Fall and Winter Weather Forecast - 1997-98

George H. Taylor, State Climatologist

Tye W. Parzybok, Research Scientist

August 28, 1997

Introduction

Winter weather in Oregon and the Pacific Northwest is influenced by the Pacific Ocean, the jet stream in the upper atmosphere, low level winds, and other factors. In recent years, the very strong influence of the tropical Pacific on worldwide weather, including the Northwest, has been identified. It is now known that the ocean-atmosphere condition in the equatorial Pacific known as the El Niño/Southern Oscillation (ENSO) plays a key role in global weather.

ENSO has two phases: the El Niño, or "warm event," involves warmer than average sea temperatures off South America and generally cooler than average in the western Pacific near Indonesia; and the La Niña, or "cold event," with cooler than average temperatures off South America and warmer than average in the west Pacific.

Typically, an El Niño will produce a drier and warmer than normal winter in the Northwest, while La Niñas are associated with relatively wet and cool winters. For the past two winters, a La Niña has been in place, and the result has been very wet winters. Occasionally, very strong El Niño events bring wetter than normal winters to the Northwest (this happened during the 1982-83 and 1994-95 winters), but conditions are drier than average during about 75% of all El Niños. Along the southern boundary of the U.S. (from southern California through Arizona, New Mexico, and Texas to the Gulf Coast), winter conditions are just opposite to those in the Northwest: wet and cool during El Niño conditions and warm and dry during La Niñas. A temperature map, from the Climate Diagnostics Center, shows areas which are typically warm (red, orange, yellow) and cool (blue, violet) during El Niños in the months of December - February. A similar map for precipitation for January - March shows the generally dry conditions across the northern tier of states and the wetter than average southern regions.

In the past 5 months, a strong El Niño has developed in the Pacific, and it appears to be one of the strongest of the century. Sea surface temperatures (SST) more than 9 degrees F above normal have been reported off the coast of Peru. Forecasters will be watching the Pacific closely during the next several months. There are indications that this El Niño peaked in June and is beginNiñg to wane. Based on that assumption, and using historical information from previous similar conditions, the following forecast for the 1997-98 winter was developed.

Previous Similar Years

This winter forecast for Oregon is based heavily on analog years (past winters with similar El Niño situations). Those years include: 1946, 1951, 1957, 1965, 1972, 1977, 1982, and 1991. While the 1982-83 event was the strongest on record, it is somewhat different than the current El Niño. For example, the 82-83 event peaked in late in the winter, while the current El Niño appears to have peaked during summer. The years which most closely resemble 1997 are 1977, 1946, and 1972. Our forecast is based on a composite of the eight years above, based most strongly on the three best-fit years.

Several notable weather events occurred during the analog years:

1972. In December, an extremely cold air mass settled over Oregon. Salem's all-time record low (-12 degrees F) was recorded, and the entire Willamette Valley had below-zero temperatures and heavy snow (15 inches at Salem in December). In eastern Oregon, many sites had readings below -30, led by Seneca at -40F.

1977. A series of very strong storms hit western Oregon in November and December, bringing high winds, heavy rain and snow (8 inches at Portland) to the state.

1982. A relatively dry, cool early winter was followed by very mild and wet weather, especially during February and March. The high-elevation snowpack, which was quite extensive early in the season, eroded quickly after the first of the year.

1991. Abundant early-season snows (October and November) in the mountains brought hopes for an end to the six-year drought, but dry, mild weather from December through the remainder of the winter led to a statewide drought emergency the following summer.

Forecast

While early September is expected to be cool and unsettled, the weather will turn pleasant and mild by mid-month and extend into early October. In mid-October, the first significant winter storms are expected, bringing rain, wind and much cooler temperatures.

The active polar jet stream and the abundance of moisture from the warm equatorial waters will help to spawn several potent mid-latitude storms in the northern Pacific Ocean during November and December. Several of these storms will affect Oregon, producing cold temperatures, high winds, moderate rains and mountain snows. While the storms are expected to be powerful, we do not expect extreme flooding as we did during the past winters.

The latest forecast models indicate moderating SSTs later this winter; thus, we expect a shift from the current strong El Niño to more normal conditions. Therefore, in January, the jet stream will move northward, and thus Oregon is more likely to see the typical influences of an average El Niño: drier and warmer than normal (unlike the winter of 1982-83, when continued strong El Niño conditions caused a very wet winter in Oregon). The storm track will likely split, sending powerful subtropical storms into California, with southern Oregon occasionally receiving glancing blows. The northern branch of the jet stream will affect areas well to the north (Alaska and British Columbia), leaving most of the Northwest dry and mild through February.

We are watching a large cool SST anomaly 2,000 miles off the Oregon Coast (near 37 N and 150 W ) which may have additional impacts on Oregon's winter weather, but at this point its effects are unclear.

Summary

Seasonal precipitation will be fairly close to normal, with above average early season rains balanced by a dry latter half of winter. Temperatures will be generally above normal, and snowpack will be above normal early and then deteriorate during mid- to late winter.

Month-by-month comparison

Western Oregon

Eastern Oregon

Legend

++ Much higher than average

+ Somewhat higher than average

N Near average

- Somewhat lower than average

-- Much lower than average

Next Update: October 1