September is the month where summer fades away and fall takes over. Much like March where winter fades away and spring gradually takes over.
Notice that last fact. Now compare it to the following graphic
Only 1 of the last 8 summers have not seen a 90 degree temperature in PDX. That was the cool summer of 2010. The last two Septembers we’ve even seen a 95 degree high in the first half of the month. It’s also interesting to note that in both cases the next day it quickly dropped to around 80 degrees. Just as in March it’s hard to keep daytime highs cold; it’s really tough to have several days of hot weather in September.
This weekend will be similar with a couple days near (or above) 90 degrees Friday and Saturday, then a dramatic drop either Sunday or Monday as a cool trough moves by to the north. That turns the dry and warm east winds back to onshore and cool.
The big picture over the next 10 days or so is for a strong upper-level ridge to linger just off the West Coast, keeping us mainly or all dry. Most likely after the cool trough early next week we’ll see another fast warm up.
The 00z GFS has no precipitation over us for the next 16 days, and the 12z ECMWF looks quite dry as well. Here is the 10 day precipitation accumulation. A tenth of an inch over the mountains, and just about all of that was supposed to be this evening and tonight. So basically no rain in the lowlands through the 11th of September.
Chief Meteorologist Mark Nelsen