|Here fishy fishy|
Let's take a look at the water temperatures at the beach closest to my house - Madrona Beach. All of these temperatures (and fecal coliform levels) for beaches in the lake can be found here.
The temperature has stayed in the low-mid 70s since July. That's absolutely extraordinary. How does this compare to recent years?
2013 was pretty impressive, with a stretch of around for the month of July, climbing into the low 70s for August. Temperatures are measured in Celsius to the nearest degree, so the decimal point is misleading. The temperature wasn't actually 71.6 degrees for five straight weeks. 2012 was a little cooler, and 2011 was much cooler, with temperatures not making it above 21 degrees C.
This data is only for one beach, and is therefore not a good measurement of the lake as a whole. However, it still highlights the relative warmth of the lake from one year to another.
It's easy to see why the lake is so warm this year. We had the second-hottest July on record with an average temperature of 69.2 degrees, 3.5 degrees higher than normal, and August thus far (as of August 20) has an average temperature of 70.7 degrees, 4 degrees higher than normal. The lake temperatures are higher than the average atmospheric temperatures because the water absorbs radiation coming from the sun, heating it further. It's the same concept as an asphalt road on a hot sunny day. It takes a lot of energy to heat water, and Lake Washington is extremely deep - over 200 feet deep in spots - and this prevents the lake from getting too warm as mixing occurs throughout the summer. However, water has a very low albedo (it absorbs almost all the radiation incident on it), so it is very efficient at storing heat. Moreover, because water has such a high heat capacity, it will store this heat well into September and October.
So there you have it. Warm atmospheric temperatures and lots of sunshine = warm water, and warm water = good fishing. And you think Madrona Beach is warm? Check out Lake Sammamish Beach in the shallower Lake Sammamish.
80 degrees? That's starting to get a little gross.