Tag Archives: tornadoes

Forecasting, Mid-latitude cyclones, Severe

Another Multi-Day Tornado Outbreak Expected

The year is off to a record start in tornadic activity, and more is on the way. Another three-day tornado outbreak is currently underway, with day 1 having brought close to 40 tornado reports and over 250 hail and wind reports.

The system is expected to bring more severe weather to the eastern part of the U.S. through Tuesday and Wednesday. At the time of this writing, a squall line has developed in the Mississippi River Delta that is expected to push east overnight, bringing strong winds, rain, lightning, and the risk of embedded tornadoes.

This scenario is more complicated than the setup for the last big outbreak, which had a single powerful system to generate the intense weather. A closed low located over Arkansas is responsible for the day 1 activity. This low formed today from a shortwave kink in an upper trough. This low is expected to be blocked by a strong high pressure system off the Atlantic coast, causing it to move north and eventually northeast to die out over the Great Lakes area. However, a second shortwave kink is expected to enter the mid-South and undergo cyclogenesis on Tuesday afternoon or evening.


Surface low at 36 hours

The combination of the current cyclone’s development, the upper-atmospheric jet that is causing all this shortwave activity (see below), and the next cyclone’s appearance on the scene will result in there being significant sources of uplift.


250 mb jet stream


700 mb upward vertical velocities

The highest values of instability in the event are currently prognosticated by the models to occur around midday tomorrow. The NAM and GFS generally agree on the areas of high CAPE, with each model forecasting at least 3000 J/kg (and it should be noted that models do not do well with CAPE and have a tendency to underforecast. Keep an eye on observations such as soundings).


Surface-based CAPE at 18 hours, NAM


Surface-based CAPE at 18 hours, GFS

For Wednesday’s event, the highest CAPE values are expected to be over Mississippi. The NAM and GFS agree on the maximum values but have the location and orientation of the high CAPE axis different.


Surface-based CAPE at 45 hours, NAM


Surface-based CAPE at 45 hours, GFS

The Energy Helicity Index (EHI) values for 18 hours and 45 hours (midday Tuesday and afternoon Wednesday) indicate the areas that the models forecast are most conducive for tornadic supercell development. Here is what the NAM indicates for the two times:


EHI at 18 hours, NAM


EHI at 45 hours, NAM

I am especially concerned about the middle part of Tennessee on Wednesday if that is accurate. Those EHI values are almost off the scale, and they coincide with an area of at least 3000 J/kg CAPE. That area has a history of tornadoes, and between the strong instability, powerful uplift, and helical pattern to the winds, I think it is quite likely that the Tennessee valley may be in the worst part of this outbreak.

The Storm Prediction Center has already put up a Moderate risk for Tuesday and Wednesday. It is thought highly likely that at least one, if not both, of these days will see that risk upgraded to High.

Forecasting, Mid-latitude cyclones, Severe

Tornado and Severe Weather Outbreak in OK/AR/MO Thursday

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I am expecting a tornado and severe weather outbreak to occur tomorrow in what I would call the heart of Tornado Alley.

However, that’s a worthless statement from any perspective—forecasting, meteorological, geographical—without a qualification of the term “Tornado Alley.” I have long thought that the true “heart” of Tornado Alley is not over Oklahoma City at all, but is instead farther east. In truth, where that heart is depends on what one chooses to look at. (For long-tracked violent tornadoes, the bull’s eye is, beyond a shadow of a doubt, over Mississippi.) Some meteorologists developed the idea of three distinct tornado alleys: Plains Alley, the traditional storm-chasing zone in the Plains states that is often referred to with the general appellative of “Tornado Alley”; Dixie Alley, a broad region encompassing much of the Southeast but not including parts of Texas that are climatologically and geologically more similar to Plains Alley; and Hoosier Alley, which is essentially the Midwest. However, if you really take a good look at it, I think you’ll find that the entire eastern region of the country between the two mountain ranges is Tornado Alley. There is no distinct border where you can say, “There are not nearly as many tornadoes in this small region as there are in either ‘Alley’ on each side of it,” and in the absence of such a thing, the only reasonable thing to do is to say it’s all one Alley. (Besides, “Hoosier Alley”? Really?) With the whole of the U.S. between the mountain ranges as Tornado Alley, it seems reasonable to expect the core somewhere east of Oklahoma City, and the meteorological data bear this fact out.

Now that that’s out of the way, let’s take a look at the science behind this and see why there will be severe weather, as well as where it should occur.

The weather-maker will be a low pressure center forming east of the Rocky Mountains. A low that is currently analyzed at 1009 mb (which is to say, not much of a low) is located over Kansas right now. This cyclone is forecast by the NAM to open up as it weakens and become more of a broad trough. This should be occurring right now, in fact. After that low dies out, a strong upper-level low (700 mb) is set to close off its circulation over the TX/OK panhandles on Thursday. Upper-level cyclogenesis should have occurred by 9:00 to 10:00 tomorrow morning (CDT). The usual vertical tilt of these kinds of cyclones will result in the surface low being over central Oklahoma at this time, and it should be at about 1000 mb by midday tomorrow.

Thermal advections will be strong, with the greatest warm-air advection (WAA) occurring in an area just west of Tulsa, OK to north TX to the AR/LA border. Temperature advection drops off in northeast Arkansas.

Surface dewpoint temperatures will be quite high, approaching 65-70 from Tulsa south along the OK/AR border in a narrow swath. A larger area of 60-65 F dewpoints will cover the area from Oklahoma City through a diagonal line bisecting Arkansas NW-SE.

However, despite all this moisture around the surface, a dry socket should be present at the critical 700 mb level in the atmosphere. This shows up on a synoptic model forecast map as a medium brownish-gray blotch of low relative humidity.

All this will lead to mixed-layer CAPE values approaching 2500 J/kg.

The instability will definitely be present, and between the thermal advection and the diffluence-based forcing from the low, lift should be present over the target area. Decent upward vertical velocities are forecast by the models to be present over the area of interest.

A cap of CIN will be in place early in the day, but as the surface heats up, the surface temperature should remove the temperature inversion, making it easier for moist parcels of air to rise.

The disappearance of the capping inversion also shows up in forecast soundings for the area bounded by Tulsa, Fort Smith, and Fayetteville. They indicate that a substantial warm surface layer will have developed by late afternoon, and with CAPE values as high as they are expected to be, this will set off convective growth.

Helicity values are also supposed to be quite high, with 0-3 km at 300-500 over the target area, at a time coinciding with the high CAPE values and the eroded cap, which will support supercellular development instead of linear. This increases the risk of tornadoes and large hail.

The Storm Prediction Center has already put up a Moderate risk for this general area, which I agree with 100%. I wouldn’t be overly surprised to see them go with a High by tomorrow morning, though this is a small enough area that they may not do it on the basis of its size. In any case, I am growing quite confident of a risk of supercells tomorrow for eastern Oklahoma, western Arkansas, and southwest Missouri. If I were to pick one city that I think has the highest chance of a significant impact, I’d go with Tulsa. However, a slight shift in any direction of the low’s movement would result in a shifting of the severe weather risk, and these events are never points, but areas. It is looking like a potentially dangerous day tomorrow for this region of the country, so people living there should make sure they keep aware of what is going on in the afternoon and be ready to act.

Severe

Severe Weather Season Starts Off With a Bang

Having lived in the Southeast for most of my life, I find it a bit dishonest to say that there is a defined “severe weather season” for this region. We, after all, are the ones who rang in the new year with a tornado outbreak. We’ve had tornadoes a couple days before Christmas, in January, during hurricanes, and occasionally even during the summer months. We have certainly had powerful thunderstorms in the summer months that have brought extreme wind, hail, and lightning. However, it is fair to say that there are periods of the year when the most destructive types of severe weather are more common than others, and perhaps these are what should be defined as the “severe weather seasons” for the South. If this is the case, then we are entering the spring severe weather season. We’ve already had a taste of it when the powerful squall line came through last week on Thursday and Friday, generating several tornadoes and 100-mile-an-hour winds in Tennessee. That, however, was just a teaser, and an event is currently in the process of unfolding that promises to be more significant. Tonight’s event has a couple of parameters in place that last week’s did not, at least not to a great extent, plus all the parameters that last week’s event did have. I am using images from the RUC model, which is a short-range model containing live observations that is run every hour.

First of all, there is a strong jet over the region.

This jet will have uplift occurring on its northeast side. Since it is a 120-knot jet, this will provide significant uplift.

Secondly, there is a decent thermal gradient and fairly strong winds at the 850 mb level.

This will provide fuel to the strengthening low pressure center that is the source of the upcoming bad weather. The advection of warm air is also a source of uplift.

There is also an area of vorticity (essentially, rotational motion of wind). Vorticity is not the source of tornadoes, despite the suggestive name; in fact, uplift associated with vorticity is not directly under the strongest area, but is instead downstream of it. The vorticity is being advected in that direction by the motion of the air.

Incidentally, that accounts for three parameters providing favorable severe weather conditions over pretty much the same spot: Memphis/St. Louis/NE Arkansas.

Now, the parameters that last week’s event did not have in place relate specifically to supercell formation. Last week’s event had plenty of uplift and wind shear, just not a lot of atmospheric instability and helicity. Tonight’s expected outbreak is a different story.

Here’s atmospheric instability, as measured by CAPE (convective available potential energy):

There are values approaching 2000 J/kg expected over, yes, the same area where a lot of uplift is supposed to be occurring. This isn’t a stupendous value, but it’s pretty significant for an early-season outbreak. Some of the highest values of CAPE I’ve ever heard of were on the order of 6000 J/kg for events that produced F5 tornadoes. Where other favorable parameters exist, CAPE is directly correlated with the formation of thunderstorms.

Here’s helicity.

The peak swath of high helicity is a little off-center compared to the rest of the favorable parameters, but a pretty significant area of it is nonetheless centered right over the area in the lower Midwest/upper South where the rest of the favorable parameters are converging.

I am expecting a tornado outbreak over this area tonight, pushing eastward into Tennessee in the early morning hours. This is not good, as nighttime tornadoes are easily the deadliest of all. (See my previous blog entry about the tornado outbreak of February 5, 2008 for more about Southern tornadoes.) I completely agree with the Storm Prediction Center’s decision to issue a moderate risk for this region tonight and tomorrow. I think it is possible that tornadoes may occur a bit farther south than currently indicated, mainly because of that high helicity index, but I am not expecting there to be a very great threat much south of Highway 82.

Severe

Lessons of the Super Tuesday Tornado Outbreak of 2008

2008 OutbreakAt the moment, Arkansas is apparently under a blanket of snow in many areas, and more cold weather and possible winter precipitation is apparently expected. This is interesting to me because at this time not too long ago, Arkansas was essentially “ground zero” for an entirely different kind of destructive meteorological phenomenon, which had also occurred in a La Nina winter. Three years ago, a violent, extensive tornado outbreak began in the South and lower Midwest. Because the outbreak began on “Super Tuesday,” the day when the largest number of state presidential primaries were held, the outbreak was dubbed the Super Tuesday Outbreak. It left in its wake 87 confirmed tornadoes, which killed 57 people—a death toll unheard of for almost 25 years.

The system that produced the outbreak, a low pressure center with a deep warm sector and powerful trailing cold front, moved from Texas to Missouri during the outbreak. Upper-level jets provided decent support, with a jet approaching 120 knots located over eastern Kansas, Arkansas, Missouri, and Oklahoma. Instability was sufficient to promote severe weather in the form of supercells, with 1000 J/kg of CAPE observed. This is not particularly spectacular for the usual tornado season, but it is not often found in cold-season outbreaks. High shear as a result of the jet further fueled the outbreak. The tornadoes occurred on the east side of the cyclone, as is usually the case.

Though it was not as intense as the Super Outbreak of 1974 that produced either 6 or 7 F5 tornadoes (depending on what source you believe), this outbreak was typical of large-scale tornadic outbreaks in that it produced a lot of violent and strong twisters, which normally occur in comparative isolation. In this outbreak, five EF4 tornadoes were observed, including a long-tracked tornado in Arkansas that had a path length of over 120 miles. This tornado alone was responsible for 13 of the fatalities. Another long-tracked tornado, this one an EF3 in Tennessee with a 51-mile path, was responsible for 22 deaths.

This relates intimately to a point that I have been making for several years, and what I think is the principal lesson of this outbreak and its calamitous death toll (for modern times). The South, particularly AR, LA, MS, TN, and AL, is subject to far more violent long-tracked tornadoes than any other region of the country. I mean it isn’t even close. Here is a PDF of a paper by two professional meteorologists about the statistical patterns indicating the real “tornado alley,” at least as far as F3-F5 long-tracked tornadoes are concerned. If you are frightened of statistics, this is a graphic from Jackson, MS that puts up the raw data, the paths of all long-tracked tornadoes rated 4 or 5. It’s not exactly the same data being looked at in the graphic and the paper, but nonetheless, the point that is coming through loud and clear is that the Deep South is the primary breeding ground for the most dangerous, destructive, and deadly kinds of tornadoes that the atmosphere can produce: strong-to-violent long-tracked twisters.

Furthermore, the South is more prone than any other region of the country to have substandard housing that is particularly vulnerable to tornadoes. There is no reason for anyone in a house to die in a weak tornado (an EF0 or EF1), but it still happens, and poor construction or deterioration is to blame for a lot of it. Mobile homes do not help the problem, and it is sickening to me (I guess I still have fragments of a social activist in me) that one’s survival in a tornado, in the United States in the 21st century, could depend on one’s financial status, but this is how it is. Also, most buildings in the South do not have tornado shelters. Even large venues in the South often have no severe weather plan. I’m going to pick on one, the Georgia Dome, which was brushed by an EF2 tornado while a basketball game was going on later in 2008. There was little official word given to the spectators about what was happening until the tornado was actually upon them, making the ceiling shake! There is an infamous YouTube video from inside the stadium as the tornado was passing by, so you can see for yourself what I mean. The only reason that didn’t result in a major disaster was that the game went into overtime, keeping the fans inside the stadium. And finally, unlike the other parts of Tornado Alley (“Dixie Alley” and “Hoosier Alley,” the South and Midwest respectively, are not really separate from Plains Alley), the South does not, in many locations, have an effective local warning system. I don’t mean the issuance of warnings. The weather office in Jackson is very effective at producing warnings. If anything, they overwarn! There is not a problem with the warnings being generated. The problem, rather, is with dissemination to the public during times when the public is most likely not able to know what is going on.

Some areas are better than others at issuing tornado alarms. I can think of one time when Oktibbeha County, Mississippi sounded its alarms for apparently the whole county when the tornado in question was on a far corner and headed out. This is how it happens more often than not when a county has tornado sirens. This kind of thing, in my opinion, is overwarning, and it can lead to complacency, but it is preferable to the alternative. I witnessed that on the New Year’s Eve 2010/2011 tornado outbreak when a very strongly rotating supercell entered Noxubee County, MS around 2:00 AM. It was quickly warned by the National Weather Service. However, the county did not sound its alarms until the tornado was actually on the Mississippi/Alabama border! It was an EF3 tornado, and by that time, it had already done its damage in the rural part of the county. I’ll repeat this: 2:25 in the morning on New Year’s Day, and the tornado alarm was not sounded until the tornado had already passed. This is inexcusable. It is fortunate that no one was killed. Some other areas do not have tornado alarms at all. The major problem in the South, which contributes to the higher death tolls from tornadoes in the South than in any other region of the country (including the other two parts of Tornado Alley), is that of public awareness.

However, in general I do not mean awareness of the event as it is ongoing. Unless it is a nighttime event or is occurring when people are likely to be occupied with something else (such as watching a basketball game or being drunk, as the case may be), I think most people are aware if there is a tornado warning for their county. The local media are still the primary source of people’s information about weather warnings (most people are not plugged into the Internet 24/7, and as horrifying as it may seem to some of you, most of the South is still on dial-up). Most TV stations here interrupt regular programming to provide information. WCBI in the Golden Triangle region of Mississippi is good at providing detailed information about what is happening (though, in my opinion, there is a slight urban bias in what events warrant cuts in programming). When I say that there is a public awareness problem, it is referring more to long-term awareness. Awareness of the risk, which (as the paper and the graphic I linked above indicate) is much higher than they seem to realize. This is the sort of awareness that might lead people to build a tornado shelter underground, designate a specific room in the house to be shored up (which, if done properly, generally provides shelter against up to EF4 winds), and have a plan.

The affordability of shelter is a separate problem, and it is one that, as I said, positively sickens me. Back in the day, after the F5 tornado of May 3, 1999 that hit near Oklahoma City, there was a government rebate for people who purchased tornado shelters. It’s difficult in this day and age to make the case that the government should spend money on ordinary people, it seems, but it really wouldn’t hurt that much for it to at least be a tax write-off.

The Super Tuesday Outbreak demonstrated that, even in the modern era of NEXRAD, real-time weather forecasts, high-speed Internet, cell phones, and PDAs, a tornado outbreak lasting essentially a day and a half can still kill over 50 people. A single tornado, one that was not even rated “violent,” can kill over 20 people. Meteorologists for several years have been saying that there is a tornado awareness problem in the South, which is manifest in public complacency, poor shelter options, and ineffective tornado siren policies. Given that the Deep South is, quite clearly, the bull’s eye for the worst kind of tornado that the atmosphere can produce—long-tracked violent ones—this state of affairs is unacceptable. The Super Tuesday Outbreak will be repeated at some point—if not in the exact same geographical location in the South, then somewhere else in the region, and it will have an aftermath that is the same or worse unless something is done about the problem.

Mid-latitude cyclones, Severe

Epic Storm to Bring Severe Weather to the South

Any “weather weenie” is well aware of the fact that a major storm is coming for the eastern United States (yes, it really is that big). Accuweather.com and the Weather Channel are beginning the hype, Twitter is abuzz with forecasts and speculation (at least in the weather-weenie corner of it), and even the stolid National Weather Service has gotten in on this, with the Storm Prediction Center calling for “Particularly Dangerous Situation” winter storm warnings. (A PDS ordinarily is issued for severe weather—typically, in fact, for the type of severe weather event that has a historic tendency to generate violent tornadoes.) Indeed, blizzard conditions will be widespread across the Midwest, probably affecting an area as large as the “Storm of the Century” of March 1993. The culprits are a low pressure center and a mass of arctic air coming from Canada. The unseasonably warm air that we have enjoyed in the Southeast is going to fuel the system, so to speak.

I am not going to touch on the snow and ice aspects of this. The area that will undergo winter storm or even blizzard conditions is simply too large for me to give anything other than a large-scale overview, for one. And two, this is primarily a Southeastern blog as far as actual forecasts are concerned, and we in the South are going to have to deal with the system in a different way: severe thunderstorms. That warm Gulf air that will feed the system is going to be generating what’s known as a “warm sector.”

Mid-latitude cyclones such as this one have three distinct sectors: one to the east of the cold front, one to the west, and one that remains to the north. The north sector is what will be producing the snow and ice; north sides of cyclones typically have a constant flow of precipitation, because the cold front associated with the low does not actually pass over these regions. The west side is the “dry” side, in general (though there are exceptions); this is also typically a cold part of the system. That’s what’s going to bring a return to winter temperatures for the South; it will follow the passage of the cold front. And the warm sector is the southeastern side—the side that will have southerly flow from the Gulf of Mexico. These 60s and 70s that we’ve been experiencing, in other words.

There will be several factors that will create severe weather for us. I am going to illustrate these with images from the North American Mesoscale (NAM) model, downloaded from TwisterData.com. First is the location and intensity of the jet stream:

That image shows that the jet at 300 mb (~9000 m) will have 140-knot winds over ArkLaTex on Tuesday evening. The jet typically does have strong winds on the order of 80 knots, which is why it is a death sentence (via shear) for hurricanes in the summer, but these same winds that rip warm-core tropical cyclones to shreds create conditions that enhance cold-core cyclones in the cool season, when the jet is much farther south. 140 is quite strong even for winter. Furthermore, the location of the jet indicates that air will be drawn to the northwest along the eastern end of the strongest points—the exit region, in meteorological parlance. The image doesn’t appear to depict this, but the southeastern flow I am referring to is taking place vertically, and this image only shows winds at the 300 mb level. This will create areas of divergence in Mississippi, Louisiana, and eastern Arkansas. Divergence leads to a “vacuum” of sorts—an area of lower pressure—and therefore to rising air.

The second factor is the extreme temperature gradient.

Again, take a good look at Arkansas. This is an image from the same time as the one depicting the jet stream, but much lower in the atmosphere (850 mb, or 1500 m). The 850-mb low is located around Fayetteville, and you can see the wind flow around it. But look at the colors, which represent temperature. That is a very strong cold front, and the reason for it is the extremely unseasonable warm weather across the Gulf states. You can see the southerly flow of warm, moist air, creating the warm sector. The wind flow will be moving warmer air into areas of cooler air (the wind barbs are crossing isotherms), a process called warm-air advection. Look at the strength of the winds at this level, too. There are 65-knot winds forecast at 850 mb! This is hurricane-force.

Finally, the pressure gradient itself:

I will direct your attention to the isobars over the Plains. The low hasn’t even fully intensified yet, but already there is a 40 mb gradient because the high is so darn strong. It is this pressure gradient that will be generating the intense winds with this system, producing blizzard conditions north of the freezing line, and severe weather south of it.

Now, the $64,000 question: What type of severe weather are we talking about? None of the above sounds like particularly good news, but fortunately (for north MS, at any rate), I do have some comparatively good news now. I do not think our tornado risk is going to be all that high, nor our large hail risk. Instead—you guessed it—I think we are in line for some very strong winds as the cold front approaches us late Tuesday night/early Wednesday morning, combined with a lot of rain and the chance of small hail. There will be some instability, but CAPE levels are not going to be much higher than about 750 J/kg across Mississippi. The helicity index, a parameter reasonably predictive of supercell formation, is not expected to be very high. I think there is a chance of supercells in south MS (let’s say south of I-20), but at the present time I am not seeing this type of severe weather event on a large scale.

However, as people in north MS can certainly attest after the repeat punches of severe weather over the past year, high winds can be damaging enough on their own, and this system has the potential to generate some very strong winds for us. All in all, though, I think we are dodging not one, but several bullets with it.

Severe

Thoughts on Mobile Homes: A Problem with No Solution?

Yesterday and today an early-season severe weather outbreak occurred in the Central and Eastern U.S. The outbreak produced comparatively few tornadoes, but under the right conditions, it takes only one to do catastrophic damage—and that is exactly what happened when an EF4 struck the community of Lone Grove, Oklahoma.

EF4 and EF5 tornadoes are classified as “violent,” and they are known for flattening “fixed” buildings. The main difference is that EF4s blow houses down and EF5s blow them away. Obviously, in these tornadoes, survival above ground in ordinary buildings is not something that can be counted on. (Sometimes, in fact, survival in basements is not guaranteed, such as in the Parkersburg, IA EF5 tornado that struck in May of 2008. It ripped houses from their foundations and filled the exposed basements with debris.) But a mobile home is absolutely the worst place that one could be. It’s not any better than being in the open outdoors in EF3 and higher events. And, indeed, most of the fatalities associated with yesterday’s tornado occurred in a trailer park.

This is really no surprise. These structures are death traps in severe weather. Last year, 56 of 123 tornado-related deaths occurred in mobile homes. Continue reading Thoughts on Mobile Homes: A Problem with No Solution? →