PROSSER, Wash. — If you thought that the cool weather would last forever, think again. Just in time for fall, summer-like weather finally arrived in central Washington in Sept.. Although the calendar now says October, the recent warm weather has many residents saying “better late than never.”“This year, many Washington locations experienced the third warmest Sept. on record, and Prosser recorded five consecutive days with temperatures reaching 90 degrees or greater from Sept. 9 to 13,” said Nic Loyd, meteorologist with Washington State University’s state-wide AgWeatherNet.
The heat wave starting in early September was the most extensive period of warmth of the year and occurred much later in the season than normal. With the exception of a couple of cool periods in the middle and end of the month, most of Sept. was unusually warm. The average daily high temperature at Walla Walla of 82.3?F was the warmest since at least the early 1990s, and was five degrees above normal for September. Overall, the central Washington September average high temperatures in 2011 were 3 to 4 degrees above normal. This continued the 2011 trend of anomalous weather, except that the weather was very warm instead of very cool as in the spring and early summer.
“Although the high temperatures came too late for most crops, it helped grapes catch up somewhat,” said AgWeatherNet director Gerrit Hoogenboom. The total growing degree days accumulated from April 1 through Sept. 30 for 2011 was very similar to 2010 and slightly less than the five-year average for 2006 through 2010.
The most notable aspect of the recent September heat, however, occurred in western Washington. Mt. Vernon’s weather station experienced numerous record warm readings during the month. “The cause of the unusual nighttime warmth was the unusually early appearance of the ‘pineapple express’ weather pattern,” Loyd said. This weather setup brings warm, moist air from the tropics into the Pacific Northwest. Normally, this occurs in late fall and winter. A warm air mass coupled with cloudy, moist and windy conditions led to record warm low temperatures. In fact, the three days from Sept. 22 to 24 were the warmest September days since 1994, when the station records begin, and the average temperature each day was warmer than the typical September daily high temperature. The average daily temperature of 73.2 degrees on the 23 was five degrees warmer than any other September day on record at Mt. Vernon, and was slightly warmer than any previous August day as well.
The September warmth was not confined to western Washington, as eastern Washington experienced several notable temperature extremes. The high of 98.8 on Sept. 9 was the second warmest September temperature ever recorded at the AgWeatherNet station at WSU’s Tree Fruit Research and Extension Center in Wenatchee. The Orondo weather station north of Wenatchee reached almost 101 degrees on the 12, which is quite late in the season to exceed the century mark. The Gramling weather station south of the Tri-Cities recorded its third warmest September day since 1989 with an average temperature 79.4?F on the 9th.
Washington crops are generally faring well despite the strange weather of the past year. The cold early growing season has left the pear crop 10 days behind schedule and smaller than last year. The 2011 Yakima Valley pear crop yield estimate is 126,000 tons.
Despite recent warmth, apples remain behind schedule by up to two weeks due to the cool spring. The production outlook calls for a harvest of 106 million boxes of apples. The main concern with a late crop is the potential of an autumn frost before completing the harvest of later varieties like Pink Lady and Fuji.
Grapes are running 10 days late, but are catching up due to recent warm weather. However, the forecast for early October suggests a return to cool and wet conditions. Since weak La Niña conditions are currently redeveloping in the equatorial Pacific Ocean, the autumn outlook calls for a slightly elevated probability of wetter than normal conditions.