August 20, 2006

Steve Pierce - Weather Specialist
Vancouver, Washington
e-mail: stevejpierce@comcast.net

Overview of last winter (2005-2006)
In the late summer of 2005, we issued our annual winter weather outlook for the Pacific Northwest, with an emphasis on NW Oregon and SW Washington. Here are some of the details that we expressed; "With temperatures over the tropical Pacific in the neutral range as of late August (and trending lower) the likelihood of experiencing another warm / dry El Nino-ish winter is looking less and less likely. Historically speaking, if an El Nino does not develop by August, the chances of seeing the tropics affect Pacific Northwest winter weather drops dramatically." This assessment was correct and El Nino failed to show during the winter months. However, the one thing we did not foresee was the rapid cooling of ocean temps in the tropical Pacific in November and December that ended up tossing all Nino region temps to borderline La Nina thresholds by Christmas. The winter began rather mildly and by December had turned much colder than normal. In the first few weeks of December, we experienced a brief shot of modified arctic air into the region which helped to drop overnight lows into the teens in many locations west of the cascades. Many water falls in the gorge were covered in ice for several weeks prior to Christmas. However, the lowlands near Portland escaped any major snowfall, with the exception of a quick shot of moisture that moved up the valley during the middle of the afternoon on a weekend in mid December. It brought about one hour of light snow. No more than an inch or so was recorded in the greater Portland area, for the most part. But, none the less, it was still an official snowfall for Portland. There were several east wind events this winter including one that produced winds in excess of 55 mph over much of eastern Multnomah and Clark counties, with some damage and downed trees reported.

January began with several strong storms moving up the coast of Oregon and Washington. Winds were strong at the coast (quite normal for an average winter) but none of them affected the inland regions with any damage. The inland valley escaped yet another year with no major inland wind event. The last major inland blow was December 1995, with several smaller events in 1999 and 2000. January also saw record rainfall amounts over most of Western Oregon and SW Washington. With almost every station west of the cascades receiving in excess of 10" (coastal regions seeing 20"+) January went down as either the wettest ever recorded or very close to it in many areas. Some remote mountain areas of both the coast range and the cascades saw as much as 30-50" of rain during the month of January. WOW! The Farmers Almanac predicted a very wet winter, with almost 30" of precip between November and March. Although not quite as severe as they predicted, they still get my congrats for predicting an extreme rainfall event. They called for February to be the brutal month for rainfall. Heck, they were close enough in my book.

The remainder of the winter was docile with nothing major to note, until early March. Very low snow levels accompanied an upper level low from the gulf of Alaska on a path directly into the Pacific Northwest. If there had been arctic air in place, this would have been a large dumping of snow on lowland areas like Portland. However, the snow stayed above 750-1000ft. which spared most of the city. Living around Portland allows me the chance to see what the weather is like at sea level and at 1,200 ft. within about 5 minutes of each other. I drove from downtown Portland to the top of the west hills near the TV towers (1,100ft or so) and it was another world up there. 6-8" of snow on the ground and schools were closed. Winter came to a close right along side the calendar. SST's in the tropics returned to normal and so did the weather.

A look ahead to this winter (2006-2007)
As we look ahead to this coming winter we look again to the tropics for a closer look of what is going on. After March, SST's began to increase back to normal and then a little above normal into the summer months. By July, SST's were averaging as low as -0.2 and as high as 0.5, depending on where you look across the basin. July was the most interesting month, as SST's came up rather quickly, but at the time of this report (late August) SST increases have shown signs of slowing, and (in some cases) even started to decrease a bit in some areas. In our assessment, this is a sign of a neutral SST pattern, with normal seasonal swings from one direction to the next. It is important to note that it is very rare to see an ENSO event start this late in the year, however, last years brief La Nina is a perfect example of how quickly the pattern can change. There has been some debate in the weather world over how quickly a change in ocean SST's will take to affect the weather pattern in the Pacific Northwest. Most experts believe there is a 4-6 month lag time from SST's changes to resulting weather patterns in our area. However, this past winter might just be the arguing case that there are situations where a 1-3 month lag time can occur. That is subject for another discussion. So...... on with the forecast.

After looking at many factors that affect Pacific Northwest weather, including analog (similar years), current and forecasted SST's in the tropical Pacific, as well as other indexes such as the PDO, NAO, the Atlantic hurricane (or lack thereof) season, sunspot activity and other various data, we come to the following conclusions:

The Pacific Northwest will likely face an overall ENSO neutral winter, leaning on the warm side of normal. Unlike last winter that leaned on the cold side of normal. Tropical SST's are warmer than historical normal, but just barely. Additionally, not all the signs of El Nino are present. Actually, as of late August only 2 of the 5 major indexes that we look at are signaling an EL Nino winter this year. The SOI has been one tool that is showing signs on an El Nino potentially developing later this year. If this were to happen, it would be one of only a few cases in the past 50 years of such an event occurring this late in the year. But, it is still possible and will need to be watched carefully during the remainder of the summer and into the fall. It is also important to note key analog (similar) years to what we are seeing as of late August. Without ranking them, here are our list of the top 5 analogs to the current pattern; 2003/2004, 1979/1980, 1990/1991, 1953/1954, 1981/1982. At first glance, these analogs may not appear to be anything noteworthy. But when looked at a bit closer, one starts to see that in these "ENSO neutral" years, quite a bit can happen with Pacific Northwest weather. Nov. 1981 saw a large and damaging wind storm strike the area. Jan. 1980 saw a massive dumping of snow in a series of 3 storms over a 7 day period. Dec. 1990 saw a severe arctic outbreak with snowfall still on the ground for Christmas. This was one of only 2 snowfall occurrences near or on Christmas, in the past 75 years in Portland. Late Dec. 2003 into early Jan. 2004 saw a the most severe ice and snow storm to hit the region in more than 25 years. So the fact remains, weather patterns can swing rapidly from one extreme to the other when there is no clear signal from the tropical Pacific. If the current pattern holds, this winter could have some exciting weather events in store for the entire region, even though the overall winter may turn out to be about normal, or even slightly warmer than normal, as has been the case in all but 3 winters since 1990. Perhaps global warming? Perhaps just a longer term cycle we are currently in? The debate will continue for years. Precipitation will likely be about normal throughout the winter. It was also interesting to note that in winters that were on the warm side of normal (ONI in the nino 3.4 region averaging between 0 and +0.5) we saw a greater chance of seeing snowfall in the Pacific Northwest lowlands over those years where the ONI went above 0.5. What about the odds of seeing a major snowfall event or potential damaging wind event this winter? This year we would put them both at about 50/50, with a slight advantage to the lowland snow event, based on SST analog (similar) years.

Here is a "first half vs. second half" breakdown of our forecast for this coming winter 2006-2007:

Synopsis
All in all this winter should bring a little bit of everything as long as the current pattern holds. If SST's warm to levels sufficient for El Nino, the result may be drier and slightly warmer than is forecasted here. Stay tuned!