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

Extreme Heat Continues to Plague South Central States

Dangerous heat is once again roasting South Central states, with heat advisories and warnings in effect through the end of the week in eight states — including nearly all of Arkansas, Louisiana, and Oklahoma. So far this month, numerous heat records have been shattered across the U.S., and for some locations in the lower 48 states, July 2012 may go down in history as the warmest of any month on record.

Forecast high temperatures for the U.S. on July 30, with the area of excessive heat outlined in red. Click on the image for a larger version. Credit: NOAA/NWS.

According to the National Weather Service, Tulsa, Okla., likely set a record on Monday for the warmest overnight low temperature since records began there, as the temperature failed to drop below a toasty 88°F through this Monday morning. The previous all-time record high minimum temperature in Tulsa was 87°F degrees set in both 2011 and 1980.

Tulsa has had 16 100-degree days so far this month, including back-to-back days with highs of 108°F on July 28 and 29. Through July 29, the month was running at 5.3 degrees above average in Tulsa.

Wichita Falls, Okla., set a daily record high of 111°F on July 21, and Oklahoma City set a record high of 109°F as well. Both records were the hottest readings in those locations this year. 

On July 29, every single weather station that is part of an extensive statewide network of weather observing instruments -- known as the Oklahoma Mesonet -- reached or exceeded 100°F. That is just the third time on record that has happened, the Weather Service said.

In St. Louis, through July 29 the month was running at 8.3°F above average, with 14 100-degree days, and just two days when temperatures failed to exceed the upper 80s.

Oklahoma high temperatures on July 29, showing that every official thermometer in the state reached or exceeded 100 degrees F. Click on the image for a larger version. Credit: NWS/Oklahoma Mesonet.

Not only is the heat dangerous for human health — heat is the No. 1 weather-related killer in the U.S. each year — it is also causing problems with weather monitoring equipment. On July 29, the National Weather Service reported that the Doppler Radar in Ft. Smith, Ark., had to be shut down due to extreme heat and air conditioning problems. Such radar units are crucial for monitoring severe thunderstorms and tornadoes, among other weather-related hazards.

The heat is exacerbating the extreme drought conditions while also feeding off the parched soil, since dry conditions can lead to more heat extremes.

Nationally, the U.S. recorded its warmest January-to-June period on record and its warmest 12-month period. The warm June followed the warmest spring on record, which was the culmination of the warmest Marchthird-warmest April, and second-warmest May. This marks the first time that all three months during the spring season ranked among the 10 warmest, since records began in 1895.

In a long-term trend that demonstrates the effects of a warming climate, daily record-high temperatures have recently been outpacing daily record lows by an average of 2-to-1, and this imbalance is expected to grow as the climate continues to warm. According to a 2009 study, if the climate were not warming, this ratio would be expected to be even.

On shorter timescales, record temperature imbalances can be much more significant, and this year-to-date, record daily highs have been outpacing record daily lows by a ratio closer to 9-to-1. Or, when you look at all warm temperature records, including overnight low temperatures compared to all cold temperature records, the ratio is closer to 7-to-1.

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