Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 231 PM EDT Thu Aug 13 2020 Valid Aug 13/1200 UTC thru Aug 17/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ...Long wave trough moving from the Northwest to the Western Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z CMC/GFS/ECMWF (greater weight to CMC) Confidence: Average after 15.12z 19z update: A sizable shift toward the trended noted in the 12z GFS was depicted in the 12z ECMWF/CMC and UKMET and GEFS. The UKMET still remains a bit south and therefore east while the 12z ECMWF is now even slower than the lifting into Canada on Fri/Sat. The 12z CMC shows greatest run to run continuity but also is slower to push the NAM to the fringe. This change moves more into line with initial meteorological thinking though the run to run variation still hearkens some remaining uncertainty even if it was more in the direction of preference. As such will favor the CMC in a blend with the GFS and lesser so the ECMWF. As the remaining elongated trof moves into the Great Lakes, the NAM appears slight too strong and would favor its removal now given better agreement into Day 3. Confidence increases to average through the entire short-range period. ---Prior Discussion--- A very strong closed low exists over NW Canada, with a stronger than normal jet between it and the amplified ridge in the Southwest US. This will continue to transport shortwaves through the northern tier, with a divergent/diffluent flow across the Red River Valley into MN building by late Friday as the ridge retrogrades west. The UKMET is fast entering this region and produces a very strong response but is also remains generally east of the ensemble suite/solutions. The 12z GFS has significantly shifted slower and much stronger in response to this favorable amplification environment; this may be too aggressive given the remaining guidance, but think this may be the right direction. The 00z ECMWF remains very weak and generally flat (which there is solid thunderstorm response), there is little surface development and so ejects into Canada rapidly, which is increasingly unfavorable. The 12z NAM/00z CMC are middle ground overall though are a tad west of the ensemble mean solutions. Overall, trends and global scale pattern would suggest a compromise very close to the GEFS/ECENS but some inclusion of the GFS would be prudent given the overall trend. As such a 12z NAM/00z CMC with lower weighted 12z GFS blend is preferred. The spread remains quite large and contingent on prior evening's convective events to have lower confidence through Day 2 (Saturday morning)...so is slightly below average After Saturday, the next shortwave/jet streak will continue to press the lingering trof energy eastward into the Midwest and Upper Great Lakes, supporting some negative tilt orientation, potentially increased by the evolution of the lingering shortwave energy in the Ohio/TN Valleys (see section below). The normally negative bias of a slower ECMWF was counteracted by the weaker evolution on Day 2, so the next wave appears to be amplifying similar to the remaining guidance (including the CMC/NAM/GFS), providing atypical increased confidence by Day 3 in the blend into the Great Lakes. ...Base of weak Rex Block across Ohio Valley lingers through Sunday before ejecting ahead of ... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly below average 19z update: There was a solid increase in clustering toward the 12z GFS with respect to the later arriving, UKMET, ECMWF, CMC and GEFS, particularity for the surface wave developing along the NC coast. This leaves the NAM a bit too deep in the Ohio Valley and then a bit too slow/east with the developing coastal wave. Normal ECMWF/CMC lingering of the filling trof across the Southeast by late Sunday is a more traditional/expected spread to have some mild increase in confidence, even though mass fields/flow remain contingent on convection and upscale chaotic enhancements. A non-NAM blend seems sufficient for now, with increase in confidence to slightly below average ---Prior Discussion--- A very muddled weak flow environment exists over the eastern third of the US with convectively induced MCVs driving additional development initially. With the greater approach of larger scale energy in the PacNW and retrograding ridge, a weak Rex Block pattern evolves by mid-afternoon Friday, though does get too blocky in nature as the ridge in Canada slides east by Sat leading to a larger positive tilt to the Rex pattern. This is leading to moderate overall spread due to the weak overall pattern. However, a shortwave entering the diffluent area today out of IA/MO, will settle and amplify across the Lower Ohio Valley. Only the 00z UKMET is very weak and leads to a different convective evolution (which feeds back on this weak wave), further east. So a Non-UKMET blend is supported initially. The ensemble suite (GEFS/ECENS/CMCE) has been trending toward a deeper and stronger solution through the Ohio Valley, which supports the 00z ECMWF which is solidly in the middle; and opposed by the remaining NAM/GFS and CMC which show greater convective development eastward and a trend to shift the trof that direction as well. Given the retrograding nature of the strong ridge, this seems counter intuitive, and so would favor the slower evolution after 16.00z. Confidence remains below average given the overall weak flow and contingency of convective/latent heat release and response to the upscale growth which is too chaotic in nature to result in any greater confidence. 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 Gallina