Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1212 PM EST Thu Jan 02 2020 Valid Jan 02/1200 UTC thru Jan 06/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper low over Mexico shearing to the northeast... ...Frontal wave impacting the Gulf Coast/TN Valley... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Only minor timing differences remain with the shortwave shearing eastward and the related weak surface low to track from MS toward the east. ...Amplifying longwave trough across the central U.S. ...Surface low from the Lower MS Valley to the Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 06Z GEFS/00Z ECENS mean (nearest to 12Z GFS/00Z ECMWF) Confidence: Below average The interaction between a northern stream and southern stream component embedded within a longwave trough progressing eastward from the central U.S. translates into a difficult forecast evolution. By 12Z/04, the 12Z GFS begins to push ahead of the remaining guidance with the southern stream axis of vorticity along the northern Gulf Coast, due to a more amplified northern stream component across the Midwest. However, 12 hours later, at 00Z/05, the 12Z NAM is fastest with its 500 mb trough axis over the eastern U.S., followed by the 12Z GFS/00Z UKMET. The 00Z CMC is slowest with the 00Z ECMWF next slowest. With a fair degree of confidence, it can be said that the 12Z NAM and 00Z CMC have a low likelihood of verifying. However, among the remaining 12Z GFS, 00Z ECMWF and 00Z UKMET, the resulting surface low shows moderate spread. While the ensemble scatter low plots are rather broad in their depiction valid 12Z/05, the 12Z GFS's surface low is on the northern side of the surface lows, while the 00Z ECMWF is toward the south side. A significant number of ensemble surface low plots are west of the 12Z GFS/00Z ECMWF/00Z UKMET position, hence the rather agreeable 06Z GEFS and 00Z ECMWF means are west of their deterministic counterparts. The preference is to be between the 12Z GFS/00Z ECMWF and 00Z UKMET but with a slightly westward track of their surface lows. Due to poor ensemble agreement and poor run to run consistency in the guidance, confidence is below average. ...Shortwave trough/low impacting the Pacific Northwest... ...Energy ejecting out across the Plains/Midwest by Sat/Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF Confidence: Average The preference is to go toward the middle of the latest ensemble/deterministic model spread. The 12Z NAM is fastest followed by the 12Z GFS as the shortwave trough tracks east into the Northern Plains. The resulting cold front shows similarly, but the 00Z ECMWF appears to be in the favored middle ground position. ...Next mid-level trough axis to reach the Pacific Northwest by Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z GFS, 00Z ECMWF blend Confidence: Average While some amplitude differences exist with an upper trough axis nearing the Pacific Northwest by Sunday, timing differences are greater. The relatively agreeable ensemble means support a faster timing when compared to the 00Z UKMET/CMC. While minor differences exist among the 12Z NAM, 12Z GFS and 00Z ECMWF, a blend of these 3 models seems best for now. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto