Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1134 PM EST Mon Dec 30 2019 Valid Dec 31/0000 UTC thru Jan 03/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation with preferences/confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Great Lakes and Northeast closed low... ...Surface low tracking along New England coastline Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average As the upper level low weakens/opens up over the Northeast U.S. over the next 24-36 hours, a secondary surface low will develop off the Mid-Atlantic to New England coast. Earlier differences with the track/timing (GFS faster) have diminished with the most recent guidance now showing fairly tight clustering of solutions. As a result, there is high enough forecast confidence and consistency to go with a general model blend for this region. ...Closed low entering Mexico and weakening Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non-CMC blend Confidence: Average Timing differences are still apparent as a closed low enters Mexico on Wednesday, with the most recent guidance showing the significant slow bias of the 12Z CMC. The 00Z GFS and NAM show similarities at 500 mb to the ECMWF/UKMET through about 60 hours, then the NAM becomes a bit slower than the other guidance. For now, will lean on a non-CMC blend given its delayed/slow scenario. ...Atmospheric River sinking into Pac NW tonight and amplifying into longwave trough across the central U.S. Thursday evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 03.00z followed by a ECMWF/UKMET blend Confidence: Slightly below average There's fairly good model agreement and run to run consistency seen in the atmospheric river event heading for the Pacific NW the next 24-36 hours. As the shortwave digs across the west and eventually carves out a longwave trough for the central U.S. mid/late week, a surface low is forecast develop in the vicinity of TX/OK. With the neutral to slightly positive tilted trough in place, wavy low pressure should lift along the south-central Plains to mid-MS River Valley. There are some notable differences in the low track/position. The low track plots at 03.00z and 03.12Z show sizable spread that lowers overall forecast confidence. A consensus approach is appropriate for the first 60 hours (general model blend) but then a ECMWF/UKMET/GFS blend is preferred, given the CMC's timing bias. ...Shortwave trough and surface low approaching Pacific Northwest Friday/Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: ECMWF/UKMET blend Confidence: Slightly below average After a brief period of ridging, another shortwave trough approaches the Pacific Northwest late in the week into this weekend. There are considerable timing differences typically seen in the guidance, where the GFS is much faster compared to the non-NCEP guidance. The 00Z GFS races out ahead of most of the guidance and is significantly faster than the GEFS mean as well (which is close to the ECMWF/ECENS solution). As such, will favor the non-NCEP guidance with a ECMWF/UKMET blend preference. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor