Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1131 AM EST Sat Dec 28 2019 Valid Dec 28/1200 UTC thru Jan 01/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with preferences/confidence intervals ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Central Plains to Great Lakes closed low/surface cyclone(s)... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly above average GOES-WV suite depicts weakening Southwest shortwave progressing through the Central Plains with very impressive mid to upper level jet/anticyclonic baroclinic shields evolving across the Northern Plains at this time as well as the sharp trof along the eastern Rockies/High Plains and the meridional jet sliding south. The guidance appears to have come into common agreement with the timing/placement of the southern stream wave and associated 150kt 25H jet into the development of the pivot of the closed low in SE SD/SW MN/NW IA by 00z Monday. The model spread begins to increase in earnest as the upstream jet rotates around the base and the associated shortwave goes negative tilt through the Mississippi Valley into the Great Lakes. Here, the GFS has been, and continues to be with the 12z run, weakening the inner core in favor of this stronger/sharper negative tilt wave...which leads it to be well east with the lower level surface low through the Great Lakes. Making it less favorable overall. The 12z NAM, continues to be a middle ground between the ECMWF/CMC/UKMET and the GFS...but continues its trend toward the non-NCEP camp, suggesting a non-GFS blend is a solid slightly above average confidence preference. ...Remains of closed low/development of coastal low into Northeast Mon-Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend thru 31.06z, Non-GFS after Confidence: Average Upstream deep closed low, will continue to break down across the Great Lakes by late Mon into Tuesday and the former pivot/inner core center accelerates eastward under the newer mature center. By this time the GFS is not preferred in the Great Lakes, however, the GFS has done a solid job in the orientation/timing of the return Atlantic moisture around the currently retrograding subtropical ridge, through Florida, relative to the slower ECMWF. Combine this with stronger moisture advection with the atmospheric river out of the Western Gulf (over 1000 units of IVT), the 12z GFS may have more utility through the incipient development of the Coastal Low through the Piedmont of VA by 30.18z and through the NY Bight by 31.12z, which has a significant influence on the warm front moving through the Mid-Atlantic and Northeast. So in collaboration with WPC QPF 2/3 and Winter Weather forecasters, model preference to include some of the 12z GFS at least initially to account for the slowness/return of the warm air/moisture flux into the Northeast was important; however, toward the end of day 3, the GFS/GEFS appear to show some of traditional fast bias issues, that shifting toward the NAM is more preferable in weighting. So a general model blend is preferred through 31.06z before going non-GFS blend afterwards. Confidence is contingent on thermals which are less congruent than the mass fields/frontal structure as a whole, and as such, is average. ...Pacific shortwave diving along the West Coast Sun-Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The Pacific continues to be very active but progressive and with an upstream sharp ridge that blocks the flow and rapidly stretches it southward along the entire West Coast of the US. There is growing consensus in the timing and shape/evolution of the wave including the timing of the wave as it closes aloft and spins up a surface low near San Francisco Bay late Sunday into Monday. Overall, the guidance is very strong that a general model blend is supported at above average confidence. However, as a point of note for marine and Baja California interests, the ECMWF's slower nature absorbs the trailing shortwave energy in the shear axis much tighter and compact leading to a SW wobble even compared to the majority of ECENS members...so it appears to be an outlier to far SW after 31.00z (though with little/no influence in the CONUS). ...Atmospheric River sinking into Pac NW by Late Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z UKMET/ECENS mean blend Confidence: Average to slightly below average After the prior shortwave digs southward, the Pacific energy shifts back into the N Gulf of AK but strong zonal flow and high energy in the northern Pacific stream, draws tropical moisture northward. The moisture and associated jet flow is relatively weaker initially and directed toward Central BC by late Monday into Tuesday. However, the overall flow/height-packing tightens and a strong 250mb -150kt jet entrance approaches and directs moisture flux toward Vancouver Island and then southward into the Pacific NW by the end of New Year's Eve. The 00z ECMWF is very strong and along with the UKMET show a faster initial wave with increased moisture compared to the GFS/NAM. Here the ECMWF/UKMET may have a bit better initial assimilation of the moisture flow/stream than the GFS/NAM due to greater satellite retrievals. So hedging this direction. However, the 00z ECMWF is very divergence in the anticyclonic (right exit) showing increased divergence aloft and supporting greater ascent and southward propagation of the convergence axis into the northwest, greater than the vast majority of ECENS members. As such will favor the UKMET and ECENS mean in timing/orientation of the AR but at average to slightly below average confidence given the spread in moisture fields and importance of timing in such a strong zonal flow. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina