By Rod Hill on 2015-05-06
Despite warm temperatures last winter, Portland will likely see a comfortable, Northwest summer! The National Weather Service projects above normal temperatures for all of the west through July and likely into August. (Shown in the brown shading.) However, my findings show temperature deviations likely to be close to normal with a slight edge toward an above normal average by one degree celsius for the three months of meteorological summer, (June, July & August).
Our current ENSO phase which looks at Pacific water temperatures, shows slightly warmer than normal readings. Looking at all summers with similar conditions dating back to 1950, roughly 40% of summer months were actually cooler than normal! When all years are averaged, the temperature expectation figures out to slightly warmer than normal. The data set used shows the average number of 90 degree days from May through September to be 10-15 days. Eleven hot days of 90 or better is considered normal. Despite the record hot summer of 2009 with 24, 90 degree days and last summer's 21 hot days, the overall trend of hot summer days is actually down when looking at 30 year averages.
The rainfall forecast is harder to pinpoint. Keep in mind, July and August rainfall typically comes from scattered thunderstorms, which can drop an inch of water in one day, putting monthly totals above average. It is not unusual for July and August to see only 1-3 days each month with measurable rainfall. Notice the National Weather Service has much of the west in the "equal chance" category for precipitation. I project June has a 50-50% chance of seeing 2.00" of rainfall, which would be above normal. I expect July and August to see mainly dry weather, but keep in mind, we only need a thunderstorm or two to go in the record book as wet!
My personal seasonal outlook accuracy dating back to 2001 shows a rating of 63%, which is roughly 15% above industry standards. You may remember last summer, I projected the warmest summer since the record year of 2009, calling for up to 20 days of 90 degree heat - PDX ended up with 21 days.
To recap: I expect a pleasant summer, possibly slightly warmer than average temperatures, but no more than 15, 90-degree days. Rainfall could be above average for June with 2.00" falling, but July and August will see typical dry weather, minus possible downpours with scattered thunderstorms.
Meteorologist Rod Hill
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