My aim here is to vet the unofficial record highs and lows for the state of Washington. By "vet" I mean examine the readings in the context of the event (utilizing data from surrounding stations) and the geography of the area surrounding the station. Erroneous readings are fairly common in the COOP data, however NCDC for a long time accepted the COOP values as legitimate. Many of the values accepted as records have turned out to be likely erroneous, commonly due to bad siting and overexposure. There are also human errors introduced by the observers as well. This is a cursory attempt to list the most credible records by month after an extensive review of the available data:
Record high: 74 at Darrington on 1/31/1940 and Mud Mtn Dam on 1/28/1940, both areas influenced by downslope warming. The former corroborated by a reading of 66 at Sedro Woolley and the latter by a 70 at Buckley the same day.
Record low: -42 at Deer Park on 1/20/1937 supported by -33 at Chewelah the same morning.
Record high: 78 at Glenoma on 2/28/1986. 73 at Centralia and 75 at Oakville on 2/27. 74 at Everett and Sedro Woolley on 2/28.
Record low: -40 at Deer Park on 2/9/1933. -38 at Chewelah and -36 at Newport the same morning.
Record high: 87 at Kennewick on 3/30/1911. 86 at Mottinger on 3/29. 85 at Kiona on 3/30. Oregon recorded its March record high a day later.
Record low: -25 at Stockdill Ranch (Okanogan Co.) on 3/4/1955. Numerous corroborating readings the same morning: -24 Chesaw, -19 Republic, -17 Conconully
Record high: 100 at Wahluke on 4/29/1926. 98 at Kiona and Mottinger on 4/28. 96 at Sunnyside on 4/29.
Record low: -7 at Lake Keechelus on 4/1/1936. A known cold wave, -1 at Bumping Lake on 4/3.
Record high: 107 at Dallesport (The Dalles Airport) on 5/30/1986. 107 at Arlington, OR on 5/30 and at The Dalles, OR on 5/31.
Record low: 10 at Paradise Mt Rainier on 5/19/1987. Camp Muir reached 2 on 5/5/2010 (automated sensor)
Record high: 114 at Hanford on 6/28/2015. Greatest June heatwave on record...113 at Chief Joseph Dam, 113 Lacrosse, 113 Walla Walla airport the same day.
Record low: 13 at Paradise Mt Rainier on 6/1/1933. Camp Muir reached 11 on 6/7/2012.
Record high: 118 at Wahluke on 7/24/1928. 114 at Kiona the same day. 113 at Omak the same day. 113 at Walla Walla on 7/26.
Record low: 20 at Paradise Mt Rainier on 7/23/1924. Camp Muir reached 18 on 7/3/2012.
Record high: 118 at Ice Harbor Dam on 8/5/1961. 116 Connell, 115 Ephrata airport, 115 Smyrna, 115 Wilson Creek on 8/4. 114 at Dayton and Othello on 8/5.
Record low: 20 at Cascade Tunnell on 8/19 and 8/20/1896. Few stations in 1896 so this reading is tough to corroborate, however Wenatchee fell to 35 on 8/10/1896.
Record high: 108 at Glenoma on 9/3/1988. 105 at Battle Ground the same day. 105 at Packwood on 9/2.
Record low: 11 at Mt Adams RS on 9/25/2000 (known to be a cool period) and at Newport on 9/25/1934. Glenwood hit 19 on 9/23/2000. West of the Cascades Olympia was 26 on 9/23/2000.
Record high: 99 at Glenoma on 10/2/1987. 97 Packwood, 96 Toledo, 95 Snoqualmie Falls on 10/1.
Record low: -5 at Bumping Lake and Republic, both on 10/31/1935. -2 at Odessa, -1 Chesaw, -1 Newport the same morning. Earliest measurable snow on record in Portland, OR on 10/30.
Record high: 83 at Kosmos (later inundated and relocated to Glenoma) on 11/2/1949. 78 at Startup the same day.
Record low: -29 at Ellensburg on 11/28/1896. -23 at Sunnyside and -22 at Moxee the same morning.
Record high: 74 at Sedro Woolley on 12/5/1939 and at Wahluke on 12/21/1933. Pomeroy hit 72 on 12/5/1939 and Lowden (near Walla Walla) reached 71 on 12/21/1933.
Record low: -48 at Mazama and Winthrop on 12/30/1968. -43 at Chesaw 4 NNW on 12/29. -38 at Republic on 12/30.