Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Dec 20 2018 Valid Dec 20/0000 UTC thru Dec 23/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Eastern third of the US through Sat/Northeast Sunday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/ECMWF/UKMET blend Confidence: Average 07z update: Small adjustments with the ECMWF and UKMET support both inclusions to the 00z GFS, while the CMC remains a bit offset from initial preference. Confidence remains average in this blend due to mild smaller scale variations between the 3 to make less than a solid/ideal blend. ---Prior Discussion--- A full-latitude neutral tilt trof begins to mature by late tonight across the eastern third of the CONUS loading heavily to the base across the lower MS River valley into the Southeast; spurring cyclogenesis out of the central Gulf and up the spine of the Appalachians. Some model differences start to unfold with this initial height-fall surge/surface wave, before the secondary and stronger surface wave lifts east of the mountains through the Carolinas Friday into the Mid-Atlantic by Saturday. The 00z NAM has trended favorably to a more stable/run to run consistent solution presented by the GFS/ECMWF and their respecting ensemble means; however, typical of bias is a bit stronger with the initial wave/cold punch across the Great Lakes. The 12z UKMET is similar and less favorable to include in this portion of the country, particularly with the QPF solutions on the colder side of the overall cyclone. Confidence in a GFS/ECMWF solution is fairly good, but with such a large system, small scale internal interactions are likely to remain moderately variable to have full confidence especially toward the end of the forecast period as the wave lifts into New England. Northern Plains Fri/Sat...Great Lakes Sat/Sun ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/ECWMF/UKMET blend Confidence: Slightly above average 07z update: No significant change in thinking/preference Upstream, a less amplified shortwave slides through the northern Rockies into the Central Plains/Southern Prairie Providences, and with downstream blocking swings the base through remaining nearly stationary with the fulcrum/pivoting vort center in Canada. Here guidance is fairly agreeable with exception of the CMC which is a bit more aggressive sliding energy southeast into the shortwave ridge into the Midwest/NW Great Lakes. The 00z NAM becomes a bit more amplified with a compact closed wave into late Saturday as well. Great Basin/Central Rockies into Central Plains ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/ECMWF blend Confidence: Slightly above average 07z update: The 00z UKMET trended faster and therefore was a bit less amplified than the 12z run, but still appears too wet/strong. While the 00z ECMWF did indicate some increased amplification of the wave especially compared to the flattest/driest GFS solution...a blend of the two remains WPC preference. ---Prior Discussion--- Further upstream, the progressive pattern emerging from the Pacific continues, flattening the ridging across the Rockies with the aforementioned trof and shedding some weak negative tilt jet energy into this increasing zonal flow. The 12z UKMET and CMC both suggest greater shortwave amplification through the Great Basin and Central Rockies into the Great Plains, while the GFS/ECMWF agree with the existence of this wave, the greater amplification seems less likely in the larger scale evolution especially without more impressive downstream ridging. As such WPC gravitated toward the GFS/ECWMF solution for this portion of the country and given agreement between these solutions and fairly stable run to run ensemble suite with this blend; confidence is slightly above average. Pacific Northwest Late Sat/Sun ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/NAM/ECMWF blend Confidence: Average 07z update: With a significant shift north, the 00z ECMWF matches fairly well with the initial preference just a tad slower than the GFS/NAM but this is a typical alignment that provides solid confidence. The UKMET/CMC also trended favorably towards a common solution, though the UKMET is a bit slow/further west especially in the lower elevations/layers to suggest keeping it out of the preference or at a very low weighting. The CMC is a bit weaker making it faster, given better agreement with the NAM/GFS and ECMWF...will go that direction for WPC preference and confidence is increased to average. ---Prior Discussion--- Progressive/strong Pacific pattern continues to try to establish a solid zonal flow into the Pacific Northwest/SW Canada as strong broad closed low takes up residence in the northern Gulf of Alaska. Models struggle with timing of next jet streak/shortwave and how consolidated the streak is. The 12z UKMET/CMC are quickest with nose of the jet and support a flatter/longer wave-length solution entering the area toward the end of the forecast period. On the other side of the ensemble suite, the 12z ECMWF is once again suggesting a slow jet approach and significant amplification to the wave and more importantly furthest south. Even within the ECENS suite, this is in the southern third of solutions. While, run to run ECENS members have been consistent in the large variation, the operational keeps flip-flopping back and forth of this amplified/southern track making it a very low confidence solution especially considering the the GEFS/CMCE members. The 00z NAM and GFS both trended to a more amplified solution, similar to the ECMWF, but further north directed toward Vancouver Island/Olympic Peninsula, while the GFS also trended much faster, typical of a negative bias. The GEFS/ECENS mean solutions appear a good compromise and the NAM/GFS seem a good but not great representation of the solutions (think GFS is too fast). As such a 00z NAM/GFS and 18z GEFS/12z ECENS mean blend is preferred but a below average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina