What's Up With The Weather?

Author: Nick Andrews

Publish Date: Summer 2010

Even for Oregon this is a wet spring. Long term residents of Western Oregon often say that you can’t rely on dry weather until after July 4th. 2010 looks like another year that could validate the saying. Scientists try to predict the future by studying natural phenomena. Weather forecasters are one the few scientists who are regularly expected to look in their crystal ball and tell us what’s going to happen next. A tough job, but on the whole they’re getting it right this year.

2010 is an El Niño year, and this explains what we’ve experienced this spring. This year we had 13.8” of rain from March through June 10 according to Agrimet data taken at Hyslop Farm in Corvallis, the second heaviest rainfall in that period since 1990 (table 1). The heaviest rainfall in that time period was in 1993 (16.5”). Experienced farmers report that it has been one of the most challenging springs they can remember for working the soil. The rain has been very well distributed and many fields have been too wet to cultivate through at least mid June. As a result, early season farmer’s markets and CSA shares might lack a little luster this year.

George Taylor (Applied Climate Services, Corvallis) was the State Climatologist for Oregon from 1991-2008. He said this spring is not a record year for wet weather or for cool weather, but “the combination of unusually wet and cool weather is remarkable”. By the 15th, June 2010 became the wettest June on record with 4.2” of rain accumulating at the Portland airport. The previous June rainfall record was set in 1984 with 4.06” of rain for the month. On June 15, records were also broken for the lowest daily maximum temperature, with a high of 57°F in Portland, the previous record was 60°F set in 1954. With well distributed rainfall and cool temperatures, soil is certainly very wet this spring! Soil moisture records don’t go back far enough to confirm whether soil moisture levels are breaking meaningful records. However, Gordon Kenyon (Crop Production Services) explained that “rainfall is recharging the soil at a much higher rate than is normal for this late in the season and cool temperatures are keeping evapotranspiration rates lower than normal so the soil isn’t drying out. This is a very wet spring and a small minority of farmers will be able to save money by irrigating less though others are unable to irrigate because river levels are too high to place pumps in the rivers. Cool, wet soils can also be problematic as many crops are either not planted, stall their development, or have disease issues in anaerobic soils. Crops that are particularly sensitive to “wet feet” will struggle the most.”

Len Coop (OSU Integrated Plant Protection Center) reviewed historical precipitation and evapotranspiration data between March 1 and June 10 at Hyslop Farm in Corvallis since 1990 when these records started (table 1). Evapotranspiration equals evaporation plus transpiration. Precipitation (PPT) minus evapotranspiration (ET) provides an estimate of how much moisture is being added to the soil from rainfall and how much is being lost from evapotranspiration, and can be used as a gauge to estimate muddiness. Water is also lost by soil drainage, but this is a very site specific factor. PPT – ET can provide a relative soil moisture index that allows us to compare soil moisture across a region and over time within different soil types. Len’s analysis confirms grower’s observations that this is one of the wettest springs in memory as far as soil moisture is concerned. PPT – ET is higher this spring (3.45”) than any other year on record except 1998 (3.87”). The winter of 1997-98 was the strongest El Niño ever recorded.

Silver lining: below average precipitation through February was followed by wet spring conditions that rescued us from possible water supply problems. Locally, the total precipitation for October-February was about 90 percent of average. However, in the Cascades, total precipitation for that period was only 74 percent of average and the snowpack at the end of February was just 40 percent of average. If we had gotten a warm, dry spring we would now be concerned about drought.

The El Niño-Southern Oscillation (ENSO) is a climate pattern in the tropical Pacific Ocean. On average, it occurs about every five years. Last winter saw moderately strong El Niño conditions. Normally winds in the tropical Pacific blow from the east (Americas) to the west (Asia), taking the surface water with them. The water heats up because it is exposed to the sun for a long time. Cool, nutrient rich water rises to the surface in the Eastern Pacific. Cooler water means less evaporation and thus drier air. Therefore, the west coast of South America has a very dry climate. The warmer water in the western Pacific promotes high rates of evaporation and generally wet climates.

During an El Niño the trade winds from east to west weaken or even reverse. Warm surface water from Asia (western Pacific) can then flow east towards the Americas and warm the eastern Pacific. The warm water also creates a warm wet atmosphere with heavy rain and thunderstorms. The generally mild winter temperatures and heavy spring rains we have seen in Oregon are typical for an El Niño winter and spring in our region. Extreme cold events are also characteristic of El Niño effects in Oregon, and could explain the “Arctic Blast” that froze so many crops in December.

La Niña is the opposite of El Niño. Water in the eastern Pacific becomes cooler, creating cooler than normal air temperatures in the Americas (including Oregon). Last winter’s El Niño ended during May, and conditions are currently favorable for La Niña conditions to develop over the summer. The Climate Prediction Center issued a La Niña watch on June 3, forecasting a 50% chance of La Niña conditions. George Taylor explains that “Summer weather in Oregon is influenced more by local conditions than La Niña conditions in the tropics. Nearly every summer we see three months or so of warm, dry summer weather, when summer starts late it tends to continue later into fall. If we do see La Niña conditions we could expect a dry and somewhat mild fall followed by a stormy mid winter with a lot of snow. Spring tends to arrive earlier after La Niña.”

Weather patterns are extremely complicated and difficult to predict, especially on a long term basis which would be useful for crop planning. Climatologists have made great strides in predicting weather and the resources below may help you better understand long term trends and prepare for weather events that may affect your bottom line.Table 1. Precipitation minus evotranspiration from March 1-June 10 (1990-2010) at Hyslop Farm, Corvallis, Oregon. Courtesy of Len Coop (OSU Integrated Plant Protection Center). + one day of precipitation data was missing and was corrected with data from Forest Grove.