Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 103 PM EDT Sun Jun 14 2020 Valid Jun 14/1200 UTC thru Jun 18/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Preliminary Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low developing in Southern Appalachians Mon/Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through Tuesday night 12Z GFS, 00Z ECMWF/CMC Wednesday Confidence: Above average Through 60 hours, there is little disagreement in the guidance evolution of a closed low dropping across the Southern Appalachians and into GA/SC where it will stall and slowly fill. A general blend is acceptable through Tuesday. On Wednesday, some divergence in the solutions begins as the NAM sends a potent vort lobe westward, dragging the weakening low center with it, while being forced in that direction as well but stronger mid-level ridging to the N/NW. The UKMET does just the opposite, lifting the weakening low quickly to the north, and while a slow northward motion is likely, something as fast as the UKMET seems unlikely since the system is completely removed from the mean flow and blocked by ridging to the north. This leaves the ECMWF/GFS/CMC as the best consensus, which also has support from the ECENS and GEFS means. Despite the comparable mass fields, QPF discrepancies do exist, especially by D3, as the ECMWF wants to spread heavier rainfall further inland into the Carolinas/Virginia than the CMC/GFS. There has been little consistency in the heaviest QPF axis recently, but note a subtly stronger jet max and LFQ rotating northward and offshore D3. This could allow for some heavier QPF/subtly deeper surface pressure falls near the coast to lift inland, so the ECMWF solution cannot be ruled out. Will maintain the preferences however, but note that this blend would wash out some of the heavier QPF potential by D3. ...Interaction of shortwave lifting through SW Canada with a closed low advecting onshore Washington Sunday... ...Secondary closed low moving into the Northwest Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC through 17/00Z 12Z GFS/00Z ECMWF thereafter Confidence: Above average through D2, average D3 Interaction of a shortwave lifting into Saskatchewan and a closed low moving onshore Washington state drives the blend through 17/00Z. The guidance is very well clustered with these features initially, with only the CMC becoming an outlier on D2 as it sends the leading shortwave too far to the east compared to consensus. This leaves less binary interaction between it and the approaching but weakening closed low, and allows the newly forming gyre/trough axis to be displaced further west. Otherwise, sensible differences are minimal at this time. Beyond 17/00Z, spread begins to expand. As another closed low drops out of the Gulf of Alaska and onshore Washington Tuesday, a large gyre will form across western Canada/northwest CONUS. Energy shedding around the approaching closed low will lift onshore prior to the closed low, and the UKMET is much too fast with this first vort lobe. This energy moving too quickly east causes the interaction of the approaching low and the previous one to be stretched/sheared too far to the southeast, and out of tolerance with the other guidance which dumbbell the vort maxes N/S around each other. Otherwise, the NAM/GFS/ECMWF are all quite similar both with the interaction, and with the intensification of the shortwave as it tilts negatively into Montana D3. The NAM does begin to lag late D3, likely in response to subtly stronger LFQ jet dynamics as a jet streak pivots around the closed feature, and for slightly higher heights blocking to the east. While the GFS/ECMWF are faster than the NAM shifting this feature eastward, impressive dynamics should be able to drive height falls into the ridge as the shortwave pivots E/NE, and the agreement combined with the ensemble means produces the suggested blend for D3. Weiss Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml