<P> This monsoon ridge is almost as strong as the one which develops over Asia during the summer . However, since the lower level moisture flow is not as persistent as in the Indian monsoon, the upper level steering pattern and disturbances around the ridge are critical for influencing where thunderstorms develop on any given day . The exact strength and position of the subtropical ridge also governs how far north the tropical easterly winds aloft can spread . If the ridge is too close to a particular area, the sinking air at its center suppresses thunderstorms and can result in a significant monsoon "break ." If the ridge is too far away or too weak, the east winds around the high are inadequate to bring tropical moisture into the mountains of Mexico and southwest U.S. However, if the ridge sets up in a few key locations, widespread and potentially severe thunderstorms can develop . </P> <P> Unlike the Pacific region of the United States and the west coast of Baja California, precipitation in the NA monsoon is not associated with large - scale mid-latitude cyclones, but with thunderstorms which have very different spatial / temporal distribution characteristics . The difficulty in understanding the variability of summertime convective activity in the southwestern U.S. and northwestern Mexico results from the complex interactions between atmospheric circulation features at both the synoptic (100 to 1000 km spatially, 1 day to 1 week, temporally) and mesoscale (several km to 100 km, hours to one day temporally) and the extremely varied topography . The larger - scale atmospheric motions may control the distribution of water vapor and the general stability or instability (that is, the tendency to form storms) in the atmosphere; nevertheless, local topographic effects are critical to the geographic and even temporal distribution of convective activity . </P>

Which statement about the north american monsoon is true