Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 134 PM EST Fri Feb 09 2018 Valid Feb 09/1200 UTC thru Feb 13/0000 UTC ...See NOUS42 KWNO (ADMNFD) for status of the upper air ingest... ...12Z Model Evaluation with Final Preferences/Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwave Moving Through The Central Rockies on Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z ECMWF, CMC, UKMET Confidence: Slightly Above Average ---18Z UPDATE--- As the remainder of the 12Z runs have come in, the CMC and UKMET have moved into much better alignment with the ECMWF, and all are close to the ECMWF Ensemble Mean. The GFS remains faster, albeit less amplified, while the NAM is the fastest and most amplified. Given the trends of the CMC and UKMET, the preference will shift to a blend of those models and the ECMWF, removing the 12Z GFS. ---PREV. DISCUSSION--- Models are in decent agreement with the shortwave that will be pushing through the Intermountain West and Central Rockies on Saturday, before ejecting into the Plains and contributing to the eventual weak cyclogenesis along the front in the eastern CONUS. The 12Z NAM shows a much more amplified wave with considerably faster timing; this is not supported by any other models and also eventually contributes to differences in the eastern CONUS (discussed below). Otherwise, there is a typical spread with less amplified models (12Z GFS, 00Z CMC) a little faster with the timing, and more amplified models (00Z ECMWF, UKMET) a little slower. Both the GFS and ECMWF have decent support from their ensembles, so at this point the preference is for a general blend that excludes the NAM. ...Extensive Front Stretching from the Southern US northeastward into New England and Broad Southwesterly Flow Aloft... ...Weak Surface Low Development Along Front on Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 60% 12Z ECMWF, 40% GEFS/EC Ens Mean Confidence: Slightly Below Average ---18Z UPDATE--- The 12Z ECMWF has largely maintained continuity with its previous run in its mass fields. The UKMET (and now CMC) continues to show a track of the surface low further inland over the Northeast. The preference remains for the scenario shown by the ECMWF, which is also close to the 12Z GEFS Mean and 00Z ECMWF Ens Mean. Will substitute the ensemble means for the CMC for the final preference. ---PREV. DISCUSSION--- As mentioned in the previous section (Central Rockies shortwave), the 12Z NAM has a much stronger wave ejecting more quickly, and this eventually leads to a stronger, further-northwest surface low in the Northeast on Sunday and Sunday Night. The preference remains to exclude the NAM for this system as well. The 00Z UKMET also has a stronger surface low further to the northwest, but it arrives at that via different means than the NAM. From a conceptual model standpoint, with a broad elongated low-level baroclinic zone and a lack of distinct, strong wave aloft arriving to focus cyclogenesis, the expectation would be for a weaker surface low that would tend to move quickly along the front (rather than curling back to the northwest). Therefore, the weaker scenarios offered by the GFS, ECMWF and CMC seem more reasonable. The 12Z GFS, however, develops a lead surface low well ahead of other models along the front. It eventually develops a secondary low more in line with the consensus timing. The lead surface low appears to be associated with an ejecting wave currently over northern Mexico, and perhaps some convection over the Tennessee Valley. Given a lack of support from the other models and the GEFS mean, the preference is to lean more in the direction of the CMC and ECMWF, with a greater weight placed on the ECMWF. ...Shortwave Undercutting Northward Extension of Ridge in the Gulf of Alaska and Pushing Into Pacific Northwest by Sunday... ...Continued Digging and Amplification of the Trough on the West Coast into Monday and Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, ECMWF, CMC Confidence: Average ---18Z UPDATE--- The UKMET continues to have the furthest south approach of the shortwave along the coast of the Pacific NW, and is even further south of its 00Z run. The ECMWF and CMC trended south very slightly, but the consensus between those two models and the GFS remains reasonably good. Therefore, there is no change to the preliminary preference. ---PREV. DISCUSSION--- Model spread still exists with the latitude that the shortwave will approach the Pacific NW. The 00Z UKMET has the most southerly solution, bringing the trough into the coast around 46-47N. The 12Z GFS and 00Z ECMWF/CMC have a decent consensus and lie in the middle closer to Vancouver Island and 49N. The 12Z NAM and 00Z ECMWF Ensemble mean are furthest north around 51N. The 00Z UKMET seems less likely as it pushes the wave east well to the south of the apex of the ridge, which would be atypical for these cases. It also is unsupported by other models. The preference in this case is to lean toward a blend of the GFS, ECMWF and CMC, which have the best consensus with the approach of the wave to the Pacific Northwest; they eventually show greater differences as the positive tilt trough builds down the West Coast, but are reasonably similar. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers