For the second day in a row, convection and thunderstorms developed over the Cascades and rolled into the western lowlands...in this case hitting southwest Washington, Portland, and the northern Willamette Valley. Mark Albright, past WA state climatologist, reported that an observer in downtown Vancouver just W of I-5 reported 0.40 inches of rain in 5-minutes from 19:29 to 19:34 PDT 26 May 2012. The highest hourly amount from the Portland Hydra network (46 sites) seemed to be the 1.34 inches over one hour from 19:00 to 20:00 PDT reported at Hayden Island just west of I-5. This is extraordinarily heavy precipitation--heavier that the downpour that hit Seattle in December 14, 2005.....the event that flooded parts of the Madison valley. Most gutters simply can't handle this intensity.
Take a look at the Portland radar at 7:26 PM Saturday....see those reds?...that is torrential rain.
Did the models get it right?....unfortunately, not....again they failed. As proof, take a look at the forecast 24 h precp ending 5 AM Sunday....not good. This summer I plan on examining this event in detail....hopefully finding the origin of this failure. But convection is hard to forecast, particularly weakly forecast convection.
Here are two good youtube videos of the event: