Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 145 PM 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 19z update: While the 12z non-NCEP solutions continue to be very strong/consistent with the initial shortwave lifting north and initial pivot point; there is a transition particular with the ECMWF/UKMET trending slower, a bit sharper with the negative tilt trof and therefore east toward the 12z NAM and closer to the GFS. While the 12z GEFS does support the GFS, preference would be toward a non-GFS solution still; even though the trends were toward it, as it still appears out of tolerance in the overall cluster. ---Prior Discussion--- 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 19z update: Like the upstream closed low, the trend in the non-NCEP solution was very much in line with faster...very near the 12z NAM, closing the gap to the fastest GFS. So, while the spread is reduced, there is still some question in incorporating the faster GFS after 31.06z. So will stick with initial preference with 12z updates that does trend that direction but not fully to the GFS. ---Prior Discussion--- 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 19z update: The 12z ECMWF was joined by the 12z UKMET in trending a bit west of the overall guidance particularly after passing Cape Concepcion and the Channel Islands. This is still fairly small and will support a general model blend but confidence reduces to slightly above average, particularly after 30.18z. ---Prior Discussion--- 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: 12z ECMWF/CMC and 00z UKMET/ECENS mean blend Confidence: Slightly below average 19z update: The 12z UKMET trended toward a stronger shortwave that actually deepens with a very strong surface reflection into Vancouver Island, typical of upscale growth influences that tend to be incorrectly diagnosed (through latent heat release) by the UKMET, so would go away from the 12z run solution for the NW. The 12z operational ECMWF slowed with the shortwave/jet streak approaching the West Coast, and delayed the southward sag of the moisture directed into the NW. This brings it much closer to the initial preference (00z ECENS). Even though this remains a bit south of the other deterministic guidance, it is generally preferred over the slower/drier GFS/NAM solutions, though the CMC or still utilizing the 00z UKMET may help account for some uncertainty. Confidence remains slightly below average. ---Prior Discussion--- 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