Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1254 AM EDT Sat Aug 15 2020 Valid Aug 15/0000 UTC thru Aug 18/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Preliminary Model Evaluation Including Preferences and Forecast Confidence ...Longwave trough expanding across the eastern CONUS into early next week... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non UKMET through 17.12Z Non UKMET/GFS D3 Confidence: Slightly above average A longwave trough will amplify across much of the eastern CONUS through the weekend and into early next week. Through 60 hours, the mass fields are pretty well aligned with the exception of the UKMET which features higher heights across the east, with a more pronounced shortwave ridge into SE Canada D1-D2. A shortwave trough or even weakly closed mid-level low will develop within the longwave trough across the OH VLY Sunday morning before ejecting eastward towards the Mid-Atlantic and offshore by Monday morning. Again, the spread outside of the UKMET is pretty small with this feature so a general blend excluding the UKMET is reasonable. By D3, Monday into Tuesday, the longwave trough axis tries to get pulled back to the SW and spread increases dramatically. It appears the evolution of this trough axis will be dependent on interaction with a shortwave, possibly convectively enhanced /MCS/ rotating atop the ridge to the west and down into the Plains Monday morning. The GFS appears to be most robust with this interaction which drives the trough to pull way SW from Sunday, and becomes out of tolerance with the remaining guidance (again excluding the UKMET from the blend). While there is little confidence in timing and evolution of these convectively enhanced shortwaves at these timescales, outside of the UKMET and GFS the spread is within the typical D3 tolerance so the remaining models can be included in the preference. ...Shortwave moving into the Pacific Northwest Sunday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non UKMET/NAM Confidence: Average A potent closed mid-level low off CA Saturday will get pinched between an expanding ridge over the Southwest and an amplifying trough digging out of Alaska. This will force the low to open and shear to the northeast, lifting onshore Washington State Sunday night into Monday morning. The NAM is quite fast compared to the remaining guidance with moving this feature to the northeast and is removed from the blend for its speed. Although the UKMET timing is well aligned with the consensus, it continues to feature heights within the ridge that are above the other globals, a bias it continues, which forces the shortwave to be displaced a bit NW of the preference. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml This product will terminate August 15, 2020: www.weather.gov/media/notification/pdf2/scn20-58pmdhmd_termination. pdf Weiss