Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1155 AM EST Sun Dec 08 2019 Valid Dec 08/1200 UTC thru Dec 12/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall Pattern Across the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z ECMWF, 00Z EC ENS, 12Z GFS, 12Z NAM Less weight on GFS and NAM by Day 3 Confidence: Average Model differences begin to emerge more substantially around Monday Night when a southern stream trough ejecting into the Southern Plains will begin to interact with an expansive trough over most of the eastern half of the country. These differences stem from variation in the strength of the southern stream wave itself, to the precise timing of the ejection to the east-northeast. The differences are not extremely large, but they do have notable impacts on the forecast given the potential for transition of precipitation types behind the accompanying surface cold front. The model preference is weighted most strongly toward the 00Z ECMWF and 00Z ECMWF Ensemble mean. Their mass field forecasts also have relatively good agreement from the 12Z GFS, so that is also incorporated. However, the GFS has a much lighter precipitation pattern despite PWAT values in the warm sector exceeding the 90th percentile for early-mid December, and the strength of the upper level jet above the 95th percentile. Therefore, the GFS did not receive as much weight in this model preference, as most other models generate more QPF. The 12Z NAM ejects the wave faster and phases it with the large-scale trough more quickly. It also has a colder low-mid level temperature profile. This seems to be partially due to a forecast of a strong frontogenetic circulation and associated cooling with the ascending branch. The strong frontogenesis is present in the other model forecasts, and thus the NAM scenario remains plausible. Nevertheless, the NAM received less weight in this model preference due to differences with the mid-upper level mass fields from the dominant model consensus. Otherwise, the 00Z CMC and UKMET are not included in the preference. The UKMET, like the NAM, is faster with the ejection of the southern stream wave and also generates considerably more precipitation in the Northeast. This has less support from the ensemble distribution. The CMC maintains a much stronger southern stream wave, which also has limited ensemble support. Several other waves will dig into the Western U.S. by mid-week, and models differ on their strength. The preference is to lean toward the ECMWF as it is closest to the ensemble means. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers