Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1233 AM EST Mon Jan 20 2020 Valid Jan 20/0000 UTC thru Jan 23/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS Evaluation with Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ ...Shortwave/front clipping the Pacific Northwest Monday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: General model blend Confidence: Above average Differences with this system are very minor and so therefore, a general model blend will be preferred. ...Amplifying shortwave crossing the Midwest through Monday... ...Closed low evolution over the Southeast by Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: 12Z ECMWF/CMC blend Confidence: Slightly above average As a shortwave amplifies southeastward across the Mississippi River, it is expected to close off along the Southeast and move out into the western Atlantic. The 00Z NAM ends up deepest and north of the favored position of the latest ensemble means. The 00Z GFS is also a tad north of ideal while the 12Z UKMET is slower. The ensemble means favor a blend of the 12Z ECMWF and 12Z CMC, through a non-NAM blend will also suffice for this system. ...Southern stream shortwave crossing California by Monday... ...Energy crossing the Southwest/Plains Tuesday and Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: 00Z NAM/12Z ECMWF blend Confidence: Average The 00Z GFS continues to appear too fast as the shortwave reaches the Great Plains and beyond as the feature approaches the Mississippi River. This is reflected in the latest ensemble guidance valid Tuesday evening as well with the GFS leading the pack of the GEFS spaghetti plots, while the ECMWF members trail the remaining guidance. The 00Z NAM is in the middle regarding timing, followed by the deterministic 12Z ECMWF (which lies between its ensemble mean and the NAM). The 12Z UKMET appears too slow while the 12Z CMC also appears a tad faster. Overall, a blend between the 12Z ECMWF with the faster 00Z NAM appears best for this system as ensemble trends appear to be approaching the middle ground. ...Upper trough/closed low over the Pacific Northwest by Tuesday... ...Energy ejecting out across the northern Plains Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: Blend of the 00Z GFS/12Z ECMWF/12Z CMC Confidence: Slightly below average Some latitude differences continue to exist with this system as it approach the Pacific Northwest on Tuesday with the 12Z ECMWF/UKMET north with the center of the closed low while the 00Z NAM is south. These differences amplify downstream into interior portions of the Northwest, downstream of a longwave ridge centered over the eastern Pacific. At this time, a middle ground is favored which currently is best represented by a blend of the 00Z GFS, 12Z ECMWF and 12Z CMC. Given above average ensemble spread and complex evolution of the downstream trough given multiple embedded vorticity maxima, confidence is a little below average for this system. ...Upstream mid-level trough offshore of the Pacific Northwest early Thursday along with accompanying surface cyclone... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: 00Z GFS/12Z ECMWF 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 00Z NAM is fastest and 12Z 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 00Z GFS and 12Z ECMWF 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 Otto