|Date of tornado outbreak:||May 1-4, 2018|
|Duration1:||91 hours, 22 minutes|
|Maximum rated tornado2:||EF5 tornado|
|Total Damages:||$65 Billion USD|
|Areas Affected:||Eastern United States, Canada, Mexico|
1Time from first tornado to last tornado
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Starting late April, a pattern shift had left most of the contiguous United States in a heat wave coupled with extreme humidity as a result of a powerful high pressure system. This had followed a relatively minor outbreak a few days prior that flared up as a result of a powerful La Nina that had suppressed tornadic activity severely for the past 4 months. Both January and February did not have a single confirmed tornado, March only had 14, and April only had 56, 31 of which came from the recent outbreak.
On April 29th, Dallas, TX recorded its hottest April day at 101°F, accompanied by 91% humidity. Strong storms often flared up randomly due to the unstable nature of the moist hot airmass, resulting in several weak to strong tornadoes by the end of the month. On April 30th, a deep low within a powerful trough dove into the Rockies. Winds within the jet were measured to be 150 MPH as a result of the broad sweeping nature of the of the jet. At 0600Z, an ENH Risk for Severe Weather was issued for April 30th as a result of this new set up. While relatively few tornadoes formed, it became evident that something bigger was at hand when taking into account the absurd CAPE and powerful jet that was sweeping down from the north. At 2000Z on April 30th, a Day 3 MDT Risk was issued for May 2nd. The sluggish movement of the High Pressure made it pretty clear that an Omega Block was eminent, and the jet would end up stalling over the South Central US for a time as a result, and would slowly work its way east as the High slowly moved out from off the coast.
On the morning of May 1st, an ENH Risk was issued for the day, with a hatched significant area for West-Central TX and NM. Due to high levels of MLCAPE and moderate Helicity, a significant outbreak was expected, but nothing too serious as a prominent capping inversion was present. This, coupled with the dense humidity, rendered most Supercells in an HP state, decreasing the overall threat for the day. While there were a total of 94 confirmed tornadoes, most of them were weak, and only two people died from the storms. One died when a tree feel on their car from an EF2, another died after being caught in a hailstorm. The main threat was flash flooding and large hail, with the most significant flooding occurring in central TX. The storms then quickly formed into a derecho that screamed across TX at 70 MPH. Significant wind damage was reported, with the highest gust being measured at 101 MPH over Dallas. This MCS would continue through TX before entering LA. While the strength of the system had decreased during this trek through the area, the sufficient amount of instability and humidity made it especially long track. The storm did not dissipate until it reached SE GA at 1200Z on May 2nd. Due to the prolonged nature of the storm, significant amounts of moisture was left in its wake. There was also a significant amount of convergence an outflow boundaries left behind as well, and due to the cooling from the storm, a significant capping inversion was now in place.
Originally predicted to be a Derecho event, due to the presence of many convergence and outflow boundaries as well as the presence of two deep Lows that formed behind the powerful MCS as it raced across the SE. Because of these Lows, the Capping Inversion created by the rain cooled air was expected to be swept away by the mid and upper level winds. At the same time, the newly formed Lows in combination with the High pressure off the coast dumped tons of moisture back into the SE, in addition with the humidity created by the MCS earlier that morning. The powerful S-SE moist inflow jets colliding with the WSW winds of the lows generated an extremely unstable atmosphere, with wind-flow intersections at 90° in some locations. This, coupled with the numerous outflow and convergence boundaries made instability sky rocket, with extreme Helicity and MLCAPE present across much of the Gulf States and into SE plains in the Arkansas-Tennessee area. Because of this strong inflow, a strong warm front formed, following the parent, upper-level Low situated over the Midwest states. As a result, a 60% hatched risk for an unprecedented Tornado outbreak was issued at 1200Z. Much of East TX, AR, and AL were included, along with all of MS and LA. Tornadoes here were expected to be long tracked and violent. The most violent weather was expected to be in E TX.
Unlike the other regions and systems, the Low in TX had access to a plentiful dry airmass in Mexico. This formed a prominent dry line that severely contrasted with the moist air to its east. This formed a Triple Point, a system of Drylines, Warm/Stationary fronts and Cold fronts. Because of the extremely volatile setup, there was great concern for East TX, prompting an immediate issuance of a PDS Tornado watch over the entire region, including parts of AR, LA, and OK. Further PDS Watches were issued for the rest of the day, and due to the sluggish movement of the system, several remained in place for the next day.
At 10:00 PM CDT in extreme SW AR, Gravity Waves and convergence boundaries began to collide with one after the other, forming numerous small systems and quickly evolved into complexes of towering cumulonimbus with severe hail and gusty winds. The first tornadoes of the main event of the outbreak formed along a stationary front situated on the AR-TX-OK border. These tornadoes were short lived, however one of them did cause EF3 damage in Winthrop, AR, killing one. Storms continued to explode over most of AR and LA as the Eastern Low dipped SE into Northern AR. The shift in wind shear and continuous collision of boundaries resulted in an explosion of hundreds of discrete supercells. It didn't take long before the first EF5 of the day formed just north of De Queen, AR. Extremely long-tracked, this storm tore through the heart of Vilonia, AR, killing 115. This tornado formed as three others merged into a 2 mile-wide wedge. The most shocking aspect of this was how quickly it spun up, as the tornado started inflicting EF5 damage mere minutes upon forming. As a result of the explosion of storms, numerous outflow boundaries collided with the numerous convergence boundaries, causing even more storms to flare up. And due to the constant collisions of each boundary, each storm was to be long lasting.
At 12:00 PM CDT, 1700Z, storms exploded over MS and LA. Numerous Violent tornadoes tore through the area, including one EF5 that formed over East TX and traveled 259 miles through LA and dissipated in MS, killing 307. Numerous violent and destructive tornadoes were reported throughout the early afternoon as Storm Spotters scattered frantically to report. By 1730Z, over 100 tornado warnings were active as storms ripped through the SE. Several long-tracked and violent tornadoes were often accompanied by numerous satellites and weaker siblings from the same storm. Triplets weren't uncommon, and occasionally some storms had as many as four tornadoes down at once. One notable complex featured a dozen weak tornadoes at the same as the massive system swept across the Ohio valley. This complex would go on to tighten into an extremely long track EF5 that tore through northern Kentucky.
By 1830Z, Storms in the east had begun to form into MCSs, with discrete cells trailing on the southern ends as they drifted NE. By 1900Z, they had formed into massive MCS that would go on to produce numerous significant tornadoes throughout the rest of the day as well as significant wind damage and flooding.
At around 1800Z, a Dryline moved in from the WNW into East Texas. As it moved along, moisture built up considerably, and inflow from the gulf had increased as a shortwave swept into NE TX/SE OK. This inflow intersected the Dryline at about 70°, and due to the speed in which the flow of air on each side of the boundary converged into one another, extreme levels of Helicity had built up at the surface and mid-levels of the Troposphere, coupled with a fast upper level flow from the SW, allowing for incredibly tall storms. This allowed for incredibly powerful updrafts, keeping precipitation suspended and the bases dry, allowing for incredibly visible and photogenic storm structure. Despite this, storms on the southern end of the Dryline moved at an ESE direction due to the Dryline becoming stationary, parallel to the coast, and perpendicular to the inflow due to the strength of the flow from the gulf from the upper low in combination with the shortwave low forcing the Dryline northward into a bowing shape, with a strong kink in the boundary along along NE Texas in the Dallas-Fort Worth area.
Along this kink in the boundary would the first storms form at 1850Z. These storms would quickly become violent, doing high-end EF4 damage in the Dallas-Fort Worth area and into Sulfur Springs before cycling and dropping several strong tornadoes into AR. At 1900Z, more storms exploded along the line, with Echo Tops as high as 65kft in minutes. Powerful updrafts lofted hailstones into the stratosphere, accumulating in size and dropping as grapefruits miles from core of the storm due to the strong upper level winds. Each one of these storms would go on to produce at least one violent tornado, as the powerful inflow generated a sustaining convective system that kept the storms dynamically driven and long-tracked, even when rain-wrapped. One of these tornadoes qould go on to tear 104 miles through Downtown Houton, causing extreme EF5 damage and claiming 4,035 lives and injuring over 200,000. This storm would go on to set the records for Widest Tornado and Fastest Winds on Earth, 2.9 miles wide and 321 MPH winds at 30ft off the surface respectively. This day saw a record 233 tornadoes confirmed, eclipsing that of the 2011 Super Outbreak on April 27, 2011, which had 218 tornadoes confirmed.
Storms in Texas and the Southeast would go on to form numerous MCSs and would go on to produce several tornadoes into the next day as they retreated quickly northward as a result of the strong, moist low-level SSE jet. This allowed conditions in the SE to return to an extremely volatile state. However without the prolonged simmering of instability that was present before May 2, conditions failed to return to the extremes they were the day before, though this doesn't change much considering the once in a life time situation in regards to May 2.
Greatly resembling April 27, 2011, this day saw 192 tornadoes, 15 of which were violent. 8 were EF4 and 7 were EF5. This is due to a vast area of Helicity of around 500 m2/s2 to 700 m2/s2, and cape values as high as 5000 J/kg. This allowed any storm in the area to develop a strong rotating updraft, and provided an environment that allowed sustained cyclogenesis. The best example of this sustainability in the conditions is a supercell that formed in central Louisiana and traveled over 700 miles before dissipating in North Carolina. During its lifespan, it produced a total of 10 tornadoes; 7 EF3s, 2 EF4s, and 1 EF5, and is responsible for the deaths of a total of 451 people and ≈$18 Billion USD in damages. At numerous points in its life, two tornadoes were on the ground at the same time, the most infamous of which being the Birmingham Twins, an EF4 and EF5 that traveled together over much of Alabama before the EF4 dissipated in far east Alabama and the EF5 dissipating shortly after crossing into Georgia.
All 7 EF5s would tear into or through Alabama, along with 5 of the EF4s also seen on this day. All of these would end up being killers, and some towns would be practically wiped off the map. The city of Cullman would be hit with a maximum bound EF4 with winds of 200 MPH; the downtown area completely destroyed resulting in 30 dead. Demopolis would take a direct hit from an EF5 with winds of 220 MPH, resulting in 52 fatalities. Many more highly populated areas would be hit throughout the day, with Decatur, Florence, and Huntsville, AL all taking hit from violent storms, resulting in a total loss of 217 lives.
In total, >1,000 will lay dead from the storm, and more than >$20 Billion USD in damages would be inflicted as storms passed through. States of emergency and economic collapse in some states after the May 2 and May 3 tornadoes would be inevitable as these two days along would be responsible for more than >6,000 fatalities and well over >$60 Billion in USD in damages.
Tornadic activity would continue on into the night and next morning due to the storms, like the nights previous congealing into a line of embedded cells that would produce numerous weaker tornadoes. This line would produce a few violent tornadoes later in the day, but the main activty was actually out towards the plains. A decently strong High over the Rockies pinched up great levels of instability in the Central Plains. Due to the eratic weather patterns and summer-like conditions in terms of temperature and humidity, instability could quickly recover even after a strong system had moved through.
With CAPE values well into the 7000 J/kg range, a slight risk was issued over a small region in the plains in anticipation for windy conditions and large hail. However, subtle stationary boundaries from the storms on Tuesday had lingered in the area since. Combined with the powerful upper level jet and tight ridging over the region, supercells exploded off of these boundaries and towered well over 60kft. The intense updrafts combined with the powerful flow of the upper level jet allowed the storms to spin up violently. Soon a massive cluster of violent thrunderstorms begain to envelope the KS-MO-NE border region. Descrete cells in the late afternoon congealed into a line of embedded supercells, whose powerful updrafts within the powerful flow spun up numerous violent tornadoes, many more than any would have suspected. At one point, one could drive parallel to the line of cells and see 5 ongoing EF4 and EF5 tornadoes. One of these EF5 storms, packing 300 MPH winds, tore through the suburbs Kansas City, killing 305 and destroying thousands of homes. This is one of three EF5s that would terrorize Kansas and Missouri, the other two developing to the west of the Kansas City storm from separate cells. The other two wouldn't go on to be as deadly, though in total they claim 27 lives.
Along with the 3 EF5s, the other 11 tornadoes in this system were low to high EF4 storms. These often came in pairs, similar to Pilger, and most were rainwrapped and brief, with their ratings based on spotty damage among the rural country in Missouri and Kansas. This storms only claimed 9 lives total and did pidly in total damages, but is still one of the most violent outbreaks in history, and the most violent outbreak per square mile than any before it.
These storms eventually fell victim to the power of the ridge and bowed out into a Derecho that raced south for several hundred miles, doing extensive wind damage and killing 5. The highest gust observed was in Little Rock AR at 101 MPH. The next day would see next to no activity, with only two tornadoes being reported, and only one confirmed, but soon the next day, a new system sweeps in to do even more damage to the already worn nation.
- Main article: Houston-Pasadena Tornado
The 2018 Northwest Houston-Pasadena Tornado was a particularly intense EF5 that touched down to the NE of Brenham in Washington County, TX and was on the ground for over 2 Hours as it tore a path of destruction from Benham to Seabrook, killing 4,035, injuring more than 200,000, and doing $17.89-billion USD in damages. To this day it remains as the costliest and deadliest tornado in history. This was one of 37 EF5s confirmed during the May 1-12, 2018 Tornado Outbreak Sequence.