Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 109 PM EDT Mon Aug 10 2020 Valid Aug 10/1200 UTC thru Aug 14/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z NAM/GFS Evaluation Including Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Longwave troughing over the Northwest from Tuesday through Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z NAM blend Confidence: Slightly above average Through the middle of the week, the model consensus is for a longwave trough to amplify as it reaches western Canada into the northwestern U.S. The 12Z NAM is the fastest of the deterministic guidance to progress the trough eastward, noted Wednesday into Thursday and the NAM also lies on the faster edge of the latest ensemble guidance. Amplified flow upstream should tend to limit a faster progression depicted by the NAM and even trends in the ensembles have been slower over the past few cycles. ...Remainder of the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend, least weight on the 12Z NAM Confidence: Average There is weak low to mid-level troughing in place over the southeastern U.S. today, downstream of a low to mid-level ridge over the Lower Mississippi Valley. There is a secondary area of ridging located over the southern AZ/NM border, forecast to expand and slowly lift northwest through Thursday. Up north, quasi-amplified flow across the northern U.S. into southern Canada will impact the movement of a couple of cold fronts into the U.S. There is fairly good agreement on a departing mid-upper level trough axis from southeastern Canada and the attendant cold front into the Great Lakes and Northeast, which extends westward through the OH Valley into the Great Plains. The greatest disagreements are with small scale, convectively driven vorticity maxima which move and amplify/deamplify within the relatively weak steering currents over the central and eastern sections of the country. While no models appear as an outlier compared to ensemble spaghetti plots, the 12Z NAM appears too strong, especially with a mid-level shortwave/vorticity max forecast to be over MO/IA Thursday morning. Over-amplification of mesoscale features can be a common bias in the NAM, so for now, less NAM weighting is preferred within a general model blend across the remainder of the CONUS (outside of the Northwest trough discussed in the top section). 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