Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 229 AM EDT Thu May 03 2018 Valid May 03/0000 UTC thru May 06/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Ejecting Southwest U.S. upper trough crossing the central Plains and upper Midwest through Thursday... ...Phasing with northern stream energy from south-central Canada on Friday... ...Surface low lifting from the central Plains Thursday to the lower Great Lakes on Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly above average There is generally very good agreement among the models with how this trough lifts into the Upper Midwest and Great Lakes. The most substantial difference is with the 12Z CMC, which shears the vorticity out in an elongated fashion and has a slower progression of the trough than other models. The other models, with a more compact and concentrated vort max, develop a stronger and more focused surface low, while the CMC struggles to do this. For these reasons, the preference is a non-CMC blend. ...Lingering trough over the Southwest on Friday... ...Ejecting east into the Lower MS Valley by Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z GFS; 12Z ECMWF Confidence: Average Greater differences exist with how the models handle the southern stream portion of the trough that will be ejecting into the central CONUS. This lingering portion of the trough has greater uncertainty. The preference is to lean toward a blend of the GFS and ECMWF which are reasonably similar and close to the ECMWF Ensemble Mean. The 12Z UKMET is flatter with the wave and offers a faster progression than the other models. The 00Z NAM is stronger and a bit slower. And the 12Z CMC seems unreliable as it keeps more of the mid-level vorticity sheared out and connected between the two ejecting waves. ...Low amplitude shortwave approaching British Columbia Friday... ...Upper level low/trough approaching the West Coast by late Saturday; interacting with the northern shortwave... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z GFS; 12Z ECMWF Confidence: Average Considerable complexity exists as the second approaching trough/low on Saturday begins to interact with the shortwave to its north near the coast of the Pacific Northwest. This leads to notable differences in the mass fields for the available models. The NAM, GFS and UKMET generally maintain a stronger northern stream wave, and eventually show a retrograde motion with the associated vorticity becoming increasingly concentrated (with a secondary low development). The ECMWF and CMC, meanwhile, show a weaker wave to the north, with the southern stream portion of the trough becoming more dominant by Sunday morning. There is no clear preference for one scenario over the other. Therefore, the model preference for this system is a blend of the GFS and ECMWF -- they handle the interaction of the troughs differently but their overall height pattern is very similar. The CMC and UKMET deviate from this consensus quite a bit in some spots. ...Upper level low developing near the Bahamas by Friday... ...Inverted surface trough developing by Friday, and extending into Florida this weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly above average Upper level low will develop at the cusp of a lobe of higher PV to the south of an anticyclonic wave breaking event over the western Atlantic. Models are in fairly good agreement with this taking place near the Bahamas by Thursday Night and Friday morning, and show very little difference in their mass fields. The 12Z CMC does show a stronger surface trough and a stronger ridge aloft to the north of the low than the other models. This was enough of a difference from the consensus that a non-CMC blend is preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers