The weekend's deadly storm outbreak in the Midwest may cost upto $1 billion, says RMS, and could rank as the most expensive ofthe top five catastrophes of its kind to have occurred in Novembersince 1950.

“Sunday's big tornado outbreak is yet another atypical storm ofwhat has been an unusual 2013 severe weather season,” said MatthewNielsen, RMS director and meteorologist in a statement. So far, thethree largest tornado outbreaks of 2013 have occurred in January,October, and November, all outliers of the usual spring/summerpeak.

More than 80 preliminary tornado reports were filed in relationto the storms that killed six people, as well as incidences ofhurricane strength winds, heavy rain, lightning and large hail. Thestorm system affected as many as 10 states, including Illinois,Indiana, Kentucky, Missouri and Michigan.

State Farm spokeswoman Holly Anderson says the insurer hasreceived 3,300 homeowners claims in Illinois, 1,900 in Idaho, 1,000in Michigan, and 400 in Ohio as of Wednesday.

The greatest damage occurred in Illinois, where tornadoes andthunderstorms flattened entire neighborhoods. One EF-4 tornado inNew Minden, Ill., was one of only 20 tornadoes of similar strengthto occur during November since the middle of the last century.

In Chicago on Sunday, thousands of customers were left withoutpower, an NFL football game was delayed, and flights were delayedat two major airports.

“It will be a week or two until there is a better sense of thelosses,” says Steve Bowen, senior meteorologist at Aon Benfield.“The main takeaway from Sunday's outbreak is that most of the majormetropolitan areas in the Midwest were largely spared fromextensive damage. Had one of those metro regions taken a directhit, the level of devastation and losses would be even greater thanwhat we're already seeing in some of the smaller communitiesimpacted in Illinois and Indiana.”

The weekend's storms were a turning point for an otherwise calmyear for convective storms and tornadoes—previously, says theInsurance Information Institute (III), only 818 tornadoes wereconfirmed this year, the lowest annual count since 1988.

Now, however, there have been five severe weather outbreaks inthe U.S., the third-highest total in the country's history behindthe 2011 and 2012 seasons. The most expensive and deadliest singletornado outbreak occurred in May and devastated Moore, OK. whichcost about $562 million in insurance payouts.

RMS says the magnitude and severity of Sunday's tornado outbreakwas caused by unseasonably strong thermodynamic instability andstrong wind shear. Spring and summer months are good hosts forstorms because of their strong daytime heat. According to the III,nine out of the top 10 costliest U.S. tornadoes occurred betweenApril and May. One occurred in October.

Winter deflects storms with acute sun angles, short days, andcooler temperatures—but late autumn provides strong upper levelwinds and sharp temperature contrasts.

Specifically, says Aon Benfield, the conditions that coalescedin supercell thunderstorms and long-track tornadoes on Sundayincluded a rapidly deepening surface area of low pressure in theMidwest/Great Lakes; moisture and heating ahead of the advancingcold front; the presence of an upper level trough; strong windshear; and a vigorous mid-level jet stream with wind speeds inexcess of 120 mph.

Furthermore, Sunday's storm was unique because of the geographicspan of its tornado warnings, which reached Wisconsin and Michigan.RMS's Nielsen tells PC360 that the powerful Sundaytornadoes in Illinois may “go into the record books” as the secondand fourth most northerly EF4s in November.

Nielsen says North American jet stream is further north thanusual in 2013, leaving the storm-struck region vulnerable toanother outbreak for another week or more, whereas averagethunderstorm activity at this time of year occurs near Arkansas andMississippi.

RMS says the storms highlight “the importance of capturing thesetypes of events from a catastrophe modeling perspective, even ifhistorical records don't.”