This will be quick... I have people to meet and places to go! UW game at 12:30 by the way, go Huskies!
Anyway, today will be warm and humid* as we have that warm southerly flow. We could see those thunderstorms in the Cascades, although it looks like we will see them in the early morning hours of Sunday, though some storms are possible later today.
The below graphic shows 3-hour precipitation amounts - you can see precipitation not associated with the oncoming front over the Cascades, particularly the eastern foothills. Click on it to enlarge the picture.
Valid 02:00 am PDT Sun, 25 Sep 2011 - 21hr Fcst - UW 12km 12z WRF-GFS 3-hour precipitation
The Monday storm looks different again... precipitation amounts vary widely over different parts of the lowlands. I'll let you know more info as the the date moves closer. There will be a noticeable change in the weather tomorrow, with steady rain at times and mostly cloudy conditions, but the real stuff will come in on Monday. One thing is sure; Monday will be very cool relative to what we have had lately - highs will struggle to reach the low 60s.
*Seattle doesn't actually get that humid