Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 118 AM EDT Wed May 02 2018 Valid May 02/0000 UTC through May 05/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS/UKMET with preliminary model preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ NAM/GFS initialization errors do not seem to degrade their short range forecasts. Shortwave moving into the Northwest Friday night ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Canadian/GFS/ECMWF blend Confidence: Above average The 00z UKMET was more amplified with this system than the other guidance, while the 00z NAM was on the weak extreme. As the system is moving over the top of the ridging across the West, the system should be fairly weak. Even so, the NAM looks too weak. A compromise of the 12z Canadian, 00z GFS, and 12z ECMWF is preferred with above average confidence. Shortwave moving into the Lower MS Valley early Saturday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: GFS/ECMWF/UKMET blend Confidence: Above average The 00z NAM is stronger with this shortwave than the other guidance, its usual bias. Meanwhile, the 00z UKMET appears weaker/more shallow at 500 hPa, though it is more comparable with the other guidance at 700 hPa. With a broad upstream ridge, believe the shortwave should have some amplitude associated with it. Prefer a compromise of the 00z GFS/00z UKMET/12z ECMWF here with above average confidence. Northern Plains system late Friday/early Saturday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Above average The 00z NAM digs a shortwave more significantly than the other guidance, which leads to a stronger low moving near the central US/Canadian border Saturday morning, its usual bias. The general flow pattern favors shortwaves with minimal amplitude, thus favor a compromise of the 00z GFS, 00z UKMET, 12z ECMWF, and 12z Canadian with above average confidence. Phasing systems near the central US/Canadian border Fri morning Surface low moving from CO through MI by Friday morning ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-Canadian blend Confidence: Average The 12z Canadian was north of the other guidance with the surface wave sweeping across the Plains and Great Lakes due to a more northerly track in the mid-levels. A compromise of the 12z ECMWF, 00z GFS, 00z NAM, and 00z UKMET stays within the broader 12z global ensemble clustering with the surface wave and is preferred with average confidence. Retrograding Upper Trough moving towards FL early Saturday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: ECMWF/GFS/UKMET blend Confidence: Average The guidance has been advertising the development of this system for about two weeks now, so feel fairly confident in its formation. An anticyclonic wave breaking event leads to the forming of an upper low/TUTT cell which moves westward towards FL under the base of northerly displaced subtropical ridge. Of the current guidance, the 12z Canadian is the slowest/deepest of the guidance, with the 00z UKMET/00z GFS the quickest. The 12z ECMWF shows some convective feedback north of the Bahamas early Saturday which leads to a sharper easterly wave that is more held back than the 00z GFS/00z UKMET. However, the guidance can sometimes retrograde mid-level cyclones too much. Prefer a compromise of the 00z GFS, 00z UKMET, and 12z ECMWF to deal with the lingering uncertainty with average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Roth/Otto