The temperature at Nowata, Oklahoma, fell to −31°F (−35.0°C) at 7:40 a.m. CST (GMT - 6) on 10 February 2011, establishing a potential new minimum temperature record for the state of Oklahoma. This replaced the old record of −27°F (−32.8°C) set at Vinita on 13 February 1905, at Watts on 18 January 1930, and at Guthrie on 4 January 1947. The temperature at several other Oklahoma weather stations fell to −27°F or colder on 10 February 2011, breaking or tying the previous state record; these were at Bartlesville Municipal Airport (−28°F), Blackwell 4SSE Mesonet (−27°F), Medford 1 SW Mesonet (−27°F), Miami (−28°F), Pryor Mesonet (−28°F), and Ralston (−29°F). The purpose of this article is to describe the circumstances of the new record-cold temperature at Nowata following the manner of previous reviews of state or regional cold records.

The Nowata station is located in Nowata County (36°44′37″N, 95°36′28″W) at 206-m elevation above sea level. The station is part of the Oklahoma Mesonet (www.mesonet.org) and was established on 1 January 1994. The Oklahoma Mesonet consists of 120 automated weather stations that transmit data in near-real time to the Oklahoma Climatological Survey at the University of Oklahoma. Temperature is measured 1.5 m above the soil surface with a Thermometrics thermometer with accuracy of 0.5°C, and housed in an aspirated radiation shield. This is similar to the temperature sensors in the U.S. Climate Reference Network that Lin and colleagues (in a 2005 study) found to be a few tenths of a degree cooler than a nonaspirated thermometer of the maximum-minimum temperature system (MMTS) while temperatures were cooling at night over snow-covered ground. Lin and colleagues attributed the small “bias” to the nonaspirated MMTS thermometer. The area within 1,000 m of the station is open, flat terrain with agricultural land use (Fig. 1). The station is not in a valley or depression that might contribute to the development of extreme cold, as exists at some other locations of state cold records.

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The weather station at Nowata taken on 11 Feb 2011, the day after the record temperature.

Citation: Bulletin of the American Meteorological Society 94, 4; 10.1175/BAMS-D-11-00169.1

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Severe winter weather was widespread across Oklahoma during the first 10 days of February 2011. A snowstorm on 31 January–1 February brought high winds and 30–48 cm of snow from Oklahoma City to Tulsa and Bartlesville, including Nowata. Another snowstorm on 8–9 February brought deep snowfall across northern Oklahoma, including a state record 24-h snowfall of 68.6 cm (27 in.) at Spavinaw, 65 km southeast of Nowata. Snow depth of 38 cm on the morning of the cold record was measured by the landowner of the property where the Nowata Mesonet station is sited. This is consistent with surrounding weather stations.

The average temperature for February 2011 at Nowata was 1.0°C, and February 2011 was about 1°C cooler than average in Oklahoma. The highest temperature for the month at Nowata was 79°F (24.4°C)—remarkably, just seven days after the cold record was established. The 110°F (61.1°C) temperature change at Nowata was the greatest in a seven-day period in Oklahoma's recorded history. [The temperature at Nowata reached 111°F (43.9°C) on 2 August 2011, creating an annual range of 142°F (78.9°C).]

Strong surface high pressure (1,032 mb) was centered over Oklahoma on the morning of 10 February 2011. The combination of clear skies, calm winds, and deep fresh snow cover contributed to rapid overnight loss of heat from the surface. At night, the active radiation exchange surface is at, or very near, the actual snow surface, and the lowest temperatures occur at the snow surface. The poor diffusivity of snow results in fast surface cooling in the overlying air and the development of intense temperature inversions based at the surface. The temperature sensor at Nowata was 1.5 m above the soil surface but only 1.12 m above the snow surface and thus closer to the effective radiating surface. The temperature sensor at 9-m height at Nowata registered a minimum temperature of −30°C on the morning of the record, giving an inversion lapse rate of 0.67°C m⁻¹ between 1.5 and 9 m. The lapse rate between 1.5 m and the snow surface may have been greater than the lapse rate above 1.5 m, but due to the lower effective height of the sensor, the impact of the 38-cm snow depth on the minimum temperature at 1.5 m is expected to be less than 0.5°C. Snow depth is not a factor in accepting a temperature as a record, but it does have a possible local effect on temperatures.

The counties where the temperatures fell to −27°F or below on 10 February 2011 are in the Tulsa or Norman NWS County Warning Areas. Forecasts issued through the day and evening of 9 February 2011 for the morning of 10 February from those two NWS forecast offices called for lows of −3° to −13°F from the Tulsa office and −8° to −15°F from the Norman office.

The Nowata station is not in the NWS weather observer network, so it is not included in the monthly summary publication, Climatological Data—Oklahoma. The February 2011 issue of Climatological Data—Oklahoma indicates the coldest temperature in Oklahoma was −29°F on 10 February at Ralston. This exceeded the previous Oklahoma cold record of −27°F. According to Climatological Data (by state) from the National Climatic Data Center, the coldest temperatures in neighboring states on 10 February 2011 were −17°F at Lipscomb, Texas; −18°F at Fayetteville, Arkansas; −24°F at Cassville, Missouri; and −26°F at Mound Valley, Kansas; and the coldest temperature in the entire United States that day was −39°F at Embarrass, Minnesota.

Possible all-time state record meteorological observations are investigated by a combination of federal and state officials. When the observations in question are brought to the attention of either a local NWS Forecast Office or the state climatologist, those officials can ask for an assembly of the State Climate Extremes Committee (SCEC). The SCEC was created in 2006 by the National Climatic Data Center (NCDC) to enhance and stabilize the evaluation of weather observations that may have either tied or broken all-time state records. Once verified by the SCEC, NCDC then becomes the official keeper of those records. The SCEC is composed of officials from the NCDC and NWS regional headquarters, local NWS forecast offices, the Regional Climate Center, and the office of the state climatologist particular to the location of the possible record observation being investigated.

The primary concerns of the SCEC in the investigation of the −31°F reading at Nowata were the siting of the Mesonet location, the calibration of the temperature instrument, and the snow depth below the instrument (although the latter would not disqualify a record). The siting of the Mesonet site was approved by the SCEC with very little discussion. Oklahoma Mesonet locations are chosen with great care on level terrain away from anthropogenic influences or natural obstructions. Photographic evidence from the day after the possible record-low reading indicates a snow depth of approximately 38 cm (15 in.). Since the temperatures remained below freezing through that time, it is unlikely that any melting occurred between the time of the reading and the time of the photographs, although there may have been some settling. The Mesonet temperature instrument was removed by Oklahoma Mesonet technicians soon after 10 February and found to be in proper working order after testing in the Oklahoma Mesonet's calibration lab. With all concerns satisfied, the SCEC voted unanimously to approve the −31°F reading at Nowata on 10 February 2011 as the new lowest minimum temperature record for the state of Oklahoma.