On the forecasting of orogenic mesoscale convective complexes

Evidence is presented to support the hypothesis that mesoscale convective complexes (MCCs) near the Rocky Mountains are more likely to form when the middle-tropospheric relative humidity is greater than average and the lower-tropospheric relative humidity is less than average, Radiosonde data for MCC events are chosen at the nearest place to first storm development and at the nearest time before first storms occurred. A sounding representing an average seasonally adjusted climatological location of orogenic MCC first storms was used to represent non-MCC days. The 500-hPa relative humidities were significantly higher for MCC events than for non-MCC days. The 700-hPa relative humidity was significantly lower for MCC events than for non-MCC days. MCC days also have somewhat less stability than non-MCC days but this factor appears to be related to higher temperatures at 500 hPa on days when the 500-hPa relative humidity is low. The values of various quantities used to assess the utility of this information for weather forecasting indicate that this method needs to be combined with other MCC forecasting methods to be useful.