Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1249 AM EDT Fri Aug 14 2020 Valid Aug 14/0000 UTC thru Aug 17/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Preliminary Model Evaluation Including Preferences and Forecast Confidence ...Long wave trough moving from the Northwest to the Western Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non GFS or UKMET Confidence: Average Longwave trough moving across the northern tier will be reinforced by a potent vorticity maxima rotating through the base of this trough and into the Western Great Lakes Saturday, with a secondary vort lobe driving eastward quickly on its heels Sunday. The globals are overall in very good agreement both with position and intensity of this lead shortwave, but the GFS continues to exhibit a progressive bias and outruns the consensus early in the forecast period. While the other guidance seems to have sped up a bit, the GFS being on the fast edge of the envelope makes it an outlier and the preference does not include it. The UKMET, while it is similar to the others with the intensity of this lead impulse, its bias of being too strong with ridges shows up with its 12Z run which causes a latitudinal position north of the other guidance and is also not preferred. The second shortwave and associated sensible weather will be highly dependent on the effects of the first, with the lingering influence driving position and intensity changes in the associated convection beneath the second shortwave. For consistency, a blend which does not change from the first shortwave seems most reasonable as small scale interactions from thunderstorms are difficult to resolve even by D2. ...Longwave trough across the eastern CONUS ... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-ECMWF/UKMET Confidence: Slightly below average Although the overall spread is pretty small in the amplitude of the trough digging into the Gulf Coast this weekend, there is much more variability in the secondary trough moving through the Ohio Valley and how it impacts surface low development by Sunday moving across the Mid-Atlantic. The ECMWF remains an outlier and is completely out of phase (shortwave ridge vs. shortwave trough) in the OH VLY Saturday aftn before eventually forming a lagging trough. Otherwise, the deterministic guidance is in pretty close agreement with the mid-level feature, although the GEFS mean is a bit deep compared to the consensus, and the UKMET continues to feature heights above the rest of the models even within the trough. The GFS may be a bit fast to push the accompanying surface low to the northeast, and a subtly lower blend near the NAM/CMC could be a reasonable solution, but interaction with a wave off North Carolina the morning, and potential MCVs diving into the trough through the weekend will all impact the evolution of both surface and mid-level features. This leaves lower than average confidence, and something between the fastest GFS and slower GEFS/ECMWF is probably most realistic at this time. 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