Solid Lifters
When it gets hotter as the night progresses, it means tomorrow will be one hot mother...
[nerd mode]
There are 4 situations that I can think of that will lead to the night mysteriously warming up:
1)A thermal trough moves into the area at night (what you are describing). A hot air mass, still very warm from the daytime sun, makes its way over an area which has cooled off. I've seen it here where it will be about 65 at some nighttime hour, and then a short time later, it will be closer to 75. Indeed, the next day is usually a stinker.
2)Clouds move in. This traps the heat which is trying to escape from the earth. This usually doesn't raise the temperature too much, but it does make the air more humid, so it feels warmer than it is, and quite muggy. This usually happens in front of a storm system.
3)Pineapple Express. This happens in the winter, when the jet stream dips way down south over Hawaii, and then swings back north to hit the west coast. This draws very warm, moist air into otherwise cold regions, warming them considerably (and usually bringing heavy rains). A couple years ago, it was about 45 degrees when I was out and about around 10 PM. Three hours later, it was 65, which is a wee bit warmer than a typical January night here. This was, in fact, the high temperature for that particular day, set at about 1 AM, as it cooled off gradually during the rest of the day.
4)Approaching windstorms. Again, a winter night phenomenon, and a very bizzare one at that. As the low pressure approaches, higher pressure is drawn towards it. In this area, the Cascade mountain passes act as wind tunnels, so when the low pressure approaches, high pressure air in Eastern Washington is funnelled through the mountains at ridiculous speeds. As the air descends down the slopes, it compresses, which warms it up (basic thermodynamics). I recall a situation where the overnight temperature in North Bend, which sits near the mouth of Snoqualmie Pass, hovered at about 70 degrees (amid 80 mph winds)....in December.
[/nerd mode]
j/k It's been super hot for Portland area as well. It hasn't reached 100 in a week or so though, thank god. I can't wait until August. Maybe I will buy that air conditioner..... if I can find it. 
Still cold though!
