Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1245 PM EDT Tue Aug 04 2020 Valid Aug 04/1200 UTC thru Aug 08/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z NAM/GFS Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Hurricane Isaias... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 15Z NHC advisory package Best model proxy: Non-12Z GFS blend There was generally good model agreement with the track of Isaias as it crosses eastern NY state on its way into Quebec by 05/12Z, though the consensus was a bit faster than the NHC track. The 12Z GFS, however, was much too weak with the system and continues that trend as it moves into eastern Canada through 05/12Z, after which time the pressure differences are not as egregious. Due to the pressure difference, the 12Z GFS was left out of the preferred blend. Please consult the latest NHC forecast advisory package for more details. ...Broad long wave trough from the Great Lakes to the Gulf Coast... ...Closed mid level low over Ontario... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The 12Z GFS becomes a bit faster than the tightly clustered consensus with the broad long wave trough tracking across the eastern US, weakening as it does so. Otherwise, there is generally good model agreement and continuity with the evolution of the long wave trough. There is also good model agreement (including the 12Z GFS) with the track of the closed mid level low as it tightens and crosses Ontario into Quebec before reaching Newfoundland after 08/00Z. Based on the above, a general model blend is preferred, with above average confidence. ...Short wave energy dropping down over the Northern Plains/Upper Great Lakes Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The 12Z NAM/GFS are close to the consensus taking short wave energy from southern Manitoba across the Northern Plains Wed, before tracking it across the Upper Great Lakes by 06/12Z. After that time, the short wave energy becomes absorbed into the circulation of the deepening closed low moving across Quebec and Newfoundland Wed into Thu. Based on the tightly clustered solutions, a general model blend is preferred with this. ...Southern stream trough moving into CA Wed/Thu... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The 12Z NAM/GFS are close to the consensus taking short wave energy near 29N 129W into Southern CA before 06/00Z. There is a subtle hint of a closed mid level circulation trying to for on the northern portion of the evolving long wave trough over Northern CA near 05/12Z, but most solutions keep the trough open. Additional short wave energy dropping into the back side of the long wave trough Thu into Fri pulls the trough just offshore of Central and Southern CA by 08/00z. While there are minor differences in the timing of the short wave energy dropping into the long wave trough position Thu/Fri, a general model blend is preferred with above average forecast confidence. ...Mid level trough crossing the Pacific Northwest Wed before reaching Alberta/Manitoba Fri... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-12Z GFS blend Confidence: Slightly above average The 12Z NAM/GFS are close to the consensus taking an evolving long wave trough from the British Columbia/Pacific Northwest coast Thu morning into south central Alberta by 07/12Z. After that time, the 12Z GFS becomes closer than the consensus, as it forms a closed mid level low over east central Alberta. The remainder of the guidance is more progressive taking the negatively tilted trough central Manitoba by the time. The GFS has been slower with this feature over the last three model cycles, so the 12Z GFS was not included in the preferred blend. Because of the consistently slower GFS, model confidence is slightly above average. 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 Hayes