Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 157 AM EST Mon Dec 23 2019 Valid Dec 23/0000 UTC thru Dec 26/1200 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: Blend of 00Z GFS, 12Z CMC, 12Z UKMET Confidence: Slightly below average ---06Z UPDATE--- The 00Z ECMWF did trend slightly closer to the consensus of the other models with the developing closed low near southern California. The preference is held the same given that the ECMWF has trended in that direction and the GFS, CMC and UKMET have been relatively consistent. ---PREVIOUS DISCUSSION--- Models are in fairly good agreement through Tuesday Night (around 25.06Z) and a general model blend for this time period would be a reasonable forecast preference across most of the country. An important divergence in model guidance occurs with the trough along the West Coast on Wednesday. A reinforcing shortwave is forecast to dig sharply southeast from the Gulf of Alaska and lead to closed low development somewhere in the vicinity of California. The 12Z ECMWF -- and the vast majority of its ensemble members -- digs the wave with more of a southerly component relative to many of the other models. This eventually leads to a mid-upper level closed low further to the west by 26.12Z, a surface low further offshore of the southern California coast, and height falls aloft constrained closer to the coast. The QPF in the interior West is also lower. Meanwhile, the 00Z GFS is faster with spreading height falls into the West and has a surface low closer to the California coast. It is joined by the 00Z NAM, 12Z CMC and 12Z UKMET. Normally as an event draws nearer in time, different models will begin to converge on a common scenario. In this case, individual ensembles have seen their spread narrow, but the gap between the ECMWF, GEFS and CMC has not narrowed substantially over the past couple model cycles. In this case, the preference is to lean toward the GFS, CMC and UKMET, given greater support from a variety of different models. This also aligns more closely with the WPC medium range forecast preference, which applied slightly greater weight to the GFS and UKMET at the beginning of the medium range period. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers