by MICHAEL PIDWIRNY

Introduction

During the winter months of December, January, and February, the climate of the northwest USA and southern British Columbia is defined by cooling temperatures and increased precipitation. Temperatures cool mainly because of the Sun’s reduced heat energy. During these months, the intensity of the solar radiant energy declines because of lower Sun angles and a shortening of day length. Figure 1 describes the winter season’s average near-surface temperature (2 meters above ground level). Temperatures along the west coast are moderated by the stored heat energy found in the waters of the Pacific Ocean.

Figure 2 describes the typical patterns of winter precipitation in North America. High amounts of precipitation occur along much of the West Coast because of the interaction between mid-latitude cyclones and orographic uplift. During winter, mid-latitude cyclones often originate in the northeastern Pacific Ocean and then move in an easterly direction. Orographic enhancement of the quantity of precipitation falling from these storm systems occurs because of mountains running from Alaska to California. Central North America is relatively dry compared to the West Coast because most of the precipitable water held in the clouds of the mid-latitude cyclones previously precipitated out, and the cold continental air masses frequently found here retain little moisture.

Variation in year-to-year winter weather of the northwest USA and southern British Columbia is strongly influenced by several large-scale cyclic climate phenomena which modify large-scale atmospheric circulation and sea surface temperatures in the Pacific Ocean. Further, these climate factors significantly impact temperature and precipitation trends during the winter months over southern British Columbia and Washington state. Seesawing on a time scale of a few years are El Niño and La Niña events located along the tropical Pacific Ocean. El Niño – usually brings warmer-than-average winters and below-average precipitation to this area of the Pacific Northwest. La Niña – is often associated with cold winters with average to above-normal precipitation. Generally, the effects of significant El Niño and La Niña are limited to one or maybe two consecutive winter seasons. Operating on a much longer timescale of one to three decades is another cyclic climate factor of importance known as the Pacific Decadal Oscillation. The Pacific Decadal Oscillation alternates between a warm or a cold state, and these states seem to amplify the climatic effects of co-occurring El Niño and La Niña events, respectively.

El Niño

El Niño is the name given to the cyclical development of warm ocean surface waters on the east side of the Pacific Ocean at the equator. This climate event usually occurs around Christmas and usually lasts for a few weeks (weak) to a few months (strong). El Niño is created by a reduction in the speed of the Trade Winds right along the equator, which results in a shift in atmospheric circulation and pressure patterns in this region of the planet (Figure 3). Sometimes, an extremely warm El Niño can develop and last for more than a year. Since 1935, significant El Niño events have formed in 1958, 1966, 1978, 1983, 1987, 1990, 1992, 1993, 1998, 2005, 2010, and 2016. Figure 4 shows the general global patterns of winter surface temperature warming and cooling associated with El Niño.

La Niña

La Niña is the name given to the cyclical development of cold ocean surface waters on the east side of the Pacific Ocean at the equator. Like El Niño, a La Niña climate event usually occurs around Christmas and lasts typically for a few weeks (weak) to a few months (strong). La Niña is created by an increase in the speed of the Trade Winds along the equator, which results in a shift in atmospheric circulation and pressure patterns in this region of the planet (Figure 5). Sometimes, an extremely cold La Niña can develop and last for more than a year. Since 1935, significant La Niñas have occurred in 1950, 1956, 1967, 1971, 1974, 1976, 1999, 2008, 2011, and 2021. Figure 6 shows the general global patterns of winter surface temperature cooling and warming associated with La Niña.

Figure 7 shows the relative strength of El Niño and La Niña events from 1930 to March 2022, according to the Southern Oscillation Index (SOI). In this figure, negative values indicate El Niño conditions, with lower values suggesting stronger events. High positive values indicate significant La Niña events.

The Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is a cyclical ocean-atmosphere climate variability pattern in the North Pacific Ocean. The PDO is detected as a change in sea surface temperatures over the Pacific Ocean from 20 to 60° North latitude. There are two phases that can last many years to several decades, as shown in Figure 8. During the warm or positive phase, sea surface temperatures in the western North Pacific Ocean become cooler, while the eastern side of this ocean warms (Figure 9). The warm phase results in a zone of warm seawater hugging the west coast of North America from Alaska down to the Baja Peninsula. During the cold or negative phase, sea surface temperatures in the western North Pacific Ocean become warmer, while the eastern part of this ocean cools down (Figure 10). Significant reversals in the prevailing phase of the PDO have occurred around 1956, 1960, 1976, 1997, 2013, and 2019 (Figure 8).

Forecast Winter 2023/24

El Niño conditions are now observed over the equatorial Pacific (Figure 11). NOAA’s computer models currently suggest that there is a greater than 54% that this event will evolve into a historically strong El Niño event during November 2023 to January 2024. That would place the 2023/24 El Niño in the top five events since 1950. There is a 60% chance that El Niño will still be present from April to June 2024.

The Pacific Decadal Oscillation (PDO) index from January 2010 to October 2023 is shown in Figure 12 (and see web page Monthly PDO Index). A sudden decline into negative territory began in October 2019 and continues today. Typically, a negative Pacific Decadal Oscillation brings below-normal temperatures to western Canada.

The last intense El Niño event occurred in 2015/16, coinciding with a positive (warm) Pacific Decadal Oscillation. The combined winter climatic effects of these two events can be seen in Figures 13 and 14. Above-normal winter temperatures occurred in western Canada, with the most significant impact seen inland. Precipitation was average for much of western Canada. A patch of above-normal precipitation was observed in the southwestern corner of British Columbia. A below-normal amount of precipitation occurred along the coast of British Columbia above the patch of higher-than-normal precipitation.

The last negative (cold) Pacific Decadal Oscillation aligned with an El Niño event was 2009/10. This El Niño was classified as a moderate event. Figures 15 and 16 describe the winter climatic effects of these two events. Above-normal winter temperatures occurred in British Columbia and the eastern half of Canada. Most of British Columbia had below-normal winter precipitation. It might be reasonable to predict that the observed winter temperature and precipitation patterns of 2009/10 will repeat this winter.

Climate Prediction Center – North American Multi-Model Ensemble Long-Range Monthly Forecasts – December 2023

There is one more critical piece of information that can provide insight into what the winter season will be like in the Pacific Northwest USA and southern British Columbia in 2023/24. National Oceanic and Atmospheric Administration’s Climate Prediction Center creates long-range seasonal forecasts based on the average of eight different General Circulation Model simulations. Figure 17 describes North America’s December surface mean temperature forecast, released in November 2023. This forecast suggests temperatures will be 1.0 to 2.0°C above-normal for British Columbia, Alberta, Washington State, Oregon, Idaho, Montana, and Wyoming.

Figure 18 describes the December precipitation forecast for North America from the Climate Prediction Center, released in November 2023. This forecast suggests below-normal precipitation for much of British Columbia. Above-normal precipitation is forecasted for California, Nevada, Utah, southern Oregon, and southern Idaho.

Climate Prediction Center – North American Multi-Model Ensemble Long-Range Seasonal Forecasts – January  2024 to March 2024.

Figures 19, 20, and 21 describe respective January, February, and March surface mean temperature forecasts for North America released in December 2023. (Figure 19). At the same time, southern British Columbia and Alberta and all of the USA’s western states will see temperatures about 0.5 to 1.0° C above-normal.

The February forecast suggests temperatures will be 1.0 to 2.0° C above-normal for British Columbia, Alberta, Montana and most of Washington State, Idaho, and Wyoming (Figure 20). Temperatures will be 0.5 to 1.0° C above-normal in California, Oregon, Nevada, Utah, and Colorado.

The March forecast suggests a continuation of conditions seen in February with temperatures 1.0 to 2.0° C above-normal for most of British Columbia, Alberta, Washington State, Oregon, Idaho, Montana, most of Wyoming, and in the northern portions of California, Nevada, and Utah (Figure 21). Temperatures will be 0.5 to 1.0° C above-normal in southern California, Nevada, Utah, and Colorado.

Figures 22, 23, and 24 describe the January, February, and March precipitation long-range forecasts for North America released in December 2023. The January forecast suggests precipitation will be above-normal for most states in the western USA (Figure 22). Below-normal precipitation will occur along the entire coast of British Columbia. In most of the interior of British Columbia, precipitation conditions will be near normal.

The February forecast suggests precipitation will be above-normal only in California and central Nevada (Figure 23). Drier than normal conditions will occur in most of southern British Columbia, Washington state, northern Idaho, western Montana, New Mexico, and coastal Oregon. Precipitation will be near average in eastern Oregon, eastern Montana, Wyoming, Colorado, Utah, northern Nevada, Arizona, and Utah.

The March forecast suggests precipitation will be above-normal in southern California, southern Nevada, central Montana, and most of Arizona (Figure 24). The area of drier than normal conditions will expand relative to February, occurring in Washington state, Oregon, Idaho, northern Arizona, the southwestern corner of Wyoming, western Colorado, and the southern half of British Columbia. Precipitation will be near average elsewhere.

 

UPDATE Jan 7, 2024 – Climate Prediction Center – North American Multi-Model Ensemble Long-Range Monthly Forecasts – January 2023

Figures 25 and 26 describe respective February and March surface mean temperature forecasts for North America released in January 2024.

Figures 27 and 28 describe the February and March precipitation long-range forecasts for North America released in January 2024.

 

UPDATE Jan 12, 2024 – December 2023 North America Temperature Anomaly Map Relative to 1971-2000 Normal

Figure 29 describes the December 2023 surface mean temperature anomaly map for North America using a baseline 1971-2000 normal.