UPDATE 10:00 p.m.: Auroral activity as forecast appears to be increasing and dropping further in latitude.  It is possible to be visible in clear-sky regions of the Pacific northwest tonight with the optimal alignment with the Northwest in the wee hours of Sunday morning.  However, low clouds will be spreading in.  Map: http://helios.swpc.noaa.gov/ovation/USA.html

UPDATE 1:45 p.m.:  The Alaska Geophysical Institute forecast calls for aurora visible "low on the horizon" for Seattle and similar latitudes.  There are many variables, including timing of peak of geomagnetic storming and, of course, cloud cover.  Link: http://www.gi.alaska.edu/AuroraForecast/NorthAmerica/2012/07/14

UPDATE 11:45 a.m.:  The latest indications point against a widespread viewing across a large part of North America.  Instead, viewing is most likely in Canada and possibly the extreme northern tier of the United States.

The shock wave from a strong, X-class solar flare approaches the earth and reached instruments onboard a satellite Saturday morning (Pacific time).    

The instruments reported a near-doubling of the solar wind speed to over 600 kilometers per second and a force that could transfer energy into the planet's magnetic field.

This could cause extensive viewing of Aurora Borealis, also known as "Northern Lights" Saturday night,  local time.

Viewing conditions in Western Washington are likely to be marginal, at best, because of cloud cover.   After dark and before midnight, some pockets of clear skies could allow for lucky folks to see the phenomenon away from city lights.

After midnight, low clouds will continue to thicken and spread, socking in most of the area from viewing the possible auroral event.

Watch this NOAA website for auroral forecasts during the event.  This forecast is very-short-range, through less than one hour.