From AMS Glossary
The development of an area in the boundary layer (e.g., often observed on surface or 850-mb charts) where moisture values become higher than in the surrounding region.
Moisture pooling typically occurs in an area of low-level convergence during the warm (growing) season, and can have a significant effect on convection initiation and evolution.
Johns, R. M. 1993. Meteorological conditions associated with bow echo development in convective storms. Weather and Forecasting. 8. 294–299.