The Idaho state record high of 118 at Orofino (1,027') on 7/28/1934 has seemed suspect to me in the past. In an attempt to determine the veracity I utilized an ad hoc event analysis for the late July 1934 heatwave.
My main questions:
- Does the reading make sense given the topography (elevation, proximity to large bodies of water, continentality, aridity, etc)?
- Does the reading make sense when compared with other local observations with similar topographical traits?
Without further adieu let's examine the topographical traits of some nearby stations.
Lapwai (891') - about 20 miles downstream from Orofino and also situated along the Clearwater in a narrow valley.
Kooskia (1,259') - 25 miles upstream from Orofino and also situated along the Clearwater in a canyon.
For comparison Lewiston (757') at the confluence of the Snake and Clearwater rivers in a broad valley.
All four are COOP stations...with a COOP station the high temperature depends on the time of day the observer reads the max/min thermometer (which is commonly during the early evening hours...5 or 6 pm local time was common). If the observations were made prior to the high temperature of the day, or right at it, the next-day's high temperature may be reflective of the previous day's high rather than the current day's high. Keep that in mind when observing highs that seemingly occurred on the same day yet are shown one day apart on the COOP obs.
It is pretty obvious the above scenario occurred at Orofino...the high on 7/27 was listed as 117 and the high on 7/28 was listed as 118...there weren't two separate days of 117+, what happened was the observer recorded the high for 7/27 which was 117 at the time he recorded it, then he reset the max/min thermometer and checked it again the next day which showed a 118 because the high for the day prior occurred after the observation was taken that day. I believe the Orofino 118 degree reading actually occurred on 7/27 rather than 7/28.
Nearby readings on 7/27 include:
116 at Lapwai
116 at Kooskia
114 at Lewiston
113 at Pete King R.S. (1,550')
Whether a station is located in a canyon or in an open valley/plain does reflect in terms of the maximum attainable temperature under identical atmospheric conditions. In a canyon you have a greater percentage of heated land-surface near the station which can produce higher temperatures on the canyon floor than would be encountered in a broad valley at the same elevation. I believe this "canyon" heating effect is the main factor allowing for Lapwai (and especially Orofino and Kooskia) to get hotter than would be expected given the slightly higher elevations versus Lewiston. All four locations are semi-arid with similar vegetation.
Lewiston actually managed to hit 117 in the pre-airport era on 7/27/1939. Slate Creek (in the Salmon River Canyon) at 1,566' hit 117 on 7/13/2002. A similar effect allows for some spots along the lower portion of the John Day River in eastern Oregon to reach the 115-116 mark during strong heatwaves (e.g. Monument, Dayville)
In conclusion I think the 118 reading is plausible at Orofino, and I feel comfortable listing it as the state record. It's not a "lone wolf" hot reading that sticks out well above other heat records in the region at locations with similar topography and elevation.
Comments are welcome, as always.