Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 132 AM EST Wed Dec 25 2019 Valid Dec 25/0000 UTC thru Dec 28/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation with Preference and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall Pattern Across the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM Blend Confidence: Average ---06Z UPDATE--- The 00Z CMC has trended closer to the preliminary preference (GFS, ECMWF, UKMET) such that it is now included in the preference. Its depiction of the evolution of the phasing waves in the Rockies now looks similar to the other models, and the surface low position in the central Plains by 28.12Z has trended north closer to the other models. It's worth noting that the 00Z GEFS Mean low position has also trended north. ---PREVIOUS DISCUSSION--- Models are in relatively good agreement with their mass field forecasts through Thursday afternoon (27.00Z), at which point more substantial differences begin to emerge. One significant source of model variability seems to be a potent shortwave currently near the Aleutian Islands that will become sheared out and dig into the Northern Rockies by Friday. Small variations in the exact trajectory and strength of the wave as it reaches the U.S. produce large differences downstream as phasing begins with the ejecting southern stream closed low in the Southwest. The 00Z NAM and 12Z CMC show a stronger northern stream wave that fails to completely phase by Saturday morning (28.12Z). Both models were near the extreme end of the ensemble envelope in terms of amplification of the northern stream wave, and the 00Z GFS has trended slightly less amplified with its new run (closer to the ECMWF, UKMET and ensemble means). Therefore, the preference is to lean toward the GFS, ECMWF and UKMET as they have greater ensemble support and are more representative of the trends over the past couple model cycles. Nevertheless, those three models still have fairly significant differences with their QPF. The ECMWF, for instance, spreads measurable precipitation well into the Dakotas by the end of the Day 3 period, while the GFS is much drier that far north (the UKMET is in between the two with respect to the northward extent of precip). Therefore, although the mass field preference is for a blend of the GFS, ECMWF and UKMET, the QPF was trended toward a compromise in terms of geographical extent while still maintaining some of the magnitude of maximum amounts in the central Plains. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers