicago.
11:50 a.m. EDT
The storms have cleared Illinois and are rapidly intensifying as they enter Northwest Indiana. At about the same time, the National Weather Service in Sterling, Va., introduces enhanced wording that there is a increased chances for thunderstorms for the Washington DC area, feeling that the thunderstorms most likely will keep rolling east.
12:14 p.m. EDT
The Storm Prediction Center (SPC) begins tracking the evolution of the developing MCS in Illinois and Indiana. However, they fail to truly comprehend the true nature and danger that is developing. At one point an operational forecaster states that the "extent of the severe threat should be limited to areas west of the Appalachian Mountains."
12:50 p.m. EDT.
The SPC issues a severe thunderstorm watch for northeastern Illinois and the northern half of Indiana.
At 1:08 p.m. EDT
The MCS has grown in both size and intensity.
At 1:17 PM EDT
The first tornado warning of the day goes up in northwest Ohio.
1:54 p.m. EDT.
Fort Wayne International Airport records a wind gust to 91 m.p.h. Emergency officials in parts of Indiana begin reporting "massive damage" following passage of storms.
2:15 a.m. EDT
The MCS has developed into a wicked bow echo as it continues to gain strength and momentum.
3:30 p.m. EDT.
The SPC upgrades parts of Indiana, Ohio, and West Virginia to a moderate risk of severe weather and warns of "significant winds."
4:03 p.m EDT.
The SPC classifies the "widespread/locally significant wind damage" as a derecho.
Between 5:00 - 5:20 p.m. EDT.
TV meteorologist go on air taking about the possibility of severe thunderstorms in the Ohio Valley and Mid Atlantic states.
5:37 p.m. EDT.
5:37 p.m. EDT.
The NWS, introduces increased chances for thunderstorms - some severe weather - for the region. They are becoming increasingly concerned about the ongoing derecho over the Ohio Valley.
6:30 p.m. EDT
The derecho has pushed across the Ohio River and has moved into West Virginia, falling trees have trapped numerous people in their homes.
7:20 pm EDT.
Yeager Airport, in Charleston, recorded a 77-mph wind gust.
7:50 p.m. EDT
The SPC realizes the derecho moving over West Virginia and warns it will continue to roll beyond the east slopes of the Appalachians and move into Virginia.
8:00 p.m. EDT.
The SPC upgraded parts of Virginia, Maryland, and Washington D.C., to a moderate risk of severe weather and issues a severe thunderstorm watch with extremely high winds likely.
A little after 9:00 p.m. EDT. (this is an interesting side note) Amazon's Cloud Service fails, this in turn took down Netflix, Pinterest, Instagram, and other services across North Virginia.
9:08 p.m. EDT
The National Weather Service enhances the severe thunderstorms warnings in Virginia using the wording warning of "destructive winds."
10 p.m. EDT.
The derecho is approaching the Washington, D.C. metro area. It is still producing powerful damaging wind gusts. Between 10:00 pm and 11:00 pm EDT. Airports in Virginia and Maryland are reporting wind gust of between 70 and 80 mph .
10:10 p.m. EDT
The NWS issues the warning for the people in Washington DC and the surrounding counties "This is a dangerous line of storms... These storms are capable of producing destructive winds in excess of 80 miles per hour. This is a serious situation. You need to take cover now."
12:50 a.m. EDT
The last two people lose their lives when powerful winds toppled a pine tree onto a tent.
1 a.m. EDT
The derecho was crossing over southern New Jersey, and the final reports of wind damage came in around 1:40 a.m. in Tuckerton, N.J., where winds were reported to have gusted to 81 mph.
A timelapse of NEXRAD base reflectivity of the 29 June 2012 derecho. The timelapse starts in Davenport, Iowa and ends in Richmond, Virginia. It can be found here.
Here we go again, lets all blame the NWS:
Another meteorologist friend of mine Matt Lanza brought an article out of the Baltmore Sun to my attention.
The article talks about the derecho and how forecasters didn't anticipate the extent of how things would unfold on the 29th. Now that much is true. As I pointed out in this blog post. forecasters didn't expect the type of storm that developed last Friday. However the way the article is written it implies that power crews, government offices and the general public were caught off guard with no warnings at all, until after 10 p.m EDT. This is far from true. As I've shown in the timeline of the storm. The SPC, the NWS local field offices, and local weather TV meteorologist were giving warnings well before that time. It always seems that when a rare event like this happens, the first thing just about everyone does is point a finger at the meteorologist and say... I didn't know this was going to happen, because you never told me... This blame the weatherman for everything is starting to wear on me. Everyone needs to listen for warnings and respond to them. This hide your head in the sand mentality is also starting to wear on me. I've wrote several post where I berated individuals for falling to heed warnings from the NWS, be it come outside to see what going on, drive to the store, or just flip the channel on the TV and sit there, during a tornado warning The general public has to except their share of the responsibility for failing to act on any warnings given. Now I agree the SPC and the NWS fumbled the ball on the June 29th super derecho; but, I'm sure the local NWS field offices issued warnings with enough lead time for people to take shelter. The SPC did underestimate the strength of the derecho. This event shows, while we understand many things about the science of meteorology, there are still many things we don't completely understand. But we're getting better every day. Matt Lanza said "The whole warning system needs an overhaul in my opinion. You need tiered warnings for events. I don't know how this gets accomplished in a way that makes sense...I just know that this is what needs to be done. Unfortunately you can't always delineate between EF-0 and EF-3 TOR on radar...which makes this idea difficult to accomplish in reality. But something needs to get done somewhere". I completely agree with this statement. I've said in the past that the severe warning net can and should be improved. But people are the weak link in the chain. For people to sit there and blame the NWS for failing to issue warning everytime something like this happens is ridiculous.
Here is one of the post I've writen abut this subject. It can be found here.
A link to the article can be found here.
Fig 6 was taken by Aviation Dave as the super derecho rolled into the Cincinnati/Northern Kentucky International airport
Fig 7 shows the derecho climatology for the United States. As you can see the Northeast normally see's a derecho once every four years.
Well that's it. I hope you enjoyed reading this post and maybe learned a thing or two. As always questions and remarks are always appreciated.