1. INTRODUCTION Recent efforts within the National Weather Service’s Southern Region (NWS-SR) to refine criteria for excessive heat revealed high occurrences of extreme temperatures in the upper Red River Valley of southwestern Oklahoma and northwestern Texas. Recent efforts within the National Weather Service’s Southern Region (NWS-SR) to refine criteria for excessive heat revealed high occurrences of extreme temperatures in the upper Red River Valley of southwestern Oklahoma and northwestern Texas. Between 2000 and 2014, several observing sites within the area recorded more than 200 instances of daily maximum temperatures ≥40° C (105° F) with temperatures ≥43° C (110° F) occurring on average two to three times per year. Between 2000 and 2014, several observing sites within the area recorded more than 200 instances of daily maximum temperatures ≥40° C (105° F) with temperatures ≥43° C (110° F) occurring on average two to three times per year. This represents a 300% increase in the occurrence of temperatures ≥40° C (105° F) and a three- to eight-fold increase in the frequency of ≥43° C (110° F) temperatures relative to the remainder of the southern Great Plains for the fifteen-year period. This represents a 300% increase in the occurrence of temperatures ≥40° C (105° F) and a three- to eight-fold increase in the frequency of ≥43° C (110° F) temperatures relative to the remainder of the southern Great Plains for the fifteen-year period. This study reviews heat wave episodes in the upper Red River Valley of Oklahoma and Texas. The North American Regional Reanalysis (NARR) is used to identify and quantify mean synoptic-scale atmospheric patterns associated with forty multi- day heat waves between 2000 and 2014. This study reviews heat wave episodes in the upper Red River Valley of Oklahoma and Texas. The North American Regional Reanalysis (NARR) is used to identify and quantify mean synoptic-scale atmospheric patterns associated with forty multi- day heat waves between 2000 and 2014. Lastly, climatological evidence is examined to determine if the frequency of heat waves in the upper Red River Valley has increased during the recent southern Great Plains drought. Lastly, climatological evidence is examined to determine if the frequency of heat waves in the upper Red River Valley has increased during the recent southern Great Plains drought. SUMMARY Meteorological composites were generated using the NARR for forty multi-day heat waves that occurred in the upper Red River Valley of the southern Great Plains between 2000-2014. Composites showed 5925 m 500 hPa geopotential heights, a 1006 hPa MSLP trough (thermal low), and relative humidity <20%. A climatological peak in heat waves was noted in July. Heat waves appear to have increased since the 1990s. Twenty events have occurred since 2010. These periods have been characterized by frequent drought episodes in the southern Great Plains. Long duration heat waves tend to be associated with higher maximum temperatures. *For additional information or questions contact: Todd Lindley [todd.lindley@noaa.gov] NOAA/NWS Norman, OK Occurrence of temperatures ≥40° C (105° F) (left) and ≥43° C (110° F) (right) across NWS-SR between 2000 and 2014. High frequencies of extreme temperatures over the upper Red River Valley of Oklahoma and Texas are denoted (red circle). T. Todd Lindley* NOAA/National Weather Service – Norman, Oklahoma Composite mean 500 hPa geopotential heights (m) for forty multi-day heat wave episodes in the upper Red River Valley of the southern Great Plains between 2000 and 2014. Composite mean 300 hPa geopotential height, 500 hPa vector wind, 700 hPa geopotential height, mean sea level pressure (MSLP), 2-m relative humidity, and 2-m temperature valid at 00 UTC at the approximate mid-point of forty multi-day heat waves between 2000 and 2014. CLIMATIC PERSPECTIVE COMPOSITE ANALYSIS Nicholas Fenner NOAA/National Weather Service – Amarillo, Texas EXTREME TEMPERATURES 2000-2014 In the Upper Red River Valley of the Southern Great Plains Composite analysis derived from forty 2000-2014 heat waves defined as: Instances of upper Red River Valley surface observing sites measuring temperatures ≥40° (105° F) during two or more consecutive days. 00 UTC NARR composites valid for approximate mid-point of each multi-day heat wave episode. Key Characteristics of Meteorological Composites 5925 m 500 hPa geopotential heights centered over western north Texas. 1006 hPa MSLP trough (thermal low) over the southern Great Plains. 2-m relative humidity <20%, resulting in low temperature/heat index variance. Heat waves per month (left) and total heat waves per decade (right) between 1950 and 2014. Maximum temperature (°F) and duration (days) for forty multi-day heat waves (2000-2014). Note: magnitude/duration for many heat waves were the same and are shown as superimposed plots.