Washington Drought Report: July 2015
Overview Although it may seem obvious to our readers, the recent weather has not been helpful for the drought situation. June was a record smashing month whose sweltering legacy persisted into early July. Unfortunately, heat and water stress are becoming increasingly serious issues as the realities of hot, dry weather and high water demand collide with the realities of low water supplies and a depleted snowpack. Growers are reporting decreases in crop quality and potential yield as a result of the parched conditions.
On July 9, Whitcomb Island concluded a period of 14 consecutive days with high temperatures of above 100 degrees that began on June 26th. Accumulated evapotranspiration (ET) values have also been above normal, even as water supplies dwindle. During the late June heat wave, temperatures were as hot as 114 degrees, while lows were as sultry as 85 degrees. Although June was historic in its own right (Figure 1), it is only the latest remarkable chapter in a series of incredibly warm months dating back to mid 2014.
Figure 1-Courtesy AgWeatherNet
- The June 2015 mean Prosser temperature was 8.1 degrees above normal, which makes it by far the warmest month (relative to normal) on record.
- The last time that any calendar month at Prosser was at least 6 degrees above normal was January 1990 (+6.1 degrees).
- No rain fell at Prosser during June.
- The June accumulated ETr (alfalfa evapotranspiration) value was 9.73 inches, which is nearly 2 inches above the recent 5 year (2010 to 2014) average value of 7.8 inches.
- Numerous locations across central Washington recorded 8" depth soil temperatures in the 90 to 95 degree range in early to mid July due to the long duration of excessive heat. These abnormally high soil temperatures represent a surprising reserve of subsurface heat in the soil.
Figure 2 shows the reason for the historic June heat: a massive ridge of high pressure (red) centered over Washington.
Figure 2-Courtesy ESRL
Outlook Unfortunately, the overall drought situation has also worsened slightly from an outlook perspective since the previous report. Many climate indicators, including various dynamical and statistical models, continue to suggest enhance odds of warmer and somewhat drier than normal conditions through mid 2016. Recent observations indicate a strengthening El Niño. In fact, only once since 1982 has El Niño been stronger in June than it was last month. Forecasts suggest a strong likelihood (90+%) of El Niño persisting through next winter. Although it is important to keep a proper perspective on short term, month to month changes of long duration cycles like the Pacific Decadal Oscillation, the PDO index did increase again last month, following a brief but promising 5 months of decline from near record values. A large mass of anomalously warm ocean water persists off the west coast at the present time.
Figure 3 shows the average snowfall by ENSO phase at Paradise, Mt. Rainier, which is one of the snowier locations in the Cascade Mountains. With El Niño expected next winter, below normal snowfall is a good bet.
Figure 3-AgWeatherNet, Data Courtesy NPS
Figure 4 is a composite image of the global weather pattern of a typical winter when the PDO and ENSO indices are both positive, which is the forecast for the coming 2015/2016 winter. The composite image is created by averaging together the weather pattern from each historical positive ENSO/PDO year. By analyzing the global atmospheric circulation during these historical analog years (years similar to this year), we can better assess the outlook for next winter. Unfortunately, an anomalous trough (purple) tends to form in the Pacific Ocean during positive ENSO/PDO winters, while a ridge (yellow/green) is present over Washington and western Canada. One can think of troughs as valleys of cool air, or dips in the jet stream, while ridges are peaks of warm air. The jet stream generally flows from west to east, and is normally forced to the south by these troughs, before moving northward on their eastern flank. As a result, air masses are likely to enter Washington from the south more often than normal, which increases the probability of a warm and low-snowfall winter. It is interesting to note that the (July 1 to June 30) 2014/2015 winter snowfall at Paradise was only 266 inches, which is by far a record low value. Records are available for most years since 1920, and the previous record low was 313 inches in 1939/1940.
Figure 4-Courtesy ESRL
Despite the recent heat relief, abnormal warmth should return this weekend, before subsiding around the middle of next week. Unfortunately, there is still no indication of any widespread or soaking rainfall events, and temperatures will generally remain above normal. If there is a silver lining in our present bleak situation, it is that strong El Niño events are generally followed by a shift toward La Niña for the following winter. Even if 2015/2016 becomes another lost winter, we can hope for better snow returns in 2016/2017, since history will be on our side. Statistically speaking, 2015 is very likely rock bottom in terms of heat and lack of snow, which means that conditions can only improve as we approach the latter part of this decade.
As is always the case, we encourage folks to check in with AgWeatherNet for weather/climate and decision aid information. AgWeatherNet's irrigation scheduler allows for the efficient utilization of water resources, such that enough water is supplied to maintain the health of the crop without wasting water. Also, our Growing Degree Day (GDD) calculator shows how accumulated heat units are progressing this year relative to previous years.