August 2014 Climate and Hydrology Summary

August 2015

Tulsa: August 2015 was the 24th coldest (79.4°F, tied 2009, 1981; since 1905) and the 21st wettest (5.16", tied 1972; since 1888) August on record.

Fort Smith: August 2015 was the 52nd coldest (80.2°F; since 1882) and the 31st wettest (4.43"; since 1882) August on record.

Fayetteville: August 2015 was the 8th coldest (74.0°F, tied 1949, 1974, 2009; since 1949) and the 20th wettest (4.41"; since 1950) August on record.

Using the radar-derived estimated observed precipitation from the RFCs, rainfall totals for August 2015 ranged from around 0.10” in Choctaw County to around 10” in Ottawa County. Most of the HSA received 2”-5” of rain this August. This corresponds to 150%-300% of the normal August rain north of Hwy 412 and across much of west central AR, only 25% to less than 5% of average across Choctaw and Pushmataha Counties, and a mix of above and below average conditions elsewhere.

August 2015 was a case of the “haves” and “have nots” with a portion of the HSA receiving well below normal rainfall while other areas were well above normal. For the first time since February 2015, no mainstem river flooding occurred.

According to the Drought Monitor from September 1, 2015, Severe (D2) drought emerged across Choctaw and far southern Pushmataha Counties in southeast OK. Moderate (D1) drought was occurring over most of Pushmataha County. Abnormally dry (D0), but not in drought, conditions were located across the far southern portions of Pittsburg, Latimer, and Le Flore Co. Despite the extreme rainfall during the spring and early summer, from July 10th-Sep. 3, Choctaw and southern Pushmataha Counties received less than 20% of the normal rainfall (4”-6” below normal; 6th driest since 1921) for that time period. Much of the rain from the spring and early summer evaporated during July and August, hence the very high heat index values of 105°-115°. The lack of rainfall combined with the evaporation has depleted the soil moisture across southeast OK. This has resulted in the development of “flash drought.”

According to the Oklahoma Climatological Survey, the 30-day period August 2-31, 2015 was the 17th wettest for northeast Oklahoma, the 37th wettest for east central Oklahoma, and the 22nd driest for southeast Oklahoma. Records go back to 1921. For the 60-day period Jul 3-August 31, 2015, northeast Oklahoma ranked as the 8th wettest, east central Oklahoma was the 2nd wettest, and southeast Oklahoma was the 21st driest period. For the Water Year-to-date, October 1, 2014-August 31, 2015, northeast Oklahoma ranked as the 10th wettest, east central Oklahoma was the record wettest, and southeast Oklahoma was the 12th wettest period.

Outlook

The Climate Prediction Center (CPC) outlook for September 2015 (issued August 31, 2015) indicates a slightly enhanced chance for above normal temperatures across eastern OK and northwest AR. The outlook also calls for an equal chance of above, near, or below median rainfall across eastern OK and northwest AR. This outlook is based on both short- and extended range weather forecasts. The first third of the month is forecast to see anomalous troughing in the western U.S. and an amplified ridge over the eastern U.S., which is a pattern generally inconsistent with the ongoing El Niño conditions. However, by the end of the month, the pattern will become consistent with El Niño. The slightly enhanced chance for above normal temperatures is supported by short-, extended-, and month long dynamical models due to the anomalous ridge.

The CPC outlook for the 3-month period September-October-November 2015, CPC is forecasting an enhanced chance for below normal temperatures and above median precipitation across all of eastern OK and northwest AR (outlook issued August 20, 2015). This outlook is based primarily on both statistical and dynamical forecast tools, but does consider El Niño conditions and possible impacts.

According to CPC, weekly El Niño conditions are currently of strong strength. The oceanic and atmospheric conditions reflect a significant and strengthening El Niño. There is a 90% chance for El Niño to continue through the upcoming winter and an 85% for it to persist into early spring 2016. Forecasting tools indicate El Niño will peak at strong intensity by late autumn. However, El Niño impacts are generally most significant in the Southern Plains during the cold seasons. An El Niño Advisory is in effect.