Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 131 PM EDT Fri Jul 31 2020 Valid Jul 31/1200 UTC thru Aug 04/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z NAM/GFS Evaluation Including Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Mid-level trough digging across the Mid-MS/TN/OH Valleys... ...Surface low tracking through the OH Valley on Saturday and into southeastern Canada Sunday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS, 00Z ECMWF and 00Z UKMET Confidence: Average Mid-level energy digging southeast across the mid-MS Valley will drive a trough in across much of the TN/OH Valley going through the weekend. This will be reinforced by Sunday by an intrusion of northern stream troughing dropping down across the Great Lakes region but a lead shortwave within the initial troughing will coincide with a surface low tracking across the MS River Valley into the Midwest and eventually southeastern Canada for Sunday night. The 12Z NAM is stronger with upper level jet energy in the base of the trough axis near the Gulf Coast Sunday morning which leads to a stronger and southeastward displaced trough axis on its southeastern side by Sunday evening. Meanwhile with the northward tracking surface low into Canada, there are only minor differences, but the 00Z ECMWF was a little faster than the consensus late Sunday. Overall, a 12Z NAM, 12Z GFS, 00Z ECMWF and 00Z UKMET blend is reasonable here, but note that the faster 00Z ECMWF should be accounted for. ...Hurricane Isaias... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: See NHC special 04Z advisory Best model proxy: 12Z NAM, 12Z GFS, 00Z ECMWF blend...through 12Z/02 00Z GFS/GEFS blend...12Z/02 through 00Z/04 From early on, the 00Z UKMET differs significantly east from the 15Z NHC track for Isaias, also resulting in a stronger system. The 00Z CMC also differs in the other direction with a more southern/western track over the FL Peninsula resulting in a weaker storm. While not an exact match to NHC, a blend of the 12Z NAM, 12Z GFS and 00Z ECMWF appears reasonably close to NHC through 12Z/02 but this blend is more inland than the NHC track by about 30 miles. Beyond 12Z/02, the 00Z ECMWF slows the speed of Isaias more than the NAM and GFS with a 12Z NAM/12Z GFS blend closer to the 15Z NHC advisory after 12Z/02 until 00Z/04. There is a need for increasing weight on the 12Z GFS's faster speed for 12Z/03 and 00Z/04, noting both the NAM and GFS are farther west of the NHC track by about 30-50 miles. ...Northern stream trough setting up over the western Great Lakes region by Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF/00Z UKMET blend Confidence: Slightly above average Differences with the phasing of, or lack thereof, mid-level energy tracking through Manitoba and Ontario affect the pattern evolution over the western Great Lakes region and Midwest for early next week. Ensemble means support a 12Z GFS/00Z ECMWF blend with the 12Z NAM and 00Z CMC indicating less phasing to the north and greater amplification of troughing near Lake Superior and down into the MS Valley on Monday. The 00Z UKMET is plausible as another option similar to the 12Z GFS and 00Z ECMWF. ...Upper low/trough approaching the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF/00Z CMC blend Confidence: Slightly below average Timing differences show up with the trough axis as it nears British Columbia and the Pacific Northwest late on Sunday. The 12Z NAM and 00Z UKMET are more noted to be more progressive with a shortwave in the base of the trough on Monday. The pattern from the northeastern Pacific into the southern half of Canada is rather amplified and may favor slower progression. This is represented in the agreeable ensemble means with a slower trough axis than even the 12Z GFS, 00Z ECMWF and 00Z CMC through Monday night. The preference is to go with the slower side of the deterministic spread at this point, noting that ensemble spaghetti plots show a great deal of spread with no defined trends. 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