User:TheAustinMan/sandbox/May 5–10 tornado outbreak sequence

The May 5–10, 2015 tornado outbreak sequence was primarily a significant severe weather event that impacted much of the Great Plains and surrounding regions in the United States. The prolonged severe activity was caused by several troughs and upper-level lows generally tracking over the same areas. Over the course of six days, at least 34 tornadoes occurred across nine states; four of these reached EF3 intensity and three caused fatalities. Widespread flooding, particularly in the South Central United States, accompanied the tornadoes and caused two additional fatalities. Strong winds and large hail were also reported throughout the Great Plains, at times causing extensive damage. Overall, the severe weather event caused 7 deaths and 70 injuries and largely contributed to the Storm Prediction Center receiving 722 reports of tornadoes, wind, and hail over the six day period. Although the bulk of severe activity occurred during this time frame, isolated instances of severe weather associated with the same overarching weather systems that caused the tornado outbreak were documented before May 5 and after May 10.

The first forecasts by the SPC illustrating the potential for a widespread and long-duration tornado outbreak were made on April 28. Forecasts of such a severe weather event continued up until the end of the tornado outbreak. Isolated severe weather occurred across the U.S. Great Plains beginning on May 2. Three days later, a spike in severe activity including tornadoes occurred over the Texas Panhandle and Oklahoma, marking the beginning of the tornado outbreak sequence. On May 6, numerous tornadoes were reported along a line extending from southern Nebraska to Texas' North Central Plains. Widespread flooding occurred in Oklahoma; the National Weather Service forecast office in Norman issued a flash flood emergency for the Oklahoma City metropolitan area following record-breaking rainfall that occurred over the area. A slight drop in severe weather occurred from May 7-8, with the bulk of tornadoes, strong winds, and large hail occurring around the Red River and Dallas–Fort Worth metroplex areas.

On May 9, severe activity increased once again, covering a large swath of the Central and South-Central United States. A large EF3 tornado in the Cisco, Texas area produced significant damage and caused one death, making it the first fatal twister during the tornado outbreak. Several other tornadoes were reported in western Nebraska and eastern Colorado. The following day, severe weather shifted eastwards away from the Great Plains. Two tornadoes—one impacting Van, Texas as an EF3 and the other impacting Nashville, Arkansas as an EF2—caused two deaths each alongside extensive damage. The tornado in Van, Texas also caused 43 injuries, accounting for most of the documented injuries throughout the tornado outbreak. By May 11, tornadic activity had tapered off as the final weather disturbance associated with the severe weather event tracked towards the Atlantic Ocean. However, thunderstorms over the Ohio River valley produced intense winds, resulting in numerous reports of thunderstorm wind damage over the region.

Meteorological synopsis
The early-May 2015 tornado outbreak sequence was the result of several upper-level disturbances interacting with a moist, unstable air mass over the central United States that was conducive to the development of widespread thunderstorms. From May 2 to May 11, the Storm Prediction Center (SPC) received 165 tornado reports, 439 reports of hail larger than 1 in, and 425 reports of strong thunderstorm winds across the United States, many of which were due to the severe weather outbreak. On April 28, the SPC began to indicate the possibility of a severe weather event for May 5-6 over New Mexico and Texas in the form of a shortwave trough. However, the predictability of the event was too uncertain to make a conclusive assessment of the probability for severe weather on that day. Confidence in the development of a tornado outbreak remained low as the event approached due to disparity between weather forecast models.

May 2–4
On May 2, a cold front originating from an area of low pressure over Manitoba swept across the Dakotas. The front was also accompanied by a shortwave trough that tracked through the same areas. As the day began, the SPC indicated a marginal risk for severe activity over a narrow swath of the northern Great Plains due to the potential for the formation of isolated thunderstorms. During the late-afternoon hours of May 2, isolated thunderstorms developed across eastern North Dakota. Some of these storms produced hail and strong winds in North Dakota and Minnesota. A 58 mph wind gust was recorded at Hutson Field Airport near Grafton, North Dakota. Concurrently, daytime heating and the convergence of air over central South Dakota and Nebraska caused by the shortwave trough resulted in the development of storms with the potential to reach severe levels. However, these storms were limited by the presence of marginal moisture, wind shear, and atmospheric instability. This cluster of showers tracked eastward and produced hail peaking at 1.5 in in diameter in Colfax County, Nebraska. Most severe weather was confined to a narrow swath of Nebraska where the amount of convective available potential energy was high enough to maintain storms of such intensity.