Moisture return is the flow of humid air from the Gulf of Mexico back north into the southern and central United States after a cold front has pushed dry continental air out over the Gulf. How fast it arrives, how moist it is and how deep the moist layer gets all shape the severe weather setup that follows.
How the cycle works
Behind a strong cold front, cold, dry continental air streams south over the Gulf of Mexico. On that trip the water below warms and moistens it, and a Monthly Weather Review study by Lewis notes that much of the moistening happens during this northerly-flow phase.
As the high pressure behind the front moves east, the flow over the Gulf turns west and then north, carrying the modified air back toward land. That turn is the return flow.
What decides the quality
Not all return flow is alike. Storm Prediction Center (SPC) research by Thompson names an important consideration: whether the returning air is still-modifying polar air or maritime tropical air near equilibrium with the sea surface. The Gulf of Mexico Experiment (GUFMEX) described return flow as modified polar air in retreat, often accompanied or followed by maritime tropical air.
Trajectory matters. Thompson contrasts air whose path crosses much of the open Gulf with air that has only a short trip over water, and notes that sea surface temperature sets an equilibrium dewpoint that can be calculated for a return flow event.
Real events do not always settle on that value. Thompson found some return flow episodes fell short of equilibrium at the buoy, while most eventually exceeded it.
How the low-level jet speeds it up
Once southerly flow sets up, the low-level jet can move the moisture north quickly. An NWS training page describes it carrying Gulf moisture and warmer temperatures north at 25 to over 70 knots and feeding moisture into developing thunderstorm updrafts.
Thompson notes that, from operational experience, most cool-season outbreaks of strong or severe thunderstorms in the southern and central United States are associated with return flows whose dewpoints run above equilibrium for much of the Gulf.
Where forecasts go wrong
Lewis notes that the returning air's moisture content and the depth of its moist layer have proved problematic to predict, and traces one model bias in the western Gulf to how air-sea interaction was handled.
Surface dewpoints alone can mislead, since they say nothing about how deep the moist layer is. Soundings and model profiles help judge whether the return is deep enough to matter.