Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1223 PM EDT Fri Apr 13 2018 Valid Apr 13/1200 UTC thru Apr 17/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Longwave trough emerging into the Plains today, gradually ejecting and becoming negative tilt into eastern CONUS by Sun-Mon... ...Deep surface low over the Plains, occluding and tracking into the Ohio Valley and Great Lakes this weekend and Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend; weighted to the 12Z GFS Confidence: Slightly above average Models all show a low closing off aloft today over the Central Plains as a deep cyclone matures and begins to rapidly occlude. They also all agree on the low becoming elongated this weekend and the longwave trough gradually ejecting to the northeast with time (also gradually becoming negative tilt as strong downstream ridging builds in to the northeast of the low). The models showed relatively consistent timing of the trough axis, with the primary exception being the 12Z NAM which lags the upper level low behind the other models. Therefore, a non-NAM blend is preferred to best represent the most likely timing. However, the model preference is also weighted toward the 12Z GFS. It keeps the upper level low and mid-upper level vorticity maximum further south (39-40N) than the 00Z ECMWF/UKMET (42-44N), and this has relatively strong support from the GEFS-ECMWF ensemble cluster analysis. The ECMWF and UKMET will still be incorporated in the blend to account for some uncertainty, as they are still reasonable scenarios. ...Longwave trough slowly dropping into the Pacific Northwest this weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Slightly above average Despite some minor timing differences, models show a very similar structure and amplitude to this large scale trough moving into the West. The preference is to be relatively close to the 00Z ECMWF Ensemble Mean, which is best represented by a broad-based blend of deterministic models. Therefore, a general model blend will be preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers