June 5th, 2013

Severe Weather Outbreak in U.S. May 28 to June 2

Posted at 1:00 AM ET

mayoutbreaksmallA multi-day severe weather outbreak occurred during the period of May 28 to June 2 affecting a large area of the United States and Southern Ontario, Canada.  The outbreak occurred along a complex and evolving frontal boundary as it pushed through the mainland into and during the weekend.  Affected areas include the foothills of the Rocky Mountains, the South and Central Plains, the Mississippi Valley, the Midwest, the Great Lakes and New England.

According to Storm Prediction Center (SPC) and National Weather Service (NWS) reports, strong nontornadic wind and hail reports were common over an exceptionally large area.  Downed trees and power line reports were most common, together with reports of light to moderate roof damage, and structural damage from downed trees.

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Hazard data illustrated in the CAT-i map was taken from i-aXs®, Guy Carpenter’s web-based risk management platform. i-aXs users can view impacted areas on any map as well as see how their portfolios were affected. Please contact your broker or GC Analytics®   representative for assistance or go to www.i-axs.info for further information.

Some strong tornadoes were also reported, including a family of tornadoes near Moore, Oklahoma (OK) on May 31.  Excessive flooding was also reported in the same area.  Severe to complete damage was found with EF-2 and EF-3 tornadoes in Michigan, Oklahoma, Kansas, Missouri and Illinois.  The impacts in the Moore, OK and St. Louis, Missouri (MO) areas were particularly severe.

No initial loss estimates have been disseminated for this event.  Damage assessments, recovery efforts and NWS tornado track surveys are still ongoing in some cases.  The aggregate financial impacts of light to moderate roof and tree damage over such a large area will be considerable.  Of course the severe tornadic damage to the Moore, OK and St. Louis, MO areas will also impose heavy financial losses.

As always, our first thoughts and concerns are with those lost or affected by the events of recent days.

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Meteorological Discussion

The severe weather outbreak was enabled by warm moist air over a very large area of the United States and the Great Lakes.  A complex and evolving frontal boundary separating polar from tropical air extended from the Rockies to the Midwest to the Great Lakes, and progressed eastward during the period.  Strong upper-level winds above the frontal boundary provided enabling wind shear for the severe thunderstorms.  The frontal boundary itself provided the necessary trigger for storm initiation.  To further complicate matters, low pressure areas developed and moved along the boundary, further enabling the severe weather threat over a large area.

Strong nontornadic wind and hail reports were most common, with hail generally of dime to quarter size, over an exceptionally large area.  Several tornadoes were also reported including strong tornadoes in Michigan, Kansas, Missouri, Oklahoma and Illinois.

On May 28, severe storms erupted along a line from North Texas to the lower Great Lakes.  Reports included excessive hail, an EF-2 tornado in Genesee Co., Michigan, and a large wedge tornado reported in Ottawa Co., Kansas.  Reports for May 29 include an EF-2 in Montgomery and Schenectady Counties,  New York.  May 30 saw a widespread series of nontornadic wind events in Wisconsin and the Upper Mississippi Valley, together with isolated tornadoes, and widespread hail.

On May 31, a chain of severe thunderstorm cells developed near Oklahoma City, OK and progressed eastward along a West-East line.  A stable layer and dryline prevented storm initiation until the hottest and most unstable part of the day.  Storms in this area were particularly severe as a result.  At least five tornadoes were reported for this event alone, including an EF-3 near El Reno (Canadian Co.), two EF-1’s near Lightning Creek Park and Southwest Oklahoma City (Canadian and Oklahoma Counties), and two EF-0’s in Southeast Oklahoma City, and Moore Oklahoma (Oklahoma and Cleveland Counties).  Moore, OK was only recently subjected to a devastating EF-5 tornado.  Excessive rainfall due to the “training” of supercells over the same areas caused flash flooding over a wide area including Oklahoma City.  Reports of quarter-sized hail were widespread, with some unofficial reports of softball-sized hail.

A separate cluster of storms on May 31 impacted an area near St. Louis, MO.  One EF-3 caused severe damage on a path from Weldon to near the City of St. Louis.  For some affected locations along this track, only a few interior walls were left.  Another EF-3 initiated in extreme St. Louis Co. and progressed into Madison Co., Illinois (IL). Heavy damage was found in South Roxanna, IL to an industrial warehouse building.

On June 1 and 2, excessive nontornadic wind was reported over a very large area of New England, and a weak tornado was reported in Southern Ontario.

Regional Summary

Oklahoma

The state of Oklahoma experienced the worst of the severe weather outbreak.  According to the NWS, five tornadoes were confirmed in the Oklahoma City area.  The largest tornado was an EF-3 near the town of El Reno, which claimed the lives of three veteran storm chasers.  They were among the 14 fatalities reported in Oklahoma due to the storms.  Interstates 35 and 40 were excessively busy as residents attempted to outrun the tornadoes in their cars during rush-hour.  This caused an undue degree of danger for these individuals and their families.  Six individuals remain missing according to the Oklahoma City Fire Department.

Missouri

Tornadoes were reported in the St. Louis area on May 31.  The strongest, an EF-3 tornado with estimated winds of 150 miles per hour, carved a 32.5 mile long path through areas west of St. Louis.  At least 71 homes were reported to be heavily damaged in St. Charles Co., and another 100 had slight to moderate damage. Fortunately, no fatalities were attributed to the tornado, however, three people drowned in the state due to flooding.  Governor Jay Nixon declared a state of emergency due to spot flooding, wind and hail damage.

Lambert Airport in St. Louis also experienced damage from the May 31 storm.  Two aircraft hangars, three out buildings and a parking lot had extensive damage.  The NWS confirmed that the damage was due to a tornado.

Arkansas

Flash flooding claimed the lives of five people in the Ouachita Mountains, about 125 miles west of Little Rock.

Illinois

At least one tornado was confirmed in the southwestern part of the state. The tornado was reportedly near the town of Roxana in Madison Co. and determined to be an EF-3. No injuries were reported. However, heavy damage was reported at a warehouse building and thousands of residents remain without power. 

New England

Torrential downpours and damaging winds were reported in New England.  Thousands are still without power in Maine, Vermont and New Hampshire.  A rare tornado warning was issued for parts of Maine when radar indicated possible rotation.  No tornado has been confirmed for the area, however.

Ontario

In Southern Ontario, the fifth tornado of the year was reported on June 1 near Kenilworth (an EF-0).  Light damage with downed trees and power lines were reported in Southern Ontario on May 29, and a downburst was confirmed in Brampton.  Under the excessive rainfall on May 29, the Don Valley Parkway in downtown Toronto was flooded.  Numerous vehicles had to be pulled from the floodwaters.

Sources: U.S. National Weather Service (NOAA), U.S. Storm Prediction Center (NOAA), Associated Press, Reuters, Agence France Presse, Meteorological Service of Canada.

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Guy Carpenter publishes CAT-i reports for major natural catastrophes worldwide. These reports cover catastrophes including worldwide tropical cyclones, earthquakes, major UK and European floods and any other natural event that is likely to incur a significant loss to the (re)insurance industry. Please email CAT.i@guycarp.com if you wish to be added to the free email distribution list.

Guy Carpenter compiles RISK-i reports for major technological or man-made events worldwide. These reports cover risks to property, transport and life including explosions, fires, crashes, engineering disasters and terrorist attacks that are likely to incur a significant loss to the (re)insurance industry. Please email RISK.i@guycarp.com if you wish to be added to the free email distribution list.

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