Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 230 PM EDT Fri Jul 06 2018 Valid Jul 06/1200 UTC thru Jul 10/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall Model Analysis for the CONUS... Overall Preference: Non-CMC Blend General Confidence: Slightly Above Average ---18Z UPDATE--- No major changes to the preliminary preference or reasoning. The 12Z CMC has moved toward the other models slightly, but still has the same trends as described below. ---PREVIOUS DISCUSSION--- Models show a high degree of similarity in their mass fields over most of the CONUS. With the ridge over the West building and expanding to the east through the period, the pattern should largely evolve to a dominant ridge over most of the country, a trough along the Pacific Northwest coast, and zonal flow clipping the northern tier of states near the Canadian border. A general model blend is generally preferred given the overall high confidence in the synoptic pattern and positioning of major features. However, the 00Z CMC was displaced a little further west (about 150-200km) from the consensus of the other models. It does not build the ridge as fast to the east, and its trough along the Pacific Northwest is displaced a bit to the west. Therefore, the CMC is not included in the preference. The primary uncertainty will continue to be related to convective processes, which is common in the summer. With a lack of strong synoptic forcing, thunderstorm outflows and related low-level boundaries can play a larger role in focusing convection in future days. For example, the 12Z NAM appears to be suffering from some convective feedback with a mid-upper level vort max that dives southwest into the northern Gulf of Mexico and then curls back northwest into Texas. This seems to be unlikely, and why a general model blend is preferred, but is illustrative of how convective processes can dominate in the absence of strong synoptic forcing. ...Tropical Disturbance off the Coastal Carolinas... Preference: 12Z GFS; 12Z ECMWF; 00Z ECMWF Ensemble Blend Confidence: Slightly Below Average ---18Z UPDATE--- The 12Z UKMET now shows a closer approach to the coast than any of the other models. The preference was for something closer to the 00Z ECMWF Ensemble Mean, and the closest models to that at this time are the 12Z GFS and 12Z ECMWF. All these models show a stalling system. The NAM and CMC continue to eject it northeast more, but the CMC has slowed down considerably. ---PREVIOUS DISCUSSION--- The National Hurricane Center has a high (70 percent) chance of tropical cyclone development in the next 48 hours with the disturbance off the coastal Carolinas. Models are reasonably close over the first 36 hours of the forecast, but then begin to diverge, and the primary differences are likely related to the strength of the upper level ridge to the west, how fast that ridge builds in, and the timing and amplitude of a trough that will be digging into eastern Canada by Monday. These factors will control the evolution of steering flow and the path of the system. A reasonable intermediate scenario for the track of the associated surface low is offered by the 00Z UKMET and the 00Z ECMWF Ensemble Mean which show very slow progress to the northwest over the next few days. The 12Z GFS and 00Z ECMWF deterministic runs drift the low a little further west or southwest, while the 12Z NAM and 00Z CMC eject the low faster to the northeast. The 00Z CMC in particular is not preferred, for reasons stated above. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers