Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1208 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 preliminary 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, 00Z ECMWF, 00Z UKMET blend Confidence: Average Timing differences are still apparent as a closed low enters Mexico on Wednesday, with the 12Z NAM and 00Z CMC 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. While there could be a flip back toward the faster side, prefer for now to stay toward the middle to faster side of the latest available guidance, or near the 12Z GFS, 00Z ECMWF and 00Z UKMET. ...Atmospheric River into Pac NW today and amplifying longwave trough across the central U.S. Thursday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 00Z ECMWF, 00Z UKMET blend Confidence: 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. The 12Z GFS, 00Z ECMWF and 00Z UKMET are in the middle but the 00Z ECMWF is weakest with an 850 mb low which is expected to track from the Southern Plains into the Midwest, but the 12Z GFS/00Z UKMET may be too strong here. A blended approach is considered best with better agreement on position compared to intensity. ...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. 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 00Z ECMWF/UKMET/CMC. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto