( CNN ) -- High temperatures in America 's Plains States are expected to rival those in Death Valley again Monday , according to the National Weather Service , which predicts a heat wave to roast the county 's midsection for yet another day as it spreads to the east . The hottest spots located in Oklahoma through South Dakota should see highs over 100 degrees Fahrenheit , and top temps are forecast in the 90 's for most of the rest of the county -- with the exception of some mountain and coastal regions . Much of middle America baked in a heat wave Sunday , and the National Weather Service declared excessive heat warnings in at least 14 states , most of those in the upper Midwest . Several daily temperature records were broken -- from Alpena , Michigan , south to Miami , Florida . `` Heat index values '' -- how hot it feels outside -- have been running over 125 degrees Fahrenheit in the worst-hit areas , the National Weather Service said . The scale designed to describe how intense heat feels also includes factors such as humidity . `` This is the hottest it 's been for the longest period of time , '' said Emily McNamara from Sioux Falls , South Dakota , where the temperatures were expected to hover in the mid-90s through the middle of the week . Extreme heat is forecast to bear down on the eastern United States by the end of this week . Jacob Beitlich , a Des Moines , Iowa-based meteorologist for the National Weather Service , said that two factors contribute to make this current heat wave especially dangerous : the lack of a significant drop in temperatures overnight to allow people 's bodies to cool down and relatively high humidity , which makes it feel appreciably hotter than the thermostat may indicate . In Iowa , for instance , he noted that the impact of mid-90s temperatures have been compounded by relatively high dew points in the upper 70s and low 80s . These combine to make the heat index -- a meteorological measure for how it feels outside -- spike so that it feels as hot as 126 degrees Fahrenheit , according to the weather service . `` That takes a toll on your body , '' Beitlich explained . `` When it 's more humid , it 's more difficult to cool down from sweating . '' Prevent heat-related illnesses . The National Weather Service notes that typically extreme heat is the biggest weather-related killer in the United States , taking about 115 lives each year . That 's why it and other government agencies urge people to minimize their time outdoors in periods of extreme heat , drink plenty of fluids , and keep especially close tabs on the elderly and young people . In Missouri scores of cooling centers have been established in libraries , senior centers , schools and other sites around the state to help deal with continuously high temperatures . At Target Field in Minneapolis , Minnesota Twins supporters stood in long lines to get water from coolers , used spray fans , wrapped wet towels on their head during a Sunday game -- anything to keep cool . `` It is blistering out , '' said one fan , Jeff Ferris . `` Brutal . I 've probably lost 10 pounds today -- it 's crazy . '' While the recent stretch has been particularly difficult , it 's all part of what 's been a continuously long , hot and in many cases dry summer in numerous locales . The weather service indicates that Oklahoma , for instance , already has had 27 days this year in which the temperature surpassed 100 degrees . Not everyone in the region sweat it out though this weekend . McNamara notes that the Wild West Waterpark , where she is aquatic manager , has had `` record-breaking days in terms of attendance '' in recent days , with several thousand flooding in to get wet and beat the heat . `` It 's awesome , '' she said . `` You see people coming out having fun . It makes for a fast day . '' CNN 's Ted Rowlands and Jacqui Jeras contributed to this report .
NEW : No break from the heat Monday , says the National Weather Service . The National Weather Service declared excessive heat warnings Sunday for 14 states . High humidity has driven up heat indices , making it feel as hot as 126 degrees . The extreme heat is forecast to move east by week 's end , affecting the East Coast .