Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 146 PM EST Sat Dec 08 2018 Valid Dec 08/1200 UTC thru Dec 12/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Central Gulf and Southeast storm... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend, no UKMET after 10/18z. Confidence: Slightly above average 19z update: The ECMWF/UKMET and CMC all trended a bit faster, allowing for a slight northward jog in the track, very near the 12z NAM, enough to consider it a bit more in the blend, perhaps at a lower weighting...but still included. As such this leads to a non-GFS blend preference at slightly above average confidence for the lead shortwave and surface cyclone. Interestingly, the slight northward jog, also allowed for similar evolution/deepening of the lingering trof over the terrain in preparation for the secondary shortwave out of the Central Plains, and allows for a slight faster adjustment for the slower ECMWF/CMC solutions very near the timing of the NAM...though both remain more reasonable in the depth (as the NAM shows typical over-deepening bias by end of day 3). The UKMET did slow a bit, but remains out in front of the other guidance sizable to remove it from the blend after 10/18z. ---Prior discussion--- As one would expect, solutions appear to be converging with the wave crossing out the central Gulf states through the southeast over the next two days. Still, the GFS remains a bit fast and suppressed to the south for full consideration in the blend. Similarly (while positively trending southward), the 12z NAM appears a shade too far north, and eventually becomes too amplified with the lingering trof on late Sunday/early Monday, leading to a southward suppression of the digging upstream trof out the Central Plains. While not precisely the same, the UKMET which handles the leading wave quite well within the ensemble suite, it is aggressive in the southward/faster progression of the Central Plains shortwave. As such in starts to fall out of favor, with lower weighting for the secondary surface cyclogenesis/frontal location of the GA/SC coast late Mon into Tuesday. The 00z CMC while, not bad with the leading shortwave/height-falls appears to wrap back more energy into the cold air across the Tennessee Valley Sunday and maintains it, leading to a slowing of the exiting of the impacting weather in the Carolinas which also negatively affects the approaching Central Plains shortwave. The 00z ECMWF solution continues to be stable, but properly trending faster with each run, and this was no different and can be favored throughout the forecast period. As such a 00z ECMWF/UKMET blend is supported initially with infusion of the 00z ECENS/06z GEFS mean at slightly above average confidence. The UKMET will fade from preference in favor of the 12z GFS around 48 to 54hrs (12-18z/10th) for the lingering frontal zone/deformation zone in the Carolinas and upstream shortwave and through 00z Wed...confidence is slightly above average for this time period as well. ...North, Great Lakes and Northeast Mon/Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-ECMWF blend Confidence: Slightly above average 19z update: The ECMWF continues to be very strong/amplified with narrowing wavelength across the Great Lakes, when even the CMC has trended a bit broader and slower, in line with the ensemble suite (including the ECENS members). As such a non-ECMWF blend is preferred for this phased wave out of the northern stream. ---Prior discussion--- Global scale trof over Northeast Canada ushers northwesterly flow out of Central Canada with phasing Arctic and Pacific streams across the Great Lakes late Monday into Tuesday. Here the 00z ECMWF is clearly too sharp with the southern portion of the phased trof with a very sharp wavelength and similar cold punch/thermal packing, and is an outlier in the 00z ECENS suite. The 00z CMC shows some similarity with a stronger shortwave and is slightly faster than the ECMWF, at least the thermal setup appears a bit more in line with the ensemble suite (GEFS/ECENS/CMCE), The 12z GFS and NAM along with the UKMET are likely a bit too flat and fast to be fully supported in a blend, and as such will incorporate more GEFS/ECENS mean in the blend with the 12z GFS/NAM and 00z UKMET to compensate. Confidence is slightly above average. ...West into Rockies... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM/CMC and *00* ECMWF/ECENS mean blend Confidence: Below average 19z update: The northern stream/polar jet focused tighter in the UKMET/ECMWF trending more GFS-like, even both outpacing the GFS in the northern stream into the Northern Plains by 00z Tuesday. This is a dramatic shift, while it is generally a dry trof through the CONUS, this is a large shift and adds more uncertainty. The CMC is a bit more like the 12z NAM, being a bit broader in width and slower in timing than the 12z UKMET/ECMWF. In addition to this more zonal jet, the bulk of guidance (exception of the ECMWF) is less apt to closing off a closed low feature along the southern half of the trof. All in all, this leads to reduced confidence and a preference near the initial one, of a 00z ECMWF and 12z NAM/CMC blend but at below average confidence. ---Prior Discussion--- Broad wave with numerous internal perturbations advances toward the Pacific Northwest coast by Sunday with fairly good agreement generally up to 10/12z nearing landfall of the mean trof. Afterwards, upstream and downstream influences/model differences dominate a departure from best agreement and the trof shears/elongates...with the 12z GFS, per usual, being faster advancing the strong post frontal polar jet across southwest Canada/Northwest US, and much weaker southwest of California. This leads to a cut-off low developing well southwest of the US and fast advancement of the northern stream trof through the northern Rockies. The 12z NAM and 00z ECMWF/UKMET all generally show similar evolution if slightly spaced different due to mild timing differences (NAM-faster, ECMWF-slower). The 00z CMC is uncharacteristically fast, leading to a more neutral-tilt through the Great Basin, starting to cut-off the low in UT/CO by 12/00z, this may be supported by a few CMCE members but the bulk of ensemble suite does not suggest this northern cut-off solution. As such a 00z ECMWF/UKMET and 12z NAM blend is supported at average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina