Updated: May 1, 2019
The National Forecast Chart such as the one shown below from the WeatherSpork Imagery view was introduced as an experimental forecast and was initially made available on the Weather Prediction Center (WPC) website. This experimental forecast will become operational on March 28, 2019. It's designed to forecast weather conditions across the contiguous United States and adjacent areas of Canada and Mexico over the next three days. The current day’s chart is updated twice a day, once in the early morning (by 5 am EST, 10 UTC), and again late in the afternoon (by 5 pm EST, 22 UTC). The charts for Days 2 and 3 are currently only updated daily at 5am EST, 10 UTC.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist