Over the last two days, severe weather has returned to the Central Plains in the U.S. This recent outbreak was by no means historic but it has become the most active severe weather outbreak thus far in 2015, with eight tornadoes, 31 wind reports and 162 hail reports, 13 of which were reported as 2″or larger. Given the tornado wind damage that occurred in the towns of Moore and Sand Springs, OK, it comes as no surprise that PCS issued its first Wind and Thunderstorm CAT bulletin of the year, although it is abnormally late for such an issuance, due to the lack of severe weather. In fact, since 2000, typically the insurance industry would have experienced just over 3, nearly 4 PCS loss events with an average of $1.3B in losses by the end of the first quarter.
Above is a look at the BMS iVision Verisk Climate hail size swath overlaid with the various Storm Prediction Center Local Storm Reports from the March 25 severe weather outbreak over the south-central plains.
There are no official tracks or ratings of the two tornadoes that impacted the cities of Moore and Sand Springs yet – those will come later today from the Tulsa and Norman, OK National Weather Service (“NWS”) offices – but, sadly, it has been verified that the Sand Springs tornado was the first deadly storm of the season. This comes later than the 20-year average for the first killer tornado of a given season (typically, February 11), but one month earlier than that of the 2014 season, which occurred on April 25.
The tornado that hit Moore, thankfully, looks to be not nearly as devastating as the same tornadoes that hit the city in 1999 (F5), 2003(F4) and 2013(F5). It is interesting, nonetheless, because it is not only the fourth tornado to hit the same general area in the last 17 years, but it also tracked in an unusual direction.
The image above, created by the NWS office in Norman from the work of Tom Grazulis, a tornado historian, shows many tornadoes that track over the Moore/Oklahoma City area travel in a northeast direction. The tornado yesterday, however, tracks in an atypical southeast direction, as the preliminary NWS map below illustrates.
The other thing that becomes apparent from analysis of the 156 documented tornadoes that have occurred in the Oklahoma City metro area (OKC), is this location appears to be a magnet for tornadoes – it experiences an average of just over one each year. Since weak tornadoes were not always documented prior to 1950, this number is likely well underestimated, according to NWS. In fact, Grazulis’ study confirms the OKC region has experienced 13 violent tornadoes (F/EF4 or stronger) since 1880, including the May 19, 2013 and May 20, 2013 tornadoes in Shawnee and Moore, respectively. Also through 2013, OKC experienced two or more tornadoes on the same day 26 separate times, with only three time periods since 1950 with an over two-year lapse between tornadoes.
However, OKC and Moore are not the only areas that have experienced similar tornado frequency. Statistical work from Florida State University’s Jim Elsner suggests there are many areas comparable to the size of Moore with just as many or more tornadoes occurring since 1950, as shown in the image below.
So, as the insurance industry prepares for the severe weather season, it is already apparent that Tornado Alley is appropriately named, since there are many areas within this region that experience the same tornado frequency as Moore. But, there is still no clear reason why, in recent years, Moore keeps getting hit by tornadoes. Studies have shown the affects of urban environments can sometimes enhance rain from thunderstorms downwind of cities (and Moore is just south of OKC), but little work has been done to determine if cities actually impact tornado formation. Future work in the insurance industry might answer these questions.