July 2013 the 30th Warmest July on Record for the U.S.

July 2013 was the 30th warmest July in the contiguous U.S. since record keeping began in 1895, said NOAA's National Climatic Data Center (NCDC) in their latest State of the Climate report. A persistent trough of low pressure over the middle of the country brought unusually cool conditions there, and strong ridges of high pressure over both the East Coast and West Coast brought unusually warm conditions to the West and Northeast. Massachusetts had its warmest July on record, and ten other states in the Northeast and West had top-ten warmest Julys on record. Four southern states had top-ten coolest Julys on record. The number of record warm highs and lows was roughly the same as the number of record cold highs and lows during July. For the year-to-date period January - July, both temperature and precipitation over the contiguous U.S. have been above normal, ranking in the upper 35% and 21% of years, respectively.

According to NOAA's U.S. Climate Extremes Index (CEI), which tracks the percentage area of the contiguous U.S. experiencing top-10% and bottom-10% extremes in temperature, precipitation, and drought, July extremes were slightly above average, and the year-to-date period January - July 2013 was also slightly above average.


Figure 1. Historical temperature ranking for the U.S. for July 2013. Massachusetts had its warmest July on record, and nine other states in the Northeast and West had top-ten warmest Julys on record. Five southern states had top-ten coolest Julys on record. Image credit: National Climatic Data Center (NCDC).

Wet July increases tropical storm flood risk
It was a very wet July for the contiguous U.S., ranking as the 5th wettest July since 1895. Florida had its wettest July on record, and twelve other states, ten of them in the Eastern U.S., had top-ten wettest Julys on record. As a result, soil moisture levels are high over portions of the Eastern U.S., increasing the chances of damaging flooding should a tropical storm or hurricane hit. The latest 2-week forecast from the GFS model keeps the Southeast U.S. under a very wet weather pattern through August, and the latest 1-month and 3-month precipitation outlooks from NOAA's Climate Prediction Center also give the Southeast above-average chances of wetter than average conditions. Lake Okeechobee in Florida is 2.1' above average for this time of year, and nearly 6' higher than two years ago. This puts the lake just 4" below what is considered high water, and Lake Okeechobee water levels are a concern as we head into the peak part of hurricane season.


Figure 2. Historical precipitation ranking for the U.S. for July 2013. Florida had its wettest July on record, and twelve other states had top-ten wettest Julys on record. Oregon had its driest July on record, and Washington and Oregon also had a top-ten driest July. Image credit: National Climatic Data Center (NCDC).


Figure 3. Soil moisture for August 12, 2013, expressed as percent average of the soil moisture observed between 1916 - 2004. Portions of the Eastern U.S. have high soil moisture levels, increasing the odds of damaging flooding should a tropical storm or hurricane hit this year. Texas and Western Louisiana are dry, and could use the rains from a tropical storm. Image credit: University of Washington Variable Infiltration Capacity Macro-scale Hydrological Model, which includes soil moisture, snow water equivalent, and runoff.

Drought conditions remained relatively unchanged during June and July. According to the August 6 Drought Monitor report, about 45% of the contiguous U.S. is still in moderate or greater drought, compared to 44% at the beginning of June. The U.S. Seasonal Drought Outlook issued on July 18 calls for little overall change in the U.S. area covered by drought conditions through October. Despite the drought, the year-to-date total acreage burned by wildfires is the second lowest in the past ten years.

Jeff Masters