Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Wed May 20 2020 Valid May 20/1200 UTC thru May 24/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Cut-Off Low Over Lower OH and TN River Valleys... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 20.12Z GFS blend beyond 22/00Z Confidence: Slightly above average Overall, the 12Z NCEP and non-NCEP guidance maintained continuity with their previous runs in 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. The GFS allows this to happen much faster than the other guidance, due in part, to the faster progression of the trough axis over the interior west and Rockies. The 12Z UKMET and ECMWF both nudged a bit to the north and a bit faster than their 20/00Z runs, they did not keep pace with the faster GFS. Given the outlier solution of the 20/12Z GFS from other global models and even from the GEFS ensemble means from 20/00Z and 20/12Z (which had good agreement in terms of placement with the ECMWF ensemble means), the WPC blend preference will remain for a non-GFS solution beyond 22/00Z, with more weight given to the latest 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, although how each individual vort max plays out differs in strength and placement. Overall, the 20/12Z GFS was the least favored because it was still faster than the rest of the guidance and because it still amplifies the two shortwaves together earlier. The NAM appears useful through about 48 to 54 hours before it starts to lag the ECMWF/UKMET and ensemble means. 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 Bann