Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 112 PM EDT Sat Aug 08 2020 Valid Aug 08/1200 UTC thru Aug 12/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z NAM/GFS Evaluation Including Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Overall weather pattern across the continental U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend, least weight on 12Z NAM across Ohio Valley after Sunday afternoon Confidence: Average The overall weather pattern across the continental U.S. will be quasi-zonal across the northern half of the lower 48, with a slow, eastward moving (and somewhat compact) mid-level ridge over the south-central U.S. The main synoptic feature on the map will be a large mid-level closed low tracking eastward across Canada, with a related cold front at the surface forecast to move across the Great Lakes region. The models show fairly good agreement regarding the timing/strength of the closed low and a shortwave spoke rounding the base of the closed low, tracking from Saskatchewan on Sunday to western Ontario on Monday. A second closed low/shortwave is forecast to enter western Canada on Tuesday. The latest guidance shows only minor amplitude differences with the second closed low, with the 12Z NAM strongest. Larger scale impact differences are tied to convectively generated/enhanced vorticity maxima in the vicinity of the Upper Midwest, Great Plains and Ohio Valley. The 12Z NAM was strongest with one mid-level vorticity maximum that tracks eastward from MO into IL by Sunday morning. The 12Z NAM maintains robust strength of this feature into Tuesday as it tracks eastward toward the Mid-Atlantic region. However, the 12Z NAM does not have much support from the ECMWF/GEFS/CMC ensembles by Monday, nor the remaining deterministic guidance. For that reason, while the NAM may have utility elsewhere, little weight should be placed on the 12Z NAM for the Ohio Valley region from Sunday evening onward. Elsewhere, model differences are considered small enough to support a general blend of the guidance. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml This product will terminate August 15, 2020: www.weather.gov/media/notification/pdf2/scn20-58pmdhmd_termination. pdf Otto