Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 325 AM EDT Tue May 12 2020 Valid May 12/0000 UTC thru May 15/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Model Evaluation Including Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Mid-level trough amplifying across the Northeast with accompanying surface low through Tuesday morning... ...Shortwave moving across the Great Lakes and Northeast Tuesday and Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Models show overall good agreement with the lead upper level shortwave and associated surface low lifting along the northern New England coast into the Canadian Maritimes Tuesday morning. Through late Tuesday, there is also generally good agreement with a second shortwave forecast to move from the Great Lakes into the Northeast Tuesday and Wednesday. By early Wednesday as the system begins to move east through the Gulf of Maine, models begin to show minor timing differences - with the NAM and UKMET slower than the GFS, ECMWF and their ensemble means. ...Upper trough/closed low along the Northwest Coast through Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models are in fairly good agreement with the mean closed mid-level low position as it meanders along the Pacific Northwest coast through early Thursday, and then as the low weakens and moves inland, undercutting the ridge centered over British Columbia Thursday into early Friday. ...Upper low closing off over western Canada on Tuesday before moving east through the Canadian Prairie Wednesday-Thursday... ...Surface cyclone moving trough the north-central U.S. Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF, UKMET, NAM and 12Z EC Ensemble Mean Confidence: Below Average A combination of shortwaves will impact the north-central U.S. from Wednesday into Thursday. The first will be relatively weak, emanating from ID/MT/WY Tuesday night and reaching eastern Montana and the Dakotas on Wednesday. Overall, models are fairly good agreement with this feature. This leading wave is forecast to lift out ahead of an upper that is forecast to develop and drop south along the eastern front of the Canadian Rockies Tuesday into Wednesday. Overall, models are in good agreement through early Wednesday as the system drops into southern Alberta and Saskatchewan. However, significant timing differences emerge as the upper system begins to move east through the Canadian Prairie Wednesday night and Thursday. The NAM, UKMET, ECMWF and EC Ensemble Mean all cluster fairly well - with solutions that are faster than the GFS, GEFS Mean and the Canadian. Of the slower solutions, the Canadian is the most notable outlier, especially by 00Z Friday. While the GFS may be too slow as well, (especially from early Thursday across the northern Plains to the Great Lakes), the general trend in the guidance, including the ECMWF has been toward a slower solution, making it difficult to completely rule out the GFS at this point. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Pereira