Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 100 PM EDT Wed Jul 29 2020 Valid Jul 29/1200 UTC thru Aug 02/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z NAM/GFS Evaluation Including Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Mid-upper level troughing over the Great Lakes and Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average Only minor timing differences were noted with this system, such that a general model blend can be used through Saturday. ...Energy advancing across the Plains through Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM/12Z GFS blend Confidence: Slightly below average Mid-level energy coming out of the Rockies into the Central Plains slowly sags southeastward toward the Ozarks on Friday and elongates with broader, larger scale troughing along the MS River through the Great Lakes region by Saturday morning. The models begin to diverge with a mid-level vorticity max located near the base of the trough late Friday. There are no model outliers, but the 00Z UKMET appears too strong here. Timing-wise, the middle of the guidance is favored, with the 12Z GFS/NAM closest to the middle of the spread. The 00Z ECMWF/CMC are faster, especially the 00Z CMC...while the 00Z UKMET is slightly slower and much stronger. ...Upper low/trough approaching the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average There are no significant differences noted with this system. ...Potential Tropical Cyclone Nine approaching southern Florida around Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM/00Z CMC closest to 15Z NHC Advisory Confidence: Refer to NHC products Potential Tropical Cyclone (PTC) Nine will track from near the U.S. Virgin Islands westward toward Cuba and southern Florida through Saturday. Model spread varies considerably with this feature with a somewhat southwestern trend was noted in the last few cycles of ensemble low plots. The best model proxy to the official NHC forecast from 15Z is a blend of the 12Z NAM and 00Z CMC. A 50/50 blend of the northern 12Z GFS position and southern 00Z ECMWF position also yields close to the NHC track. Meanwhile the 00Z UKMET appears to be slower than the NHC advisory. For the latest information on PTC Nine, please see the updated NHC advisory. ...Cold front moving through the north-central U.S. Friday and Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average There is relatively good agreement for the trough axis and related cold front impacting the north-central U.S. through Saturday. While minor timing differences are evident, such as the 12Z GFS being slower, the differences are small enough for a 60 to 84 hour forecast to allow for a general model blend at this time. 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