Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 143 PM EST Mon Dec 30 2019 Valid Dec 30/1200 UTC thru Jan 03/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation with final preferences/confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Great Lakes and Northeast closed low... ...Surface low tracking along New England coastline on Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z GFS blend through 00Z/01, general model blend afterward Confidence: Slightly above average ...19Z update... No significant changes were noted with the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. No changes are recommended for the final preference. ...previous discussion follows... The more significant differences with this system pertain to the secondary surface low tracking along the New England coast for Tuesday. The 12Z GFS was weaker (noted at 925 mb) with the low resulting in less low level northerly flow into the Northeast. By Tuesday evening however (00Z/01), the differences become less noticeable as the low deepens and pulls away from the lower 48. ...Closed low entering Mexico and weakening Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: near the 12Z ECMWF Confidence: Average ...19Z update... Only minor adjustments were observed with the 12Z non-NCEP guidance relative to their previous 00Z cycles. There were some adjustments to the timing of the subsequent longwave trough however, which are discussed below. ...previous discussion follows... Timing differences are apparent as a closed low enters Mexico on Wednesday with the 12Z GFS leading the pack with a faster track. The 00Z CMC ends up slower through Thursday morning. Ensemble trends had been a bit faster but backed off any significant timing trend with the last cycle (initialized 00Z/30). The faster GFS/GEFS and slower CMC are reflected in the latest ensemble distribution with the 00Z ECMWF/ECMWF mean near the middle. The preference is to be near to slightly ahead of the middle ground, or near the 00Z ECMWF at this time. ...Atmospheric River sinking into Pac NW tonight and amplifying into longwave trough across the central U.S. Thursday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend weighted toward the 00Z ECMWF Confidence: Slightly below average ...19Z update... The 12Z ECMWF and 12Z UKMET slowed down the timing of the longwave trough axis approaching the Great Plains Thursday morning while the 12Z CMC sped up...relative to their previous 00Z cycles. The latest ensemble spread supports the preliminary preference given a 12Z non-NCEP blend weights toward the western side of the latest ensemble spaghetti plots by Thursday evening. ...previous discussion follows... There are relatively minor differences with the atmospheric river event into the Pacific Northwest early on with the 12Z GFS weaker and south of the better model clustering valid 00Z/01 to the north of Vancouver Island regarding a surface low. This results in a slightly weaker magnitude to the low level flow into Washington and Oregon. As the mid-level energy evolves downstream, a longwave trough forms over the west-central U.S. with a few embedded shortwave impulses. Differences in timing are noted with a 700 mb trough axis into the central U.S. by Thursday evening with, similar to the above system, a 00Z ECMWF middle ground being favored. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto