Steve wrote:
Seems like every day in recent months, the forecasted high temp in
Seattle is 5-10 degrees below what it actually turns out to be. And
these are forecasts from just the day before. Today's forecast, for
instance, was for a high of 75. It's now 86. Any idea why this wide
variation?

Forecasting the high temperature near a coast during summertime is
inherently very tricky. So much depends on whether or not a sea breezes
reaches your location, which in turn depends very strongly on (a)
whether there is an offshore pressure gradient and how strong it is, (b)
your distance from open water, (c) local topography, and (d) countless
other subtle factors that are difficult or impossible to accurately
account for in a computer forecast model. Sounds like the forecasts
were anticipating an onshore flow that didn't kick in quite as hard as
expected. Or maybe you are a mile or three more distant from the water
than the forecast was written for.

I used to live in San Diego, where the temperature could be in the 70s
right on the coast and over 100 just 10 miles inland. The precise
position of the boundary between the two airmasses was impossible to
predict accurately on any given day. But at least in that case, the
official forecast would say something like "70s on the beach, 90-100 at
inland locations", leaving it to you to figure out where you fell
between those extremes.