Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1238 PM EDT Sat Jul 27 2019 Valid Jul 27/1200 UTC thru Jul 31/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including the NAM/GFS and preliminary model preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Northern Tier of the U.S... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preliminary Preference: General model blend Confidence: Slightly above average One shortwave is expected cross Ontario and Quebec, skirting northern New England Sun morning, with a trailing cold front lingering from the Central Plains to the Great Lakes. Meanwhile, a second and more energetic shortwave and associated cold front are moving into the Pacific Northwest this morning, expected to cross the northern Rockies and Northern Plains tonight/Sunday and the Upper Midwest/Great Lakes Sun night/Mon before the energy consolidates into an upper low near James Bay Tue. The GFS showed a difference from the other guidance with respect to QPF across the Upper Midwest/Upper Great Lakes on Day 2 (Sun night/Mon), with most other guidance showing a band of potentially heavy convection that is notably absent from the GFS. This difference does not seem driven at the synoptic scale but likely rather by differences at the mesoscale. Thus, while the GFS mass fields seemed usable for the most part, it was not preferred for QPF across the Great Lakes on Day 2. Finally, an upper low is forecast to dive southward off the British Columbia coast next Tue, with leading height falls just reaching the Pacific Northwest by 00Z Wed. Model differences were minor with respect to these systems through the short range, and a general model blend is preferred. ...Remainder of the CONUS including the Southwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preliminary Preference: General model blend Confidence: Above average A strong upper ridge is expected to persist across the Four Corners region through the short range. Models show slightly varying intensities for the ridge, but a compromise approach is preferred at this time. Farther east, models show a relatively weak and elongated mid/upper trough axis developing from the Great Lakes to the Southeast by Mon night/Tue south of the deepening James Bay upper low. Solutions vary on the amplitude of this feature, but spread among ensemble members suggests that either the slightly more amplified NAM/GFS or the weaker 00Z UKMET are all possibilities, and thus a multi-model blend approach is likely best at this time. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Ryan