Falcon Heavy postponed until Thursday due to high winds
SpaceX's Falcon Heavy rocket stood down for launch today due to high upper level winds. The launch kept getting pushed back into the window and sat waiting at the end in case the winds improved. Fueling never started so the launch was considered postponed instead of scrubbed. Weather was predicted to be 80% go, but these weather forecasts don't include upper level winds as they can be unpredictable.
The threat that high Upper Levels Winds poses to rockets is that it can veer the rocket off the plotted trajectory that it tries to stay on. If winds are aggressive enough, it could slam into the rocket like a sledgehammer and cause it to tilt past its limits and disintigrate. Weather balloons are launched during the launch window to monitor these wind conditions in the upper atmosphere to ensure a safe flight into space.
Falcon Heavy's launch window opens at 6:35pm EDT and extends to 8:31pm EDT Thursday, April 11th.