Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1205 AM EST Tue Dec 17 2019 Valid Dec 17/0000 UTC thru Dec 20/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS Evaluation with Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Eastern CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z NAM, 00Z GFS, 12Z ECMWF blend Confidence: Slightly above average No major differences noted across the eastern U.s., concerning a surface low and trailing cold front to slip just south of the New England coast Tuesday night. The 12Z UKMET's surface low was a bit more offshore than the latest deterministic/ensemble clustering. The 12Z UKMET and 12Z CMC are also slightly slower than the consensus regarding a deep closed mid-level low to cross New England on Wednesday but these differences shrink as the center of the closed low moves into the Canadian Maritimes Thursday morning. Overall, a 00Z NAM, 00Z GFS, 12Z ECMWF blend looks reasonable, but the 12Z UKMET/CMC may also have utility at times. ...Western to central CONUS mid-level trough... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z CMC / 12Z ECMWF blend Confidence: Average Timing differences show up rather early with a 500 mb trough reaching the California coastline early Wednesday morning with the 00Z NAM slowest. By Wednesday evening, the 00Z GFS begins to edge out ahead of the remaining deterministic guidance. Through Friday morning, the 12Z ECMWF and 12Z CMC favor the preferred middle ground with the 12Z UKMET on the slow side of the latest ensemble guidance. Trends in the ensembles have been a bit inconsistent with slower trends regarding the mid-level trough axis through the 12Z/15 cycle, followed by a quicker trend with the 00Z/16 cycle and a slower trend again for the 12Z/16 ensemble cycle. Given some of the shifting, prefer to stay toward the middle, again, closer to a 12Z ECMWF/CMC blend. ...Onshore flow into the Pacific Northwest...... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z NAM, 00Z GFS, 12Z ECMWF blend Confidence: Average A cold front will reach the Pacific Northwest on Wednesday followed by surface low development along the trailing cold front in the eastern Pacific Thursday morning. This wave will take aim at the coast of Washington followed by the arrival of its trailing cold front through Friday morning. The result will be a prolonged period of onshore flow into the Pacific Northwest. Mass field differences are relatively small but there are some timing differences with the surface low and frontal timing into the Pacific Northwest coast. A favored middle ground near the ensemble means is supportive of a blend of the 00Z NAM, 00Z GFS and 12Z ECMWF. Confidence is average given relatively good agreement with the trough placement but only fair agreement with the surface low and frontal timing. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto