Mt Rainier Recreational Forecast MOUNT RAINIER RECREATIONAL FORECAST. NATIONAL WEATHER SERVICE SEATTLE WA. 355 AM PDT THU APR 20 2017. .SYNOPSIS...AN UPPER LEVEL TROUGH WILL MOVE THROUGH THE PARK TODAY WITH SHOWERS AND A SLIGHT CHANCE OF THUNDERSTORMS. AN UPPER LEVEL RIDGE WILL BUILD OFFSHORE TONIGHT AND MOVE OVER WESTERN WASHINGTON ON FRIDAY BRINGING SUNNY AND WARMER WEATHER.