Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 126 AM EST Wed Dec 05 2018 Valid Dec 05/0000 UTC thru Dec 08/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall Pattern Across the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Broad-based model blend, weighted to 12Z ECMWF Confidence: Slightly Above average ---06Z UPDATE--- No change to the preliminary preference. The 00Z ECMWF and UKMET continue to be similar to the NAM in the sense of showing a more amplified trough, and both models actually trended slightly closer to the NAM (more amplified) on this run. Nevertheless, differences with amplitude remain, and the approach of a broad-based blend still seems reasonable. ---PREVIOUS DISCUSSION--- The most significant system of interest is a closed low approaching the California coast now. This will eventually open up into a trough by Thursday, and finally begin to accelerate east toward the Four Corners region and Southern Plains on Friday. Most substantial precipitation in the next few days should be produced by this system, as the NBM is only forecasting 0.10+ inches of QPF in extremely limited areas elsewhere in the CONUS (predominately downstream of lake effect snow belts). Given the overall good model agreement elsewhere, and localized nature of precipitation areas, a general model blend was preferred in those regions. For the primary trough in question, there is more model variance, particularly as the trough axis reaches 110W and begins to push into the Plains. This seems to be primarily related to the amplitude and sharpness of the wave (and related mid-upper level vorticity near the base of the wave). The overall timing is fairly similar, although the 00Z GFS did show a faster eastward progression of the surface low on the Gulf Coast (and related eastward displacement of the surface high in the Great Lakes). Faster progression is a typical bias of the GFS, and thus less weight was placed on the 00Z GFS in this case. Regarding other factors related to the strength of the wave, there is no clear preference at this time. The 12Z CMC and 00Z GFS were slightly faster and weaker, while the 00Z NAM by contrast shows a mid-level low closing off over southeast New Mexico by Saturday morning (08/12Z). Although the NAM does have a bias toward more amplification and strength of fairly progressive troughs, a number of other ensemble members from all of the available ensembles (NAEFS and ECMWF) show greater amplification than the deterministic global models and thus it does not seem prudent to discount the NAM in this case. Therefore, the preference is to lean toward a broad-based model blend with no clear preference on the strength of the approaching wave, with less weight on the 00Z GFS and more on the 12Z ECMWF. The model QPFs do show some differences, but are still relatively similar in their larger patterns and thus a broad blend should not be extremely different from any one model. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers