Destructive Moore, Oklahoma Tornadoes
A very strong storm system moving across the central United States has produced deadly tornado touchdowns and significant property losses. The National Weather Service reported 16 tornado touchdowns on Saturday 18 May, 29 touchdowns on the 19 May and 31 on 20 May. Two tornado touchdowns south of Oklahoma City (OK) were responsible for an estimated 26 fatalities (2 fatalities from an EF4 touchdown on 19 May, and 24 fatalities from an EF5 touchdown on 20 May). Insured property losses are expected to range from $2 to $5 billion. Advance tornado forecasts and warnings were not sufficient to reduce the loss of life from these events.
Tornadoes are violently rotating columns of air that extend between a cloud and the surface of the earth and are generally spawned by thunderstorms. The central plains see the greatest tornado activity in the US, and at an approximate rate of 7.5 tornadoes per 10,000 sq. miles per year, Oklahoma has one of the highest densities of tornado activity in the country. Tornadoes range broadly in intensity, and the Enhanced Fujita scale is frequently used to characterize the intensity of touchdowns.
Tornadoes identified as EF4 intensity carry winds estimated to range from 166 to 200 mph, and EF5 tornadoes carry winds in excess of 200 mph. In comparison, a Category 5 hurricane includes maximum winds higher than 155 mph – EF4 and EF5 tornadoes carry much higher wind speeds, but over a much smaller affected area. Historically, tornado intensity has been assigned based upon associating the damage observed to estimated winds, so there are few precise estimates of the peak winds from these events.
The tornadoes over the period 18-20 May have damaged thousands of buildings, many of them completely destroyed and insured losses are expected to tally to between $2 and $5 billion. The Moore, OK tornado alone has caused damage to approximately 13,000 structures and is where the bulk of the losses are expected.
Tornado activity so far in 2013 has been below the long-term average, with only 342 tornado touchdowns to date (23 May). Activity is only about one-half the average over the last 8 years, and less than one-third the pace of the record activity years of 2008 and 2011. Overall tornado activity does not trend well with losses to life and property however, as most tornadoes do not impact populated areas and produce small monetary losses.
NOAA’s Storm Prediction Center produces the most broadly distributed weather forecasts, and the forecasts during this period did correctly identify the precursors for severe tornado activity, with an outlook for 20 May that included:
ANOTHER ROUND OF SEVERE STORMS IS FORECAST AS A STORM SYSTEM MOVES FROM THE SOUTHERN ROCKIES THROUGH THE SOUTHERN PLAINS TODAY AND INTO THE OZARKS AND MIDDLE MISSISSIPPI VALLEY TONIGHT. THE AREA OF GREATEST RISK APPEARS TO EXTEND FROM FAR NORTH TEXAS INTO CENTRAL AND EASTERN OKLAHOMA...FAR SOUTHEAST KANSAS...NORTHWEST ARKANSAS AND SOUTHERN MISSOURI. A VERY MOIST ATMOSPHERE WILL BECOME QUITE UNSTABLE AGAIN TODAY. THIS COMBINED WITH STRONG FAVORABLE WINDS ALOFT WILL RESULT IN A RISK OF A FEW STRONG TORNADOES...VERY LARGE HAIL...AND DAMAGING WINDS IN THE MOST INTENSE STORMS.
The forecast for 20 May included a graphical representation of the areas of highest risk, and this area can be compared to the touchdowns that were reported that day.The area of highest warning extended over most of Oklahoma, and included parts of Arkansas, Kansas and Missouri.
Satellite photos of the central US in that time reference clearly identify the area of thunderstorms but highlight the challenge in precise warnings – the tornadoes of 20 May impacted less than 10 square miles, but the thunderstorms of the day covered thousands of square miles. Doppler radar – based warning systems did provide about 15 minutes of precise warnings in the affected areas, but the results show that even this amount of time is – a major improvement over the last few decades – however, it still is insufficient to eliminate fatalities.
The current climate warning models do not necessarily imply increased thunderstorm activity. The warmer, moister atmosphere forecast by the climate warming models do provide more fuel for thunderstorms, but thunderstorms also need wind shear to produce strong tornadoes and it is not yet clear whether average wind shear will increase in tornado-prone regions and seasons, with the current climate models implying a decrease in average wind shear.
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