Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 312 PM EDT Sat Sep 21 2019 Valid Sep 21/1200 UTC thru Sep 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep trough crossing the Plains/Midwest this weekend... ...Energy closing off over the Northeast early next week... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend...through 60 hours Non-GFS blend...after 60 hours Confidence: Slightly above average Model differences with the northern Plains to upper MS Valley trough are rather small through most of the weekend as the energy advances across the Plains and Midwest. By later Monday and Tuesday as the energy crosses the Great Lakes and moves into the Northeast, the 00Z-12z GFS becomes a bit more progressive than the remaining guidance. The 00Z-12z non-NCEP guidance is now all very well clustered and close to that of the 12Z NAM with the timing of the 700-500 mb low. Given the majority of models/ensemble means cluster with a slower motion that is not as progressive as the deterministic GFS, a general model blend is recommended through about 60 hours, and then a non-GFS blend thereafter. ...Upper trough digging into the West by Monday... ...Closed low evolution over the Southwest by Tuesday... ...Moisture/energy associated with Hurricane Lorena... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS/UKMET/ECMWF Confidence: Average The initial trough over the southwest deamplifies as it moves northeast out of the southwest into the plains. The National Hurricane Center's forecast for Hurricane Lorena has the system coming onshore into northwest Mexico and weakening. Among the current guidance the 12z NAM matches the NHC track the best, so it should be given more weighting. A second trough comes onshore in the Pacific northwest Sun and and the amplifies Monday as it amplifies into the southwest US. Several models/ensemble members indicated a closed 500 mb low develops, with the models then showing timing differences developing Tue. The 12Z ECMWF has nudged its 500 mb low track east and now clusters well with the 12Z GFS/12Z UKMET/00z Canadian solutions, which are now pretty well clustered with the 12z GEFS Ensemble Mean. The 12z NAM and 12z Canadian global are the farthest east solutions by the end of the period. Given an upstream ridge is likely to lead to slow forward motion, the Nam and 12z Canadian should be given the least weighting. Based on the latest clustering of solutions, a blend of the 12z GFS/12Z UKMET/00z Canadian and 12z ECMWF will be preferred. ...Upper trough over the northern High Plains by Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models have good agreement overall with the evolution of a broad upper trough departing the northern Rockies on Monday and into the Plains Tue, along with its associated cold front. There are typical modest timing/amplitude differences, but the model differences are modest enough that a general model blend is preferred. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Petersen