UPDATE 2, 234AM EASTERN: The SPC has pulled the trigger and said that a derecho may develop Wednesday across the moderate risk area. In addition to several strong tornadoes early on in the outbreak, the SPC indicates the risk of a derecho:
"INCREASING WIND FIELDS WILL FAVOR A FORWARD PROPAGATING CONVECTIVE SYSTEM...POSSIBLY A DERECHO CAPABLE OF WIDESPREAD DAMAGING WINDS. HOWEVER...EMBEDDED AREAS OF ROTATION MAY ALSO EXIST PERIODICALLY...INDICATING EITHER TORNADOES OR ENHANCES SWATHS OF DAMAGING WIND POTENTIAL. LARGE HAIL IS ALSO LIKELY...POSSIBLY WIND DRIVEN."
This could be a potentially dangerous situation as the afternoon and evening hours unfold. I'll cover the system extensively here on DailyKos when watches are issued, and won't stop until the outbreak is over. Derechos are dangerous and unfold very quickly.
The original diary continues below.
Note: This diary addresses the derecho risk that local and national news stations are doing a miserable job of explaining and qualifying in their reports. Read the entire diary for a full idea of what's predicted to happen tomorrow.
The Storm Prediction Center has issued a moderate risk for severe weather tomorrow across the Ohio Valley in anticipation of a major severe weather outbreak during the afternoon and evening hours.
All modes of severe weather (tornadoes, winds, and hail) are possible, but damaging winds in excess of 70 MPH are going to be the biggest threat.
Here is the severe weather risk for tomorrow. Red indicates a moderate risk for severe weather, yellow a slight risk, and green indicates a risk for general, non-severe thunderstorms.
Here are the severe weather probabilities for tomorrow. The shaded areas indicate a xx% risk for at least one severe weather report (one tornadoes, one report of wind damage, or one report of large hail) within 25 miles of any point within the shaded area. The black hatched area indicates a risk for significant severe weather, which seems to be a risk for widespread damaging winds in excess of 70 MPH.
That all being said, many local and national news stations are trying using the term "derecho" to describe tomorrow's event. Do not focus on the word "derecho" just yet. I put the "d word" in my title so that people worried about a derecho would know to click and get useful information as opposed to a low-information puff piece by news stations meant to get ratings and nothing else.
What is a derecho?
A derecho is a term for a long-lasting, very strong bow echo that consistently produces wind damage over a widespread area (240 miles of constant damage is the official definition).
The term is borrowed from the Spanish word "derecho," which translates to English as "straight," because a derecho has straight-line winds. Straight-line winds are winds that barrel across the land in roughly a straight line, as opposed to rotating like one would find in a tornado. This wall of wind can produce immense damage, mowing down entire forests (resulting in an event aptly called a "blow down"), damaging even well-built buildings, and knocking out power to millions as we saw last June in the Ohio-DC Derecho.
Several derechos occur every summer, and few are as strong as the one that occurred last summer across the east.
Types of Derechos
There are two types of derechos -- serial derechos, and progressive derechos.
Serial derechos are the most common types that affect the United States. Most people know them as long lines of storms that form along cold fronts and sweep through during the spring and fall months. The most memorable serial derecho in recent memory occurred in April 2011, resulting in a nearly-unprecedented 1,476 reports of severe weather in one day.
Progressive derechos are the most common type during the summer, and they're the most dangerous. They can pack winds of over 100 MPH and move at highway speeds, giving little time for people to receive the warning and take action to ensure their safety.
I discussed this type of derecho in depth last summer after the Ohio-DC Derecho (here, here, and here), but I'll summarize it in this diary, borrowing some images and paragraphs from my previous diaries. Text borrowed from my previous writings will appear in blockquotes.
Tomorrow's Setup
The event that could take place tomorrow is a progressive derecho. A progressive derecho requires four main ingredients to develop: wind shear, instability (heat and moisture), a focusing mechanism, and a clear shot.
The setup over the eastern United States tomorrow involves a ridge of high pressure over the southeast, a warm front moving into the Ohio Valley, strong vertical wind shear, and a deep pool of moisture and instability sitting atop the ridge.
Here's the 500 millibar (roughly 18,000 feet) map from this morning's NAM model, valid for tomorrow afternoon:
A low pressure system is expected to move over Iowa by tomorrow afternoon, with a warm front extending off the east side of the low across the Ohio Valley and Mid-Atlantic region. This warm front is predicted to be located along the top-end of a ridge of high pressure over the southeast.
Thunderstorms are expected to rapidly develop along and near the warm front tomorrow afternoon in Illinois, and atmospheric wind shear (winds changing speed and direction with height) are favorable for these storms to form into one or more lines.
This line of storms will begin to move east along the warm front -- think of the line of storms as a train, and the warm front as the tracks -- using up as much instability as it can. The environment in the area where the line of storms is expected to form will be extremely moist and unstable.
Here's the precipitable water for tomorrow afternoon. Precipitable water shows how much rain would fall, in inches, if all the water in the atmosphere over a particular area fell as rain. The higher the number, the more moisture. The region from Illinois to Ohio is extremely moist, meaning that the storms will have no problem ingesting moisture during development:
CAPE -- Convective Available Potential Energy, or essentially the fuel storms use to form updrafts and become strong -- is expected to be between 3000 and 3500 j/kg across Illinois and the Ohio Valley tomorrow. 1000 to 1500 j/kg is sufficient for storms to develop, so anything over that adds to the ability for intense storms to develop.
When these storms develop into a line, as they're expected to do, there is a real possibility that the result will be a derecho. A derecho is only a term used for a line of storms that lasts for hundreds of miles. Even if it's not technically a "derecho," the storms could still do damage. The strength of the line depends on how long it stays in the region of instability, and how clear of a shot it has as it races off to the east. If there are storms (and, subsequently, rain-cooled, stable air) ahead of the line, it could weaken and not produce as much severe weather as it could otherwise.
The severity of the line (or derecho) also depends on how well the structure of the storm develops. Here's an explanation of a derecho's structure from a diary I wrote last summer:
The technical term for this kind of system mesoscale convective system, or MCS. An MCS is simply a line of organized thunderstorms that move in sync with each other.
Many MCSes develop into what's called a bow echo, or a line of thunderstorms that has such strong winds within them that it takes on the shape of an archer's bow.
A derecho is a bow echo/MCS that produces extensive wind damage over very long distances. The "official" word from the National Weather Service is that a bow echo/MCS is considered a derecho when it produces extensive damage over a 240 mile swath. Seems like an arbitrary number, but that's what it is.
The difference between an MCS and an individual storm is subtle at first, but becomes a big deal later on in the storm's life. I'm sure everyone is familiar with how a thunderstorm develops...
Unstable air rises and condenses, creating cumulus clouds. This rising of unstable air, called an updraft, continues to build the cumulus cloud as far up into the atmosphere as it can. The taller and bigger it gets, the more likely it is to be a pretty big storm.
Rain, snow (yes, snow), hail, and ice pellets form in the clouds as the updraft continues to gain strength. Eventually, the weight of the precipitation gets to be too much for the updraft to handle, so it starts to fall to the surface:
As the precipitation falls to the surface and gets more intense, it drags cooler air down through the thunderstorm. This eventually creates what's called a downdraft. As the precipitation gets harder, the downdraft strengthens, which in turn increases the amount of precipitation, and so on.
All that cold air the downdraft drags to the surface has to do something. Since cold air is denser and more stable than hot, moist air, it pools up beneath the storm. This bubble of cold air is aptly called a cold pool. The cold pool is more commonly known as an outflow boundary or a gust front.
This cold pool can do one of two things. It can either
1) kill the updraft and kill the storm, or
2) race away from the storm.
This step is key to the formation of an MCS.
As the cold pool races away from the thunderstorm, it acts as a mini cold front by scouring up surrounding warm, unstable air and forcing it to rise. When it rises it creates more thunderstorms and intensifies the ones that are already there.
This process tilts everything, and that's what keeps the line of storms alive. Thanks to the cold pool, the updraft and downdraft are now tilted, and they don't interfere with each other. The thunderstorms start moving in unison with the cold pool and continue to gather energy virtually unimpeded.
As the cold pool moves across the surface, it starts to go much faster than the air around it. The air in the cold pool creates friction with the air above it, causing a horizontal rolling motion:
If you've ever been in a swimming pool and raced your hand through the water, you probably noticed little tiny whirlpools develop. This is the same concept, but on a much larger level.
This horizontal rolling motion creates what's known as a rear inflow jet, which is essentially a jet of higher winds a mile or two above the surface. The faster the cold pool moves, the stronger the horizontal rotation will get, and the stronger the rear inflow jet will be.
This rear inflow jet usually occurs near the center of the MCS, and causes it to bow out (hence the name bow echo). When the line of storms starts to bow out, either end of the storm starts to curl back because of friction. These curls, known as bookend vorticies, cause vertical rotation (going up and down as opposed to parallel with the ground) through the atmosphere.
This concept is shown in this horribly drawn map. Theoretically, this is what it looks like on radar:
As the updrafts are tilted backwards, they're ejecting precipitation tens of miles back behind the main line of storms. This creates an area called the stratoform region. This area of light to moderate rain can extend up to 100 miles behind the main line of storms.
This stratoform region drags more cold air to the surface, which strengthens the cold pool, which strengthens the rear inflow jet, and so on and so forth. As you can probably tell by now, it's all linked together in a delicately balanced system. This is why derechos don't happen too often -- everything needs to be perfect for them to flourish.
Now, as the rear inflow jet races at speeds sometimes exceeding 100 MPH, it's got to go somewhere. That direction is down. The jet of air hits the updrafts at the front of the line of storms and gets shoved down to the ground:
The result is surface winds of 60-130 MPH, as many of you experienced back on June 29th.
Now, what kills a derecho? If the cold pool gets too far out ahead of the line of storms, it can cut off the updraft. If the updraft gets cut off, there's no more unstable air to keep the thunderstorms going, so they die. Without thunderstorm activity, there's no precipitation to sustain the cold pool, so it dies. Eventually, all wind and precipitation will stop, and all that's left is the damage.
Meteorologists at the Storm Prediction Center will get a better idea of exactly what will happen tomorrow once this evening's model runs come in. Once storms form tomorrow, events will unfold very, very quickly. Many people were caught off-guard last year when the derecho hit because it happened so fast.
Prepare, but don't panic. Secure all loose items outside, as they will easily blow around in winds. Grills, patio furniture, garden gnomes, and whatever else you've got should go inside a shed, come inside, or get tied up against a sturdy object.
Make sure your cell phones are charged and you've got gas in your car.
Wear sneakers/tennis shoes/whatever you call them tomorrow in case you have to walk through broken glass or debris outside.
Make plans in case you lose power for an extended period of time.
Keep a very close eye on the Storm Prediction Center and your local National Weather Service office. Like I said, it'll happen very quickly once the storms form and the line(s) develop. People closer to the Appalachians will have more time to react than people in Illinois or Indiana.
Hopefully a derecho will not happen, but the risk exists, and it's better to be informed than scared by the news stations that can't be bothered to do in-depth reporting.
I'll continuously post updates tonight and tomorrow
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