Atmospheric water vapor is known to affect many processes, including cloud formation and precipitation. Water vapor can be measured using both in situ instruments, including radiosondes and remote sensing instruments, including Raman lidar. Radiosondes provide measurements of atmospheric water vapor and temperature that are some of the most widely used in numerical weather prediction models. They have high vertical resolution but poor temporal and horizontal sampling since they are launched every 12 hours and radiosondes are launched regularly in the U.S. from sites at an average separation of approximately 100 km. Therefore, there is a paucity of information on the horizontal, vertical and temporal variability of water vapor and temperature. Sensitivity studies indicate that severe storm forecasting is limited by a lack of accurate observations of water vapor aloft in the lower troposphere. Therefore, the availability of fine-resolution and accurate 2D and 3D water vapor field measurements would substantially improve numerical weather prediction model initialization. Microwave radiometers have the capability to measure atmospheric water vapor and temperature at sufficiently high spatial and temporal resolution to aid in advance forecasting of the onset, timing and location of severe weather.