500mb Winds:

The 500mb pressure level (18,000 feet) is critical for monsoon forecasting. At this height, large-scale atmospheric vertical motion changes, both upward and downward, are maximized and the influence of mountains on these changes is more limited. This level is where the location of the summertime high pressure becomes well-defined, and winds around this high are able to transport moisture long distances, and strongly influence thunderstorm movement. Thus the location of highs and lows, as well as the wind speed and direction, are watched closely at this level to better predict monsoon conditions.