By Rod Hill on 2018-10-12
Before I begin, it is important to share with you my track record of accuracy for seasonal forecasting, dating back to 2000-2001. Last winter my forecast was a success, largely because of my 67% confidence call for a below normal water year. In fact, PDX recorded a water year ending September 30th of 30.03 inches, (normal is 36.03). My error last season was a projection of below normal temperatures, which was clearly incorrect. Other points, correctly called for a quiet valley snow year and a call for no significant valley windstorms. Overall, my seasonal outlooks rate a 66% accuracy rating, compared to an industry standard closer to 55%.
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HERE IS MY 2018-2019 WINTER OUTLOOK (followed by research discussion)
1. TEMPERATURE: Normal to above average, 73% confidence
2. PRECIPITATION: Near to below normal. Best chance 35.40" at PDX (Normal 36.03")
3. VALLEY SNOW: Less than 6.00 inches, a large snowstorm is unlikely.
4. VALLEY WIND STORMS: 40% chance of a 60 mph wind event, a significant or above average chance.
5. MT. HOOD SNOWPACK: Below normal year, projecting season snowpack at 66% of normal.
MY RESEARCH NOTES:
The above conclusions are largely based on the NOAA Enso Outlook calling for a weak to moderate El Nino this winter. I ran a set of winter seasons dating back to 1950, that are on record with the Enso temperature range of positive departure expected in the coming months. I also ran years following summers with an extreme number of 90 degree days and multiple year patterns, matching present day. Below are my findings related to the above Winter Forecast.
1. TEMPERATURE: 11 of the 15 winter seasons I consider as analog years showed near to above normal temperatures. In fact, the last cool year of consideration was the winter of 1979-1980, making a cool winter unlikely.
2. PRECIPITATION: The 15 analog seasons have an average of 35.43 inches, which is near normal and only slightly below. When looking at weak to moderate El Nino winters following hot summers, average precipitation for the complete water year is 35.36 inches. It is worth mentioning, my research shows a possible range between 25.87" and 43.03 inches. Again, my number of highest confidence is 35.40 inches.
If I may, my highest confidence number for last season was 30.10" and PDX received 30.03 inches!
3. VALLEY SNOW: Analog seasons show an average of 7.3 inches. My research gives a 50% of winter snow in Portland being less than 3.00" and a 50% of winter snow being over 8.00". Average snowfall following a hot summer is 5.4", (last winter recorded 7.6" and the winter of 2016-2017 received 11.7"). The "kicker" for me is my finding that it is extremely rare for PDX to go three winters in a row with 6.00" or more of total snowfall each year. In fact the last time was 1977 - 1980 and the time before was 1949-1952! The result is an 8% chance for PDX to receive 6" or more of total snowfall this coming season. Therefore, I am calling for less than 6.00 inches of total season snowfall for 2018-2019.
4. VALLEY WIND STORMS: 6 of the 15 analog years had a significant wind event, which is 40% and considered to be an elevated chance of seeing a period of 60 mph wind gusts or higher. In fact the analog winter of 2014-2015, which followed a hot summer, had a December 11, PDX south wind gust of 67 mph, the strongest wind since 1981. The event lasted 4-5 hours with 40-55 mph consistent wind speeds. It is worth noting that history overwhelmingly shows Portland's biggest wind storms take place in La Nina and Normal Enso winters, neither of which is expected this year.
5. MT. HOOD SNOWPACK: Analog years show a 50% chance of a 100% of normal snow year, so there is reason to be hopeful for a good season! However, the analog average is a snowpack of 66% of normal and it is alarming that two of the lowest snowpack seasons ever recorded show up in my data set. The winter of 2014-2015 produced the lowest season snow total on record at just 25% of normal. Also in the analog years, is the winter of 2004-2005 when the snowpack was 44% of normal. Mt. Hood Skibowl's front face was closed much of both seasons. It is worth pointing out that last winter's snowpack was 77% of normal, meaning a forecast of 66% of normal would only be slightly less snow. A little perspective shows a 25% of normal snow year at Timberline would dump a range of 240-290" of snowfall for the season, depending on water content or weight of the snow, while a 100% of normal year may dump more than 600" at the lodge! Lots of factors go into labeling a resort ski season as good or bad, including timing of big snowstorms, weekend weather conditions and overall freezing levels on the mountain.
I hope you find my Winter Outlook to be a valuable tool as you plan the winter season. As a forecaster, I live in the relm of possibilities and am often reminded that averages are a good method to determine odds of wet or dry, but sooner or later the entire data set range of what is truly possible comes true.
Happy Winter Season -Rod Hill
July 2018 High Temperature Ties for Warmest All-Time