Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1214 AM EST Wed Feb 26 2020 Valid Feb 26/0000 UTC thru Feb 29/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Eastern Half of North America... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z CMC blend Confidence: Average A closed low over the southern/central Great Plains will open up into a mid-level trough during the day on Wednesday and track toward the Northeast for Thursday with a negatively titled axis in the middle and upper levels. An upstream northern stream trough will move across the upper Great lakes and phase with the energy moving into New England. A corresponding surface low will rapidly deepen as it tracks through Pennsylvania and New York Wednesday night, with secondary low development likely along the northern New England Coast on Thursday. After the system reaches maturity and begins to weaken, a deep 500 mb low will anchor over southern Quebec, setting up a favorable lake effect snow event downwind of Lake Erie and Ontario. The models have been flip flopping with the timing of the lead shortwave crossing the Appalachians late Wednesday into Thursday morning for the past few days. The differences are relatively minor through Thursday morning, but the 12Z CMC begins to depart from the remaining consensus on Thursday. The 12Z CMC ends up with a larger 500 mb low compared to the latest ensemble consensus and is also weaker with 850 mb flow across New York. The position of the 12Z CMC 850-500 mb low center also differs from the remaining model consensus, so a non 12Z CMC blend is preferred. The non 12Z CMC guidance is not identical, but a blend of the remaining guidance should help to smooth out smaller scale differences. This blend holds true for a pair of weak mid-level shortwaves diving southeastward through the Great Plains and into the southern U.S. where a non 12Z CMC blend represents the middle of the timing/position differences noted. ...Western U.S... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z CMC blend Confidence: Slightly below average A somewhat sharp mid-level shortwave will reach the Pacific Northwest coast sometime late on Friday, tracking into the northern U.S. Rockies and southern Canadian Rockies by Saturday morning. The ensemble means are in relatively good agreement with the timing of the trough but ensemble spread is modest. No clear trends have been identified regarding timing, but there has been general convergence toward the middle over the past 4 12/00Z cycles. The 12Z CMC is slower than the means, while the 00Z GFS is slightly ahead. No model is an outlier with respect to the individual ensemble members, but the 12Z CMC stands out the greatest. A blend of the remaining models regarding timing and strength is preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto