2/1/2010

Did you leave your jacket at home for a majority of the month? Felt like perhaps we skipped winter this season? Noticed buds on trees? If so, you're not alone! January was remarkably warm across the state. Average temperatures ranked in the top ten for many locations - average January temperatures, the temperature departure from normal, the rank from warmest to coolest, and the record length is shown for some locations in WA in Table 1.

Table 1: Average January temperatures around WA, the temperature departure from normal, the rank from warmest to coolest, and the period of record for each station. Temperature is in degrees Fahrenheit.

So why has it been so warm? Our warm WA weather has resulted from a combination of factors. The strongest El Niño since 1998/1999 is occurring in the tropical Pacific Ocean, which generally implies warmer temperatures in the Pacific Northwest. Looking at the SeaTac January temperature record, the average temperature during recent El Niño years (1958 through 2005) is 41.7 degrees F, or 0.8 degrees F above the normal of 40.9 degress F. This year's January average temperature at SeaTac, 47.0 degrees F, greatly exceeded that, so El Niño is only part of the story. An important factor was a period of unique weather in the middle of the month. During this time, an anomalous low pressure center formed off the WA and OR coast. This system produced winds from the southeast, drawing warm air up from CA and producing warm, downslope winds along the Cascades. The low pressure system, combined with high astronomical tides, also caused high sea levels throughout the Puget Sound and coastal regions. Figure 1 shows the composite mean of the sea level pressure from January 18 through January 23, 2010. The lowest pressure falls between 980 mb and 984 mb, right off the coast of WA. Figure 2 shows the composite anomaly from January 15 to January 23 using the 1968-1996 normal. The low pressure was a remarkable 24 mb lower than usual, hence providing less weight on the ocean and allowing the sea levels to rise. Two pictures of the high tides are shown below (from the Department of Ecology). The first was taken on January 21 in Olympia, WA by George Kaminsky. The second was taken on the same day in Harper, WA by Carey Ensign.

Figure 1: The composite mean of the sea level pressure from January 18-23, 2010. Please click on the figure to see the full-size image.

Figure 2: The composite anomaly of sea level pressure from January 15-23, 2010 using the 1968-1996 normal. Please click on the figure to see the full-size image.

What ultimately caused the anomalous weather pattern in January? That is much harder to determine. El Niño could have ultimately forced the pattern or even the negative phase of the NAO that is contributing to the colder weather on the east coast could have played a role. Furthermore, the global average temperature was among the warmest during January, and the overall warming of the climate could have also played a role in the warmer than normal local temperatures.

Image 1: Higher sea levels than normal in Olympia, WA on January 21, 2010. Photo was taken by George Kaminsky and was orginally posted by the Department of Ecology.

Image 2: Higher sea levels than normal in Harper, WA on January 21, 2010. Photo was taken by Carey Ensign and was orginally posted by the Department of Ecology.