So the NWS is forecasting 7” of snow in Pittsburgh with a 1% chance of 8” or more? (8” at the airport with a 12% chance of 8” or more.) Something has to give.
Essentially they’re issuing a watch saying “Allegheny County will likely receive 6-9 inches of snowfall” but their probabilities say it is very unlikely that most locations in the county will receive 8 inches. At the end of the day, winter storm watch/warning criteria are 8 inches in 24 hours, correct? The question basically boils down to, do you issue a winter storm watch for an area when the chance of that area meeting warning criteria is somewhere from 1-10%?
Alternatively, the messaging essentially reads “we don’t believe Allegheny County will meet warning criteria, but quite a few models say it will, so we’re going to issue a watch to cover ourselves in case it does happen.” The stakes are low when issuing a watch because you can always downgrade to an advisory.