Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1214 PM EST Sat Jan 04 2020 Valid Jan 04/1200 UTC thru Jan 08/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper trough crossing from the Midwest to the East Coast... ...Surface low near New England today/tonight... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similarly with this system. ...Shortwave ejecting out across the Plains, reaching the Great Lakes by Sun night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z GFS, 00Z ECMWF blend Confidence: Above average There remain some minor placement differences with the mid-level shortwave as it nears the Great Lakes. The 00Z UKMET and 00Z CMC are farther south with the 500 mb trough axis, whereas the ensemble means are in good agreement supporting the reasonably similar 12Z NAM, 12Z GFS and 00Z ECMWF. ...Split-flow trough near the Pacific Northwest by Sun... ...Clipper type low approaching the Great Lakes by Tue... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF blend Confidence: Slightly above average Timing and latitudinal differences show up with the northern portion of a mid-level trough amplifying through the central to eastern U.S. Monday night. The 12Z NAM and 00Z CMC are slower while the 00Z UKMET is displaced northward with the vorticity maxima compared to the ensemble means and remaining deterministic guidance. The 12Z GFS and 00Z ECMWF are thought to be closer to the middle of the model spread and are preferred at this time. ...Southern stream trough/low near the TN Valley by Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM, 00Z ECMWF and 00Z UKMET Confidence: Average The southern portion of a broadening mid-level trough over the eastern two thirds of the nation Monday night into Tuesday is expected to coincide with a weak surface low to cross the Tennessee Valley. The 12Z GFS becomes significantly faster by Tuesday morning while the 00Z CMC shows slowest of the deterministic guidance. The 12Z NAM, 00Z ECMWF and 00Z UKMET show good agreement and are favorably positioned with respect to the ensemble means. ...Shortwave/fronts approaching the Pacific Northwest Tue... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 00Z UKMET blend Confidence: Average Moderate spread continues to exist with a shortwave trough to approach the Pacific Northwest late Monday into Tuesday. A warm front is forecast to reach western British Columbia and coastal Washington Monday morning as a low nears just north of Vancouver Island. There are some minor placement differences with the surface low, the 00Z UKMET is toward the southern side of the deterministic spread while the 00Z CMC is farther north. The 12Z NAM, 12Z GFS and 00Z ECMWF are in reasonable agreement here. However, larger differences show up due to the approaching shortwave to the west. With this feature, the 00Z ECMWF was faster while the 12Z NAM was weaker with the trough. The 00Z CMC was considered a bit slow with the feature while the 12Z GFS and 00Z UKMET showed reasonable agreement to one another and a blend of the latest ensemble means. Therefore, a blend of the 12Z GFS and 00Z UKMET will be preferred with this system given differences from the 00Z UKMET's surface low influence is felt farther north away from Washington. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto