It’s been a very mild summer. As mentioned in previous posts areas most exposed to the onshore flow of Pacific air have been the coolest, but even areas well inland have only been average at best. What’s been missing is a real absence of hot spells. We’ve only had one period, July 7th-9th, where temps jumped into the mid 90s here in the Portland/Vancouver Metro area. We seen 4 days at/above 90 degrees this summer. In an “average” year we see about 13 @ PDX.
In a normal year during the period from early June through early September we go through periods of hot weather followed by cooling onshore flow…some years more heat than others. One thing that tends to change after mid-August is the longer nights start to catch up with us. Overnight temperatures start to drop off a bit and it’s also a bit harder to heat up the land surface each day as the sun angle drops as well. As a result when you look back at Portland records you generally don’t see long periods of 90+ weather the 2nd half of the month. I did find two years where that happened. Notice that both in 1967 and 1977 (both in the top 5 warmest Augusts) there were long periods of heat around mid-month. 8 days of 90+ in August 1967, and check out 1977! In the first 17 days of the month, there were only two days at 85 degrees; the other 15 were 88 or higher! But then the following two weeks? Never got above 80 the rest of the month…summer just “stopped” after mid-month in August 1977. This coming Monday and Tuesday’s record highs are from that period…102 & 105 degrees. Those will be tough to beat, but we might get really close!
For the real weather geeks out there, I checked out the 1977 heat wave. Easterly wind never made it to PDX, so apparently that is not required with a hot enough airmass; similar to last year’s 106 degree day in early August. Even more geekery…at 11pm that night wind went SW 21 mph at PDX with a drop to 75 degrees; a major southwesterly marine push kept highs the next day below 80 degrees.
So what’s changed in the last 24 hours? It appears that instead of just a few days at/above 90 degrees, we’re going to see a major “late summer” heat wave. I say late summer because of the reasons mentioned above. Yes, I know summer doesn’t end until September. Anyway, models have made a change, keeping a large area of low heights in the upper-atmosphere much farther to the west (offshore) through the middle of next week. The result is the 4-Corners High pokes right up into the Pacific Northwest Monday through Wednesday. Heights go up to around 590dm. at 500mb (or even higher on the ECMWF). That combined with offshore flow REALLY heats us up. Both GFS and ECMWF have 850mb temps between +22 and +26 from Sunday afternoon through Wednesday. For some perspective, I see only 2 August days in the last 10 years have seen offshore flow AND 850mb temps at/above +23. Both days were 100+. Due to that we put 100 degree temps in for Monday and Tuesday.
It’s now safe to say that we probably have a 5-6 day stretch of 90+ weather on the way, at least 4 of those probably 95+. That’s warmer than anything we’ve seen this summer. The good news is that we very likely won’t see anything like it again as we head towards the end of August.
Chief Meteorologist Mark Nelsen