It is well established that the negative phase of ENSO ("El Nino") brings wetter winters to the Southwest and drier winters to the Northwest, and that the positive phase brings approximately the opposite. This pattern is also reflected in an accentuated form in streamflow patterns. The wetter winters in the two regions are a result of more days with precipitation, and more precipitation per wet day. Persistent episodes are more likely, and thus more precipitation is likely to appear as runoff. Composites of upper air patterns before and after persistent precipitation episodes reveal that the associated circulation patterns are fundamentally different between El Nino and La Nina. For example, upper air patterns during negative phase persisting events in Arizona are much more zonal than the (fewer) higher amplitude patterns associated with positive phase persisting events