A look at weather extremes and the big-picture climate connections.

Early Season Heat Wave for Memorial Day Weekend

Memorial Day weekend marks the unofficial start of summer, but a large swath of the U.S. will skip right to mid-summer heat this weekend, likely breaking records and leading to one of the hottest Indy 500 races on record.

Forecasts for Sunday's Indianapolis 500 call for a high of 93°F. According to the National Weather Service (NWS), only five Indy 500 race days had highs in the 90s, and the all-time record high for the month of May is 96°F. That means that Sunday's forecast would rank as one of the warmest race days in history, and one of the warmest days in May on record.

In Indianapolis, high temperatures may reach or exceed 90°F on at least two consecutive days this Memorial Day weekend. That has only occurred 18 times during the month of May since weather records began there in 1871, the NWS reported.

Indianapolis won't be alone. The heat will stretch from the southern Plains to the Mid-Atlantic, affecting cities ranging from Kansas City to Washington, D.C. The weather system responsible for the heat is an area of high pressure that will establish itself over the Southeastern U.S., pumping hot and humid air northward around the backside of its clockwise circulation. Thunderstorms will flare up around this high, dumping heavy rains with the threat of damaging winds across the Upper Midwest, in particular.

In Peoria, Ill., the normal high temperature for Memorial Day is 78°F, and the all-time record high for the holiday is 98°F, which occurred in 1934. The forecast for this Memorial Day calls for a high of 89°F.

In Chicago, Memorial Day temperatures typically range from 70°F to 79°F, the NWS reported. The highest temperature on record for the holiday is 93 degrees, which occurred in 1942 and 1953. Last year, on May 30th, 2011, the high temperature was 88°F. The current forecast for Memorial Day calls for a high temperature near 90°F in the Windy City.

Other cities likely to be affected include  Little Rock, Ark., Louisville, Ky., and St. Louis. According to The Weather Channel, if Memphis reaches 97°F or greater this weekend it would break the record for the hottest temperature so early in the season.

Studies have shown that early season heat waves carry more public health risks than those that occur during mid-summer, largely because people have not yet acclimated to warmer weather. Climate change studies show that extreme heat events are becoming more likely as overall temperatures warm, although this particular taste of mid-summer will be brief.

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