The eclipse will be cool - literally. NOAA predicts Northwest temps could drop 3-6 degrees

Seattle on Aug. 20, 2017.

If you're one of the gobs of people heading outside to witness the eclipse on Monday, you might be wondering: Will I need a jacket?

The sun will be about 90-95% blocked for a brief period just after 10 a.m. in Western Washington and yes, that lack of sunshine will pause the day's heating.

But how much? Scientists at NOAA are taking this unique opportunity of an eclipse that's traversing the entire Continental U.S. to see its effect on weather. Researchers have tweaked the calculations in new HRRR model -- a short term forecast model that refreshes every hour -- to try to account for the large swath of the moon's shadow wiping out a bit of daytime heating across the nation.

They ran a test running the model on Aug. 4 and plugged in what would have happened had the eclipse happened at the same time that day to see what the model would have predicted. In some cases, the model predicted the eclipse would make the temperature drop as much as 9-12 degrees F in clear sky spots!

It wasn't quite as dramatic in the Northwest… about 3-6 degrees F, with Western Washington on the lower end, but! There were two factors in play there -- first, the cooling wouldn't be as dramatic in the Northwest versus areas farther east because our eclipse is happening in the morning. And even though the clock will say it's 10 a.m., remember we're on Daylight Savings Time so as far as the sun is concerned, it's happening at 9 a.m. relative to solar noon.

Second, August 4th was a rather unique day around here -- it was during our heat wave when the high would eventually reach into the low 90s across Western Washington. On that morning, the temperature was heating rapidly and the brakes applied by the moon would have a more profound effect. You can see this effect play out around the Rockies too where they had clear skies that day, it was also super hot (so, rapid morning warming), and a later eclipse. The purple splotches on that map are the calculated 9-12 degree cooling.

On actual eclipse day-Monday, the weather around Western Washington is expected to be much closer to a typical day with a more gradual morning warm up, which in turn I'd hypothesize the eclipse's effect will be more muted. So you probably won't need to don a jacket during the eclipse unless you felt the morning needed one anyway (probably marginal for most.) Full-sunshine temperatures are expected to be in the low-mid 60s for mid- Monday morning.


The model has finally run and was predicting actual temperature drops across Western Washington of about 4 degrees:

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