Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 322 AM EDT Wed May 20 2020 Valid May 20/0000 UTC thru May 23/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Model Evaluation Including Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 07Z update: No significant changes noted with the rest of the 00Z guidance and no changes to the WPC model blend preference. ...Cut-Off Low Over Lower OH and TN River Valleys... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 00Z GFS blend beyond 22.00Z Confidence: Slightly above average Models have converged on a similar solution for the evolution of the large cut-off low over the lower OH and TN River Valleys through Friday morning. Gradually the low begins to fill and with the approaching upper trough across the central Plains, its energy becomes absorbed. However, this is where the models differ, where the GFS allows this to happen much faster than the other guidance, due to the faster progression of the trough axis over the interior west and Rockies. The other guidance (ECMWF/UKMET) show a slower progression with the low remaining cut-off through much of the period. The 12Z CMC was unavailable for analysis and not included in this discussion. Given the outlier solution in the 00Z GFS, the WPC blend preference will remain for a non-GFS solution beyond 22.00Z, primarily composed of the ECMWF/UKMET deterministic solutions. ...Deep upper trough/closed low over the Western U.S. Wednesday... ...Secondary upper trough over the Pacific Northwest by Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: NAM/ECMWF/UKMET blend Confidence: Average The latest guidance shows average to slightly above average agreement for the longwave trough over the West that will lift toward the northern Rockies over the next 1-2 days. Toward day 2/3, another shortwave dropping southeast over the Pac NW absorbs the initial wave and forms a large closed low with several lobes of vorticity pivoting around the center. In the large scale sense, the models all have this scenario, how each individual vort max plays out differs in strength and placement. Overall, the GFS was the least favored as its 1) faster than the rest of the guidance and 2) amplifies the two shortwaves together earlier. The NAM appears useful through about 60 hours and then shows some lagging compared to the ECMWF/UKMET and ensemble means. The CMC was not considered in this analysis due to data flow problems. Overall, the WPC blend preference is for a near equal weighting of the NAM/ECMWF/UKMET. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor