Jim Holcomb & Bob Robinson, Clearwest, Inc.
The main features of the 2003 spring frost season were: 1. The early start due to a mild winter; 2. An outbreak of cold air in late February; 3. The relative lack of cold nights during the spring in the Okanogan district compared to areas farther south, and 4: A cold period in mid-May, which attacked areas of the Central Basin that usually do not have frost this late in the season.
A very mild winter led to early bud development in the southern districts and soft fruit growers needed protection beginning in February. No cold outbreaks of Arctic type air occurred during the main winter period, until late February. On February 23, cold air originating over north central Canada, poured across the Rocky Mountains into the Columbia Basin. While this was not a severe Arctic outbreak , minimum temperatures dropped into the low and mid teens during the next two or three mornings and protection was needed for advanced soft fruit.
Temperatures through the following spring featured generally cool days, but close to the normal number of cold nights. Clouds and wind generated by frequent upper level storms that developed just offshore, kept minimum temperatures warmer than would have been expected, considering the coldness of the air on many nights. This was especially true for the Okanogan district. The Okanogan averages more cold nights in April and May compared to the Wenatchee or Yakima districts. This spring, it had less than its normal, and fewer cold nights than either the Wenatchee and Yakima districts. The Wenatchee and Yakima districts had slightly more than the normal of cold nights but temperatures, for the most part, were not too severe. Only on the mornings of April 4th and 5th did the temperatures fall below 25 degrees during the main spring period.
A late spring cold period occurred from May 15th to the 19th. While cold temperatures are not unusual in the colder Cascade valleys this late in May, they usually don’t occur in the warmer areas of the Central Basin. An area around Quincy experienced an unusually cold morning on May 16th, especially in the western part of the Quincy basin. The temperature at the Quincy PAWS fell to 29.8, the latest in May that temperatures have gotten this cold at this station since the record began in 1989. It was unusual in that other stations in the Basin farther east remained above freezing. This was likely due to two factors. One was the very cold air mass that moved in during this period, the coldest air that has moved into Eastern Washington this late in May in the past 20 years or more. This was deep cold air and well mixed by the wind so little or no inversion formed. This type often brings coldest temperatures to higher elevation sites if winds become light. And the second factor affecting cold temperatures on May 16th was that the wind died out or became too light to hold temperatures up in the higher elevations of the western Quincy Basin, while the wind blew stronger and kept temperatures warmer in the areas farther east.
ARE MORE COLD
NIGHTS OCCURRING IN MAY?
A LOOK AT THE
POSSIBLE RELATIONSHIP BETWEEN SPRING COLD NIGHTS AND THE PACIFIC
(inter)DECADEL OCCILATION (PDO)
Like El Niño, the PDO is a short-term climatic shift indicated by changes in sea surface temperature. The PDO, however, is over a much longer time period, and focuses on the north Pacific, usually lasting two to three decades (20 to 30 years) before changing phase. While El Niño focuses on tropical waters and changes phase on the order of 6 to 18 months.
The PDO was publicized in 1997 in connection with its historical corrleation to Alaskan salmon population in the north Pacific Ocean. The PDO has two phases, a warm and cold, which have been identified over the past century. The most recent phases are: 1925-1946 (warm), 1947 to 1976 (cold), 1977 to 1997 (warm), and recently, the shift to a cold phase beginning in 1998. Unlike other indices, PDO can vary widely within a phase, even changing polarity for a short time. This makes it difficult to identify exactly when the phase of the PDO changes.
Using the only consistent long-term frost key station data, TFREC Wenatchee, the number of cold nights in May were plotted versus time on the attached graph. Averages of the number of cold nights for each phase of the PDO were plotted as horizontal lines through each PDO phase.
Because of the very few phases of the PDO this century, there is not sufficient historical frost data to provide statistically significant conclusions. However, for this short period a definite shift in the average number of cold nights in May can be seen, from the warm to cold phases. It should be noted that there are a number of years during both the warm and the cold phase where no frost days occur in May. However, there is considerably more variability in the number of frost days in the cold phase (zero to 8 days in the cold phase and zero to 3 days in the warm phase.)
If a true correlation does exist between PDO and number of frost days in May, what can be said about future May frost days? Since we are at the beginning of a cold phase of the PDO, it is possible that the future frequency of May cold nights, on the average, will increase over those experienced from 1977-97. In colder orchard locations this frequency may be more pronounced than at a moderate location like the TFREC. And although very few years have elapsed in the current cold phase, some growers have already experienced a more pronounced increase since 1998. In addition, the number of May cold nights may vary greatly from year to year. Using history as a guide, a few years may still have no cold nights, while other years could have an extreme number.