Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Wed Aug 12 2020 Valid Aug 12/1200 UTC thru Aug 16/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Longwave trough over the Northwest to the Northern Plains through Friday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: NAM/CMC/ECMWF blend Confidence: Average A broad upper level trough, in conjunction with a closed low over southwestern Canada and a closed mid- and upper-level high along 155W, will be in place across the northwestern U.S. through the middle of the week. Overall, the models are in agreement with the idea that the trough gradually shifts eastward and be in place across the northern Plains by the weekend. This will sustain a cold front tracking from the northern Rockies to the Dakotas. The 12/12Z GFS becomes faster and more amplified with this trough by 12Z Friday across the Dakotas than the 12/12Z runs of the UKMET and ECMWF. This fast GFS solution tends to be a known bias...so will continue to favor a blend of the 12/12Z NAM/CMC/ECMWF preference. Northeastern U.S. upper trough ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: ECMWF/CMC/GFS blend into Saturday morning...GFS/CMC by Saturday Afternoon Confidence: Average An upper level trough axis gradually builds in across New England and eastern Canada by the end of the week, downstream of a building ridge axis across Ontario and western Quebec. The latest NAM was the strongest with the trough, while the GFS was slightly east of the model consensus. Blending the ECMWF and CMC still yields a suitable starting point in the process into Saturday morning. However, the 12/12Z ECMWF closes off a mid- and upper-level high over Maine by Saturday afternoon/evening while the GFS continues to move an open wave eastward...resulting in 500 mb heights being nearly 80 meters lower in the GFS over Down East Maine compared with the GFS. Prefer to give little weight to the ECMWF idea by Saturday afternoon. Southeastern U.S. upper trough ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12/12Z GFS/NAM/CMC Confidence: Below average A weak closed low currently over Missouri will slowly track eastward and evolve into an open trough as it reaches the Ohio Valley and southern Appalachians by this weekend. The models have a better handle on the evolution of the flow pattern aloft but some rather large differences develop at 850 mb and...to a lesser extent...at 700 mb with respect to where a closed low forms. The 12/12Z ECMWF placed its low level low in Tennessee on Saturday morning...a northward shift compared with its 12/00Z run...while the NCEP guidance and CMC favored low pressure north of the Ohio River. The approach of a mid-level wave from the Upper Midwest and the western Great Lakes region appears to favor the more northerly solution...prompting a shift in our preference to the NAM/GFS/CMC solution. This does not preclude there being a secondary area of low pressure over the Tennessee valley given its position downstream of a closed mid- and upper-level high over the Southwest. Confidence remained below average. 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 Bann