It's not just in your head. This humidity has been hanging around like that dude at the gym you wish you hadn't asked to spot you on the bench press.
Looking at the observations for Portland over the past few days, I had to go back to Wednesday August 8th during the evening to find a dew point reading below 64 F. Is that CRAZY humid by most standards? No, but in Maine that's a pretty extended stretch of noticeably sticky air.
Unfortunately the high humidity will hang around through the work week and the associated pattern is one that involves a lot of shower and thunderstorm activity. The problem here, as far as getting rid of the high dew points, is that storms are approaching us mainly from the south and staying there as precipitation begins. Therefore we can't get a clean cold front to pass from west to east over Maine in order to drag in some cooler, drier air.
Today: A good deal of sunshine continues through the early afternoon but showers and thunderstorms pop up over the higher terrain. These showers are already visible on the satellite picture and will push eastward as the day continues. The associated disturbance is weak so the showers will be as well. Nonetheless be advised to the chance of showers this afternoon and early evening, especially over western and interior Maine. The chance of showers will be less the closer to the coast you live. Otherwise it will be humid and mild with highs in the mid 80s.
Tonight: Any remaining showers will fizzle by 9 PM and partly cloudy skies will build in. Overall it will be a quiet evening with just some patchy fog towards the morning hours. Lows will end up in the low to mid 60s, a bit more comfortable than the weekend.
Tuesday: With no real forcing mechanisms in the area (i.e. front, trough) look for mostly sunny conditions. Temperatures will push into the mid 80s with dew points in the upper 60s, so it will feel a bit warmer than the mercury would indicate. There is SMALL chance of a shower or two popping up over western Maine in the afternoon, but it's in the 10-15% range at worst. So, I'll mention it, but I'm not going to put it in the overall forecast because it would be deceiving for most of the state.
Wednesday: After a sunny start look for widespread showers and thunderstorms by the afternoon. There is a "baggy" (weather nerd slang for a weak and poorly organized storm) low meandering over southern New England which will provide quite a bit of instability. The heaviest activity will be over the mountains but there will the chance of showers statewide.
This low continues to pull offshore by Thursday morning so showers and thunderstorms will likely open up the day before some afternoon clearing takes place.
Friday should be pretty decent with dry conditions and at this point the weekend is still up for debate. There is a cold front slated to come through on Saturday but the speed and timing of the front are still in question. With any luck it will move offshore by Saturday morning and we will have a good looking weekend. Unfortunately, as it stands I think the front will stall and give us unsettled weather through the majority of the weekend.
By the way, if you don't go the gym (I know it's hard to believe I do, trust me...I know) my opening analogy may have fallen flat. Either way, we've all started social interactions that we immediately regret when the person becomes a stage five clinger. Example: Women 20-40 yrs old: Any guy at the bar you thought "seemed nice" and then proceeded to lurk creepily behind you on the dance floor until 1 AM.