Climate
Dallas has a Mediterranean climate (Köppen Csb) with warm to very warm, dry summers with cool mornings, and cold, rainy winters. Occasionally frigid weather will reach the Willamette Valley due to very cold continental air from Canada being driven over the Cascades by a low-pressure system to the south, as occurred repeatedly in January 1950 when temperatures reached as low as −11 °F (−23.9 °C) on January 31, 1950, and 68.0 inches (173 cm) of snow fell. However, snowfall is generally very rare, with an annual mean of 4.9 inches (12 cm) and a median of zero.
Rainfall is generally heavy during the winter months, averaging over 6.50 inches (170 mm) from November to February, when rain falls on around seventeen days in an average month, and on all but one day in November 1983. The wettest month has however been December 1996 with 21.93 inches (557.0 mm) and the wettest "rain year" from July 1973 to June 1974 with 80.01 inches (2,032.3 mm). As with most of Oregon, the driest "rain year" was from July 1976 to June 1977 and saw only 23.78 inches (604.0 mm).
Spring arrives slowly with pleasant afternoon temperatures and less heavy rainfall by April, although showers are common until into June. High summer in July and August is very warm in the afternoon and generally dry, with no rain falling for 79 days, between June 23 – September 9, 1967, which saw the hottest month on record in August 1967 where the mean maximum was 92.1 °F (33.4 °C). Low humidity and pleasant mornings make this season comfortable, although airflows from the hot continent can bring spells of sweltering and arid weather, with 106 °F (41.1 °C) reached on July 19, 1956, and on August 8, 1981. On average, eighteen afternoons will top 90 °F (32.2 °C) but only two can expect to reach 100 °F (37.8 °C), while 62 mornings fall below freezing, but only two spells (in January–February 1950 and December 1972) have ever seen temperatures as low as 0 °F (−17.8 °C).