Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1238 PM EDT Sun Apr 29 2018 Valid Apr 29/1200 UTC thru May 03/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Preferences...with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low moving through the Northeast through Monday... ...Associated surface low over New England through Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Model forecasts for this system are very similar, so a general model blend is the preference with a high degree of confidence. ...Closed low opening into a trough over the Intermountain West today, with resulting shortwaves ejecting northeast into the Northern Plains on Monday and Tuesday... ...Lee cyclogenesis over the Plains with a surface low pushing through the Upper Midwest and into Ontario... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS; 00Z ECMWF Confidence: Slightly below average Models continue to struggle with consistency for this system, particularly as the resulting shortwaves eject northeast into the Plains. This process will be complex, and as a result there are a variety of model forecasts. In general, these shortwaves will be competing with an expanding ridge over the central CONUS as well, in response to the digging wave into the Southwest. The 00Z UKMET continues to amplify the downstream ridge more substantially than the other models, and therefore is not preferred. The 00Z CMC may be somewhat affected by a different forecast of the digging wave into the Southwest, and also shows a more substantial secondary shortwave digging into the Northern Plains from western Canada by Wednesday Morning. The CMC is also not preferred. The remaining models -- 12Z NAM, 12Z GFS, and 00Z ECMWF -- are largely similar in their depiction of the overall flow pattern, and are relatively close to the GEFS Mean and ECMWF Mean with their height fields aloft. They still have differences, however, with the ejecting shortwaves and the associated surface low development. The 00Z ECMWF notably has a more potent mid-level vorticity maximum that moves almost north from NE CO into ND and leads to a much stronger surface low over Minnesota. This affects the QPF forecast, both in placement (further north and northwest) and intensity. The ECMWF Ensemble has a similar placement, with the GFS/GEFS further south, although the ECMWF Ensemble mean is much lower with its QPF amounts. For more details on these differences refer to the QPF discussion. However, given the overall similarity in the broader pattern and support from ensemble means, the preference for the mass fields with this system will be a blend of the GFS and ECMWF, which is also a continuation of the preference from overnight. ...Shortwave digging from the Gulf of Alaska to California by Monday with low possibly closing off aloft by Wednesday morning in the Southwest... ...New surface low development in the central High Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS; 00Z ECMWF Confidence: Average Once again, the forecast for the UKMET is likely affected by its amplification of the downstream ridge, and as a result it has the trough axis further west than other models as it digs into the Southwest. The 00Z CMC and 12Z NAM appear to dig the base of the trough further south, and this has less support from the ensemble means. The preference, therefore, is to lean toward a blend of the GFS and ECMWF, which will also provide a consistent preference for the evolution of the broader trough over the entire western CONUS. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers