Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Sat Jun 20 2020 Valid Jun 20/1200 UTC thru Jun 24/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Final Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Closed low over Manitoba/Ontario and associated parade of shortwaves across the Great Plains to Midwest ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/GFS blend Confidence: Slightly Below Average 19Z update: The 12Z global consensus on the Ontario low is now to do something similar to the 00Z ECMWF with ejecting much of the energy from the low currently near the south-central Manitoba/Ontario border northeast to James Bay by Monday and allow low redevelopment just north of Lake Superior on Tuesday. Multiple impulses still round the trough over the north-central CONUS through this time with differences among the guidance. Felt the 12Z GFS and ECMWF are similar enough to warrant a blend of them for the preference. The 12Z UKMET is farther south with the resultant low (over Lake Superior not north of) on Day 3 while the 12Z CMC is more open and farther east on Day 3. The 12Z NAM has the shortwave trough strength issue, so it is still ruled out. Previous Discussion... The main weather feature for the CONUS for the next three days is the closed low currently centered over Manitoba which is on the east side of an Omega High centered over Alberta. Shortwave energy rounds this low as well as shortwave troughs/impulses rounding the trough extending down the Great Plains which merely drifts east through Tuesday. These impulses crossing existing surface fronts provide much of the focus for convective storms across the central US through early next week. First, with the low center itself the 00Z ECMWF is most unique in that a more amplified shortwave rounding the base of the low tonight dislodges much of the energy and ejects it northeast to James Bay, leaving behind an open trough over northwestern Ontario extending to the eastern Dakotas by late Monday. All other deterministic global guidance maintains a closed low over western Ontario. However, the resultant trough positions are similar, so this does not rule out the 00Z ECMWF. Second, there are a number of shortwave troughs/impulses that round this trough through early next week. Global guidance struggles with the resultant convective processes from these impulses which is seen across the board in the latest available guidance and makes specific sensible weather forecasts particularly difficult. Welcome to summer (literally, astronomical summer begins this afternoon). The only model to greatly diverge with shortwave trough/impulse advection is the 12Z NAM which is more amplified with a shortwave trough reaching Vancouver Island later today and ejecting southeast from BC early Sunday and rounding the trough, crossing Nebraska late Monday. Therefore, the 12Z NAM is not preferred. Otherwise the synoptic patterns are similar among the other global guidance with the caveat of caution with convective processes. Troughs approaching the Pacific Northwest Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Average A deep Gulf of Alaska low persists near Kodiak Island through Tuesday. Troughs rounding this low cross the WA coast later today and approach on Tuesday. The 12Z NAM does stand out for amplitude with the trough axis reaching Vancouver Island later today, but the effect on QPF is minimal. The wave approaching Tuesday remains far enough offshore to not result in QPF over WA by 24/00Z, the end of this forecast period. Therefore a general model blend should be sufficient. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Jackson