A Red Flag Warning is issued when critical fire weather conditions (dry fuels, winds of 25+ mph and less than 15% RH for 3+ hours) are happening or are about to happen. Avoid burning, be careful around open flames, safely dispose of cigarettes. Fires can spark and grow very quickly.
A PDS Red Flag Warning is issued when a combination of extremely critical conditions (very dry fuels, single digit RH or 50+ mph winds) are present along with significant and/or frequent wind shifts. These are rare and are only issued when historic or life threatening conditions are forecast to occur. Avoid any activities that create sparks. Any fire that develops may rapidly spread and become extremely dangerous.