Your Eclipse Forecast!

Can you believe that there are less than 48 hours to go until the Great American Eclipse of 2017? I get giddy just thinking about it. The weather is looking spectacular for nearly everyone – the coast will likely be shrouded in low clouds for the viewing, and there’s still some uncertainty about a weak marine layer sticking around in the Portland metro area/Puget Sound during eclipse time.

A substantial ridge of high pressure currently sitting off the West Coast will move inland over the next few days, and by eclipse time, it will be directly positioned over the Pacific Northwest.

Ridges are fantastic for viewing eclipses because they direct major storm systems to our north and are associated with sinking air. This sinking motion does two major things – it stifles convection and warms the atmosphere, both of which decrease the likelihood of cloudy skies, albeit in different ways. Convection creates cumulus clouds, and a cool lower atmosphere makes it easier for a marine layer of stratus clouds to form west of the Cascades.

One of the most useful tools for forecasting cloud cover is the column-integrated cloud water chart from the UW WRF model. This chart shows the amount of water you would have if you took all the cloud droplets in a vertical column of air extending throughout the entire atmosphere.

At 7 am, a thin marine layer of stratus clouds extends into the the Strait of Juan de Fuca, the Chehalis Gap, Puget Sound, and parts of the Willamette Valley, while a thicker layer remains near the coast and further offshore By eclipse time (10:17 AM), these clouds have burnt off over the Willamette Valley and have lessened significantly over the Puget Sound region, but coastal communities will likely still be shrouded in cloudy skies. If you were planning to see the eclipse on the coast, see if you can travel just a few miles inland – it shouldn’t take long to get out of the clouds, especially if you can get a little bit of elevation.

Here are the cloud cover forecasts from the Seattle (top) and Portland (bottom) NWS offices. Seattle’s forecast is valid at 11 AM PDT (eclipse is at approximately 10:20, depending on where you are), and I think Portland’s forecast is close to eclipse time, but they’ve only labeled their forecast as “Monday morning.” Note how the Portland office keeps a significant amount of cloud cover in the Portland/Longview area Monday morning, something which is not reflected in the UW WRF.

This brings me to an important point: the WRF does not take the dramatic decrease in solar radiation due to the eclipse into account. Since daytime heating is the forcing that helps “burn off” our marine layer, the eclipse could delay this process in spots. The HRRR (which forecasts 18 hours out) takes the decrease in solar radiation into account, so I’ll have an update tomorrow with the HRRR’s cloud cover forecast.

Wildfires:

Though there are still plenty of wildfires in the Oregon Cascades and near the Oregon/California border, folks in Eastern Oregon should still have excellent viewing conditions. Not only does smoke not have a huge impact on the eclipse viewing itself, but most of the smoke is south of the path of totality, and with decreasing WNW winds expected over the weekend, I wouldn’t expect that to change too drastically.

Bottom line:

Your best chances for viewing the eclipse will be in the Willamette Valley south of Portland and Eastern Oregon. The coast will likely be shrouded in low clouds, but you should be treated to good viewing conditions if you head a couple miles inland and gain some elevation (plan ahead to account for traffic though!)

There’s still some uncertainty whether the marine layer in Puget Sound and the Longview/Portland corridor will burn off in time to view the eclipse. The warmth due to our ridge will be balanced by the cooling due to the moon eclipsing the sun – something that is not reflected in current weather models. Given the strength of our ridge, I am optimistic that we will have relatively clear conditions here, but I wouldn’t bet the farm on it. If you don’t want to take a chance, head into the Cascade foothills – the clouds will burn off more quickly there.

I hope you are all as excited for the eclipse as I am! Check out my previous post for more info explaining the science behind solar eclipses.

Charlie