Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1157 AM EST Mon Jan 20 2020 Valid Jan 20/1200 UTC thru Jan 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ ...Amplifying shortwave crossing the Midwest through Monday... ...Closed low evolution over the Southeast by Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: 12Z GFS/00Z ECMWF blend Confidence: Near average A shortwave impulse currently over the Midwest is tracking southeastward on the east side of an upper level ridge over the Rockies. This is forecast to evolve into a closed low near the Southeast Coast by Tuesday morning and then track eastward out to sea. This will induce surface cyclogenesis along a frontal boundary situated well offshore. Except for some light rain near coastal areas, this should mainly be a marine impact. The 12Z NAM and to a lesser degree the 00Z UKMET are considerably farther west than the ensemble means, and the NAM is a strong outlier. These differences become apparent mainly after Tuesday afternoon. ...Southern stream shortwave crossing California by Monday... ...Energy crossing the Southwest/Plains Tuesday and Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: general model blend Confidence: Above average The 12Z GFS/NAM appear slightly more progressive with the shortwave trough compared to the non-NCEP guidance as it crosses the Southwest through Tuesday. The 00Z CMC is a little weaker at the 700mb level with the shortwave and the GFS slightly stronger. There is now enough agreement among the guidance to merit a general blend of the deterministic guidance with this shortwave perturbation. ...Upper trough/closed low over the Pacific Northwest by Tuesday... ...Energy ejecting out across the northern Plains Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: Blend of the 00Z ECMWF/EC Mean Confidence: Slightly below average Some minor latitude differences continue to exist with this system as it approaches the Pacific Northwest on Tuesday with the 00Z ECMWF/UKMET north with the center of the closed low while the 12Z NAM is slightly south. As the upper level energy emerges over the northern High Plains, the 12Z GFS becomes more amplified. ...Upstream mid-level trough offshore of the Pacific Northwest early Thursday along with accompanying surface cyclone... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: 12Z GFS/00Z UKMET blend Confidence: Average Timing differences appear to be the significant reason for model differences with an upstream, shallow amplitude longwave trough to near the Pacific Northwest Wednesday night. The NAM/ECMWF are fastest and 00Z CMC slowest with the leading shortwave. There are modest differences reflected in the ensemble spread and given the source region for this system is an area of higher uncertainty, will prefer to go in the middle of the spread toward the 12Z GFS and 00Z UKMET at this time. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick