Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 148 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 Model Evaluation with Final 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: non 00Z NAM 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 but quicker trends from the 00Z UKMET/CMC (relative to their previous 12Z cycles) along with the consistent ECMWF supports a general non-00Z NAM blend for this system. ...Southern stream shortwave crossing California by Monday... ...Energy crossing the Southwest/Plains Tuesday and Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: 00Z NAM/ECMWF/UKMET 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 00Z ECMWF and 00Z UKMET (which both sped up from their previous 12Z cycles and are faster than the ECMWF ensemble mean). The 00Z CMC did not change much from its previous 12Z cycle and continues to appear too fast. Overall, a blend between the 00Z ECMWF with the 00Z NAM and 00Z UKMET should work 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/ECMWF/CMC Confidence: Slightly below average Some latitude differences continue to exist with this system as it approach the Pacific Northwest on Tuesday with the 00Z 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, 00Z ECMWF and 00Z CMC, noting that there was a small trend to be faster as the trough axis reaches the Northern Plains in the 00Z ECMWF/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/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 00Z 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 00Z 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 Otto