Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 151 PM EST Tue Dec 31 2019 Valid Dec 31/1200 UTC thru Jan 04/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation with final preferences/confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Low departing from New England today... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similarly with this system. ...Closed low entering Mexico and weakening Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/UKMET/CMC blend Confidence: Average Timing differences are still apparent as a closed low enters Mexico on Wednesday, with the 12Z NAM and 12Z ECMWF slowest while the 12Z GFS and 00Z ECMWF are slightly faster. Trends in the ensemble spaghetti plots of 500 mb heights show a sudden slowing valid 00Z/04 after what had been fairly consistent timing from cycle to cycle. The 12Z ECMWF/UKMET/CMC are consistent with the 00Z/31 ensemble cycle trend to be slower, which warrants a shift toward that trend. A nudge slower than the preliminary preference is recommended, which constitutes a blend of the faster 12Z GFS and slower 12Z UKMET/CMC. ...Atmospheric River into Pac NW today and amplifying longwave trough across the central U.S. Thursday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/UKMET/CMC blend Confidence: Slightly below average A longwave trough develops over the Great Plains Thursday into Friday with model differences with the evolution of shortwaves within. The 12Z NAM stands out the greatest with a more positive tilt to the southern portion of the mid-level trough axis by Friday evening. A slower trend in the 12Z ECMWF is a move toward the 12Z NAM but less extreme. Meanwhile the 12Z UKMET/CMC also trended a bit slower/deeper with the longwave trough making the 12Z GFS stand out from the deterministic guidance. Regarding an 850 mb low which is expected to track from the Southern Plains into the Midwest, somewhere between the 12Z GFS, 12Z CMC and 12Z UKMET is preferred which places a surface low along the IN/OH border at 00Z/04. ...Shortwave trough and surface low approaching Pacific Northwest Friday/Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: near the 12Z NAM Confidence: Slightly below average The ensembles show a fair degree of spread with the timing of the next shortwave to near the Pacific Northwest by the weekend. Upstream flow becomes zonal over the central Pacific which would imply a faster progression downstream into the Pacific Northwest. Indeed, trends have been faster in the ensemble spaghetti height plots over their past 4 00Z/12Z cycles and in the 12Z UKMET/CMC with a similar 12Z ECMWF to its 00Z cycle. However, caution is advised with the fastest and often biased GFS which is out ahead of the GEFS/ECMWF/CMC ensemble members. The 12Z NAM appears to be a better fit which is toward the faster side of the deterministic guidance but not as slow as the 12Z ECMWF/UKMET. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto