Equations were developed to forecast probability of precipitation (PoP) from Eta model forecasts. PoP is defined as the probability of the accumulation of 0.01" (liquid equivalent) or more in a given (6/12/24h) period. 6h PoPs are available every 6 hours for projections valid 12 to 60 hours after model initializations; 12h PoPs are available every 12 hours for projections valid 24 to 60 hours out; and 24h PoPs are available at the 36, 48, and 60-hour projections.
Model data for development were available at 0000 and 1200 UTC daily on a polar stereographic grid projection oriented 105°W with a grid resolution of 90.75 km at 60°N. Predictors offered include 6- and 12-hour precipitation amounts, mean relative humidities in various layers, mean sea level pressure, precipitable water, moisture convergence, u- and v-wind components, vertical velocity, K index, harmonics (sin/cos) of the day of the year, and station elevation, latitude, and longitude. Model data were available every six hours from 6 through 48 hours beyond initialization. Data were offered at all projections necessary to cover the period of each forecast; however, for projections beyond 48 hours, only data up to the 48-hour projection were offered. The developmental sample spanned 4 cool seasons (October through March) from 1997 - 2001.
Prior to the final development, a test development was made. For the verification of this test development, the 2000-2001 cool season was held out and used as independent data. The test equations were tested on the independent sample and compared to both the operational NGM and AVN MOS PoPs (neither were available for the 24h PoPs) in terms of percent improvement over the Brier score of climate. (The Brier Score is similar to mean square error, but is used for binary events like precipitation/PoP.) Generally, the ETA MOS PoPs are superior to the AVN MOS PoPs through the early projections (F12-36), showing significant skill improvement over the NGM MOS. In the later projections, the ETA MOS PoPs' skill decreases, and the AVN MOS PoPs show the most skill, but the ETA MOS PoPs still show significant improvement over the NGM MOS.
Some of the most commonly selected predictors in the final development include the 6- and 12-hour precipitation amounts, the various mean layer relative humidities, and the various u- and v-wind components. Precipitation amount (QPF) guidance will be developed soon for the ETA MOS package. The ETA MOS package and message is still under development, but will likely resemble the new AVN MOS (MAV) package.
Please select a product from the menus on the left.
Developer: Joe Maloney
Last Updated: 10 August 2001