By Rod Hill on 2011-10-02
October starts the process of wondering about winter. All outlooks continue to call for a wetter than normal October and wet months on average through December. In fact precipitation outlooks call for mostly wet months through March! Temperature outlooks show less confidence but are leaning toward the cool side.
A weak La Nina began to take hold back in August and will likely strengthen somewhat into winter. It is true that La Nina's typically bring normal to above precipitation to the Northwest as the Pacific Jet takes aim at our coast line. This year's La Nina is expected to be weaker than last seasons. This fact points at less water than we saw last winter. Below are 3 factors I have researched:
1. An active hurricane season often means a wet winter for the Northwest. To date, the current tropical season has 16 named storms, 11 is average. Thus, WETTER THAN NORMAL WEATHER IS LIKELY.
2. La Nina facts: The last 5 strong La Nina's have averaged 40.04" of precipitation. (Last year was a strong episode with 44.74" of water; 8.71" above normal). The last 5 weak La Nina's, which we expect this year have averaged 35.07" of water. Thus, YOU WOULD EXPECT 5.00" LESS RAINFALL THAN LAST SEASON.
3. Our place in history has Portland coming off the longest streak of non-soaking years in the record book. The water year ended Sept. 30th was the wettest since 1998-1999. The last time Portland went more than 10 consecutive seasons without a wet winter, the following years brought some of the heaviest rains on record, including the floods of 1996. MY POINT IS THAT WE ARE DUE FOR A NUMBER OF WETTER THAN NORMAL WATER YEARS. My benchmark number for a wet year is 40.00" or more.
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My conclusion: Normal to above normal rainfall for the valley. I expect a drier year than last, in fact 5-8" less precipitation this water year looks likely. My forecast calls for PDX to receive 36.00 - 40.00" of total water through Sept. 30, 2012.
Mt. Hood at Timberline Lodge received 696" of snow last season. This year I would look for another good year with season snow totals 600 - 650" of snow. I have no guess as to when the season will begin, but current forecast projections look favorable for Thanksgiving weekend!
So, look for a wet winter but not as wet as a year ago. Temperatures will likely be normal to below.
Rod Hill
Portland the wettest since 1998-1999