Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 332 PM EDT Sat Jun 13 2020 Valid Jun 13/1200 UTC thru Jun 17/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Final Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low developing in Southern Appalachians Mon/Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average 19Z Update: The ECMWF has continued to trend south and is a bit south of the consensus by the end of D3. Otherwise, the spread is minimal enough outside of the more amplified NAM that the GFS has been added back in and the GEFS mean is also reasonable. The CMC remains on the NE edge of the spread, but has been consistent and is within the envelope expected by D3. Previous Discussion: A shortwave dropping through the Great Lakes tonight will eventually get cut off from the flow and begin to drop nearly due south into early next week as a closed low. The trend in the guidance over the past several runs has been for a more southerly position of this closed feature by Tuesday, with the consensus now favoring a suppression to Georgia. While the guidance is in very good agreement in the initial development of the potent shortwave dropping into the Ohio Valley through 36 hours, there is some spread that develops thereafter as the low opens into a more diffuse but still closed feature aloft. The NAM remains a strong s/w outlier, which has considerable impacts on QPF placement and intensity, which as noted by the previous forecaster is a known bias of this model, despite its potential to resolve mesoscale intensity features better than the global guidance, while the GFS features less run-to-run consistency than the non-NCEP suite. Otherwise, despite a subtly more NE position and evolution of this closed feature by the CMC, the spread amongst the non-NCEP models is minimal, and has support from the ECENS. ...Interaction of shortwave lifting through the Rockies with a closed low advecting onshore Washington Sunday... ...Secondary closed low moving into the Northwest Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NCEP blend through D2 ECMWF/CMC D3 Confidence: Slightly Below Average 19Z Update: Biggest change with the 12Z non-NCEP suite was with the UKMET becoming much more amplified with the closed low/gyre across the west by the end of D3. There is quite a bit of spread in the amplitude of that trough after 60 hours, likely due to complex interactions noted below. The ECMWF/CMC appear to be very closely matched and have shown less run-to-run discontinuity suggesting higher confidence in their evolution. Previous Discussion: A complex series of shortwaves and closed lows will interact across the Northwest through early next week. The leading shortwave will move through the Great Basin today and into the Canadian Rockies tonight, slinging around a closed low dropping into Washington. The guidance has trended a bit faster with this feature as the geostrophic flow becomes pinched to the east, but the GFS remains a strong and fast outlier with the leading shortwave. This creates a situation in which the stronger/further north energy pulls the weakening closed low northward during its interaction, and produces a closed low across parts of SW Canada. The NAM is a bit weaker but shows a similar evolution, with the GEFS mean just a bit weaker still. The non-NCEP camp shears out the Great Basin shortwave more quickly, leaving less energy during the interaction and allows the weakening closed low to remain dominant producing a sheared and elongated NW to SE trough by late Sunday. The weaker solution of the primary shortwave is more reasonable due to strengthening mid-level shear, so the non-NCEP camp is preferred into D2. Thereafter, yet another closed low dives out of the Gulf of Alaska, and the binary interaction of this feature with the trough ahead of it will leave a large gyre spread from western Canada into the Pacific. The GFS is actually now on the slow edge of the guidance in dropping this next closed feature southward, against its typical bias, and much different from previous runs. The NAM remains on the fast side as the low gets pulled eastward, likely due to a stronger lead feature during the interaction. For continuity, and due to the better agreement/consistency of the non-NCEP models, will maintain a preference of the non-NCEP suite but by D3 a little less weight on the UKMET which is also showing its fast bias, but note that despite differences in the mass fields, only the UKMET really suggests significant differences in QPF. Weiss Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml