Being about 15 miles from Puget Sound, though, my experience has been that my garden's daytime high temperatures are a few degrees higher, and my nighttime temps a few degrees colder, than in many locations closer to Puget Sound. Our winter weather is rainy the vast majority of the time. With mulch, these can be depended on to overwinter. An August 1st sowing still give useable, but smaller, roots. With carrots there seems to be big differences that are just related to how particular varieties grow as the days get shorter.
Bitterness decreases with frost, and varies from variety to variety.
I sow in late September. Basically, if the ground isn't frozen, you can put them in. This entry reflects my experiences with Durabel. This applies to Allium cepa types of scallions. A. fistulosum types are much hardier and non-bulbing, but also are less tender and hotter in flavor.