Here are a couple charts I put together with average first frost dates:
Everything is lining up well for a perfect night of radiational cooling. Skies have cleared out, dewpoints have lowered, and we have no significant pressure gradient either up/down the Willamette Valley or east/west through the Gorge. So no stirring of the wind. Those dewpoints are a little higher than I’d like for low temps in outlying areas to drop into the upper 20s. But some drier air above should work down during the night, plus some moisture will condense out as either fog or dew. So my forecast of 37 at PDX and the coldest spots (SPB & HIO) at 28-29 SHOULD be okay. We’ll find out in the morning.
Chief Meteorologist Mark Nelsen