Heatwaves lead to 4th hottest July in US history

Thursday 11th Aug 2011 by theWeather Club

It’s official, (well almost). Last month in the America was very, very, hot. The persistent heat waves in the central and eastern regions of the United States were so intense that they dragged up the national average, making it the fourth warmest July on record nationally, according to scientists at NOAA’s National Climatic Data Centre.

The average U.S. temperature in July was 25°C, which is 2.7°C above the long-term average based on records for the years 1901-2000. Oklahoma and Texas had their warmest months ever on record, with average temperatures of 31.6°C and 30.6°C, respectively. Oklahoma's state-wide average temperature was the warmest monthly state-wide average temperature on record for any state during any month. 41 of the lower 48 states had above-normal, much-above-normal, or a record warmest July. Only seven of the lower 48 states – all west of the Rocky Mountains – experienced a July with temperatures near or below the 20th century average.

The South climate region - Arkansas, Kansas, Louisiana, Mississippi, Oklahoma, and Texas - had its warmest single calendar month for any climate region on record. The average temperature of 30.1°C broke the previous record of 29.9°C set in July 1980. Dallas exceeded 37.8°C (100°F) on 30 of the 31 days in July. In Oklahoma City, July was the warmest single calendar month, with an average temperature of 31.8°C, beating the previous record of 31.5°C set in August 1936. Washington, D.C.’s Reagan National Airport had its warmest single calendar month on record, with an average temperature of 29.2°C, breaking the previous record of 28.4°C set in July 1993and July 2010.

The July heat wave was characterized by unusually warm minimum temperatures, during nights and early mornings. This is typical of U.S. heat waves in the last decade, and consistent with increasing warm summer night time extremes observed across much of the country beginning in the late 20th century. Oklahoma had its warmest May-July period, the South climate region had its second warmest, and in 18 other states temperature averages were in their top 10 for the same three month period.

The hot weather was caused by a persistent trough/ridge weather pattern set up across the U.S., bringing above normal temperatures to the eastern half of the country and below normal temperatures to the western third. So while the citizens of Dallas were sweating under the sun’s fiery onslaught, Washington State having its coolest May-July period on record, and the Northwest climate region tied with its second coolest.

While these are based on preliminary figures and there will be some minor revisions, the overall picture will remain the same, large areas of the USA were warmer than the long term average for this time of year, a trend that has become increasingly common in recent years.