For hydrologic purposes, QPFs are prepared by local NWS offices based on guidance products from NMC and TDL and other information. Forecasts are made routinely once a day and more frequently during flood situations. In general, there are two approaches.
In the service area of the Northwest River Forecast Center in Portland, Oregon, which has been using QPFs since the 1970s, the forecast is for a station and three days. It specifies estimates of precipitation amounts for eight 6-hourly subperiods, during days one and two, and one 24-hourly subperiod, day three. Forecasts for 10 stations are then processed in the same manner as actual rain gauge observations to obtain forecasts of basin average precipitation amounts which are input to a streamflow model. Because of the strong orographic effects of the Cascade Mountains, temperatures are forecast as well to predict freezing levels that separate rainfall from snowfall and to estimate snowmelt runoff [ Hughes, 1993].
In the service area of the Ohio River Forecast Center in Wilmington, Ohio, which has been using QPFs since the late 1980s, the forecast is for an area and one day. It provides graphical estimates of isohyets of the precipitation fields for four 6-hourly subperiods. By integrating the fields over basins, estimates of basin average precipitation amounts are obtained. Temperature forecasts are used to classify the precipitation as rain or snow [ Eiben and Yess, 1993].
These two approaches have been detailed here not just to
illustrate the NWS operations, but foremost because the experience
gained by field forecasters preparing QPFs has proven invaluable
in developing new
[4]
methodologies. Precipitation forecasting is an experimental science,
and practice in experimentation is often needed to formulate gainful
research objectives.