Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1223 PM EDT Thu Sep 20 2018 Valid Sep 20/1200 UTC thru Sep 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall preference for the entire CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Exclusions: In the West; Non-NAM thru 22/12z, inclusion after Less 12z GFS weighting after 23/12z. Confidence: Slightly above average Models have shown solid agreement over the last few cycles, particularly with the cyclone deepening across the Midwest today before shifting eastward over the next few days. This agreement became all that stronger as the 12z GFS shifted slower, ie westward by day 2 over Eastern Canada to better match the 00z UKMET/ECMWF/CMC; while not significantly reducing the timing of the front across the Northeast on Sat. While there is the expected timing differences with the trailing PV tail/flattening frontal zone across the TN Valley into the Mid-Atlantic... the 12z GFS is weak/fast, the 12z NAM a bit stronger slower matching the 00z ECMWF in placement; The UKMET/CMC are similar variants to have solid to above average confidence in a general model blend for this system. In the West, a compact shortwave and associated surface low is trekking toward Vancouver Island/Strait of Juan de Fuca by early Sat, and for this system the 12z NAM is a bit more compact and slow supporting a surface wave that is displaced SW of a very tight clustering, leading to less confidence in this evolution through about 48-45hrs. Thereafter, as the lead wave fades to upstream strengthening jet streak, the 12z NAM falls back into the center of the ensemble cluster, as the kicker wave rounds into the Pacific NW on early Sunday. Orientation of the of two features and highly amplified jet structure will support elongation/positive tilting of the trof. Here, the slow CMC becomes well out of phase dragging a bulk of the energy back toward the anticyclonic wave break into the eastern Pacific, clearly away from ensemble mean and run to run cycle. The ECMWF/UKMET both present a more sensible depth/speed of the wave with the ensemble trend and are accompanied by the NAM by Sunday. The 12z GFS, like prior runs, remains slightly faster having a tighter proximity to the lead wave, swinging it through OR a bit faster than other guidance, which leads to an earlier deepening of the surface wave in the Great Basin...in line with the GFS typical fast bias. As such the GFS is quite useful through the forecast period, but reduced weighting should occur in the West by Sun. All in all, a non-CMC blend is favored, with exception of the NAM through 54hrs and the GFS after 72hr in the west. Confidence is slightly above average given the agreement in the remaining guidance and ensemble suite. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina