Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 311 AM EDT Mon May 18 2020 Valid May 18/0000 UTC thru May 21/1200 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 now available and no changes needed to the model blend preferences. Tropical Storm Arthur ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: National Hurricane Center official track Best Model Proxy: 00Z ECMWF/00Z GFS blend Confidence: Slightly above average Tropical Storm Arthur, currently well offshore the SC coast will brush the Outer Banks early Monday, before turning northeast away from the eastern U.S. Tuesday. Deterministic model guidance has converged on a solution with less spread noted than 24 hours ago. The best model blend proxy to the official NHC track is a blend of the 00Z GFS and 12Z ECMWF, close to the model consensus, which takes the system to about Cape Hatteras and then on a north/northeast turn away from the shore. Consolidation of Midwest and Gulf Coast shortwaves into cut-off upper low ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average A pair of shortwaves over the central US, one over Iowa and the other along the central Gulf region, will consolidate over the next day into a large cut-off low that settles over the Tennessee Valley early next week. Model guidance has improved in the last couple of cycles and there has been some tightening in the model spread such that from a model mass field perspective, a general model blend is sufficient. Deep upper level trough building in across the western U.S. followed by secondary shortwave trough approaching Pac NW ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 20.12Z, then ECMWF/UKMET blend Confidence: Above average With the amplifying upper level pattern across the continental U.S., an amplified upper level trough is forecast to become established over the West Coast and then the Intermountain West by the middle of the week as a big upper low from the northeast Pacific tracks southeastward. This will also be in response to a building downstream ridge axis across the Plains as an omega block pattern develops. By day 3, a secondary shortwave trough is forecast to drop down toward the Pacific Northwest and here, there are some model differences. The GFS is on the faster side of the spread. A blend composed of the ECMWF/UKMET is preferred here for day 3. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor