Anticipating Heavy Rainfall: Forecast Aspects

Thirty years of climatological data were used to compare operational NCEP Eta model forecasts to parameters associated with heavy rainfall. The preliminary results suggest that the model often produces forecasts of key parameters, such as 850 hPa specific humidity, that depart significantly from the 30-year average during heavy rain events. Preliminary results show great promise in using these data in anticipating rainfall events which may result in heavy rainfall and flooding.

In addition to the specific humidity fields, the low-level and upper-level jet appears to play a key role in identifying areas susceptible to heavy rains. Anomalously strong V-wind components at 850 hPa, in association with abnormally high specific humidity, appears to be a good indicator of the potential for heavy rainfall. Similarly, strong U-wind components at 250 hPa, in confluent flow is another good predictor. These results are consistent with previous forecast studies of flooding events which emphasized the concept of model forecasts of 850 hPa moisture flux, warm advection, and equivalent potential temperature advection.

The presentation will include examples of model forecasts compared to the 30-year climatology and how these data can be operationally used to better anticipate heavy rainfall events. Experimental graphics related to this study are operationally available at http://eyewall.met.psu.edu/ensembles/eta.html. These graphics show the departures from normal of the forecast fields in the operational Eta. These data show great potential in anticipating heavy rainfall events in synoptic scale and mesoscale precipitation events.