An Evaluation of Ensemble MOS Temperature Forecasts from the Medium Range Ensemble Forecast System

In September 2001, the National Weather Service Forecast Office in State College, PA, began producing a suite of graphical and textual derived ensemble MOS guidance products. Ensemble MOS bulletins were produced for all available MOS sites. Graphical products were limited to METAR station locations in and around the county warning area. These derived products are constructed using 0000 UTC MOS guidance produced for each of the twelve members of the National Centers for Environmental Prediction (NCEP) Ensemble Forecast System (EFS). One of the forecast elements in the derived MOS guidance is temperature. The 12-member mean and one standard deviation, as well as the maximum and minimum temperature values amongst the 12 members, are calculated for each station. MOS forecast data are also available from the operational Global Forecast System (GFS) for each station.

This paper will present derived ensemble MOS bulletins and describe several forecast applications using these bulletins, with the focus on the verification of the operational GFS MOS and derived ensemble MOS temperature and probability of precipitation forecasts. Preliminary results suggest that for the first 72 hours of the 0000 UTC forecast cycle, the operational GFS MOS produces more accurate temperature forecasts than the derived ensemble MOS mean. Beyond 72 hours, the derived ensemble MOS mean temperature forecast skill was comparable to the operational GFS MOS. Also noted in the preliminary results was the observed temperature fell outside the maximum/minimum range of the derived ensemble MOS more than 25% of the time at most of the station locations verified. This suggests there is not enough spread in the current NCEP EFS.