And when temperatures are plotted on maps, a warm bull's eye is often apparent around Yakima.
Let me show you. Here is a plot of the number of days over the last two years that the temperature exceeded 90F. There is a localized maximum (dark red) near Yakima (a map, with Yakima County indicated, is also shown for reference).
What about average minimum temperature? Another localized warm maximum around Yakima. In fact, Yakima has the warmest low temperatures in the state!
A plot of the high temperatures yesterday around the Yakima area, show the Yakima Airport (YKM) is warmer than any place else in the neighborhood (63).
So what is going on at the Yakima Airport? It is something of a mystery. The National Weather Service says they checked out the sensor and it seems to be calibrated ok. The Yakima instrument suite is typical of U.S. airports: NWS/FAA ASOS (Automated Surface Observing System).
Using Google Maps, there is an image of the airport area, with downtown Yakima to the north and east. The oval shows the location of the Yakima weather instruments.
And here is a blow up of the weather instruments area. The instruments are in a vegetated area between two runways.
So what is going on? The airport is relatively low, so that would contribute to daytime warmth, but not at night. The vegetation is dry, so it would warm up more than irrigated fields, but that doesn't explain the nighttime warmth. And besides, some of the cooler neighborhood weather instruments are on unirrigated land or in urban areas. The neighboring runways could warm up during the day and perhaps release heat at night.
The high resolution UW WRF model forecast for 5 PM Monday (below) does not suggest a warm bullseye over the airport--so it does not indicate some strange (but real) local weather feature.
There have been a number of reports (and publications) bringing into question the sensor used in the NWS ASOS stations: the H0-83 hydro-thermometer (see picture). These papers (example here) suggest a number of possible problems, including insufficient ventilation of the enclosure (the white object below).
So the origin of the warmth at Yakima is still not clear, but the reported start in 2011 does suggest some instrumentation issue.
Finally, this example explains something many of you have asked in the comments...why are there isolated warm and cool areas on the climatological maps I show (like the average maximum temperature map over the past two years below). The answer is probably bad sensors or unrepresentative locations (like a sensor near a building or over concrete). My discipline needs to do better.