Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1253 PM EDT Fri Mar 16 2018 Valid Mar 16/1200 UTC thru Mar 20/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep Cyclone over the Northeast U.S. and Eastern Canada... ...Digging Shortwaves Wrapping Through South Side of Cyclone... ...Gradual Shift to the E/NE this Weekend and Early Next Week... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 06Z GEFS Mean, 00Z EC ENS Mean Confidence: Average As the broad cyclone continues to gradually shift east and northeast into the Canadian Maritimes, the Northeast U.S. should largely be in a northwesterly flow regime with a surface ridge building in by Monday. Models are in good agreement with this general progression, but show quite a bit more variability to the northeast closer to the center of the cyclone. A general, broad-based model blend would probably work best in the Northeast. Further to the northeast, models are struggling with how to handle the interactions between several waves that will be digging southeast and wrapping into the overall circulation. The overall preference for the system, therefore, is to rely on the GEFS and ECMWF ensemble means, which will best account for the variety of detail differences. The 12Z GFS and 00Z ECMWF are probably the closest operational model approximations of these means. ...Strong Plains Cyclone Weakening as it Shifts East into the Ohio Valley and Carolinas this Weekend... ...Upper Level Shortwave Shearing Out and Deamplifying... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Above Average Over the past several model cycles, models have moved into much better agreement with this system and now show tight clustering in many of the mass fields. Most of the surface low positions by 18/00Z are within 150-200km of each other in the Carolinas. Aloft, the 12Z NAM is still the fastest model and shows the strongest wave as it is being sheared out, but has trended substantially toward the global models. The NAM is close enough that it can't be discounted as a reasonable possibility, but should represent the stronger end of depictions of the wave aloft. ...Large Closed Low over the Northwest U.S.... ...Breaking Down into Peripheral Shortwaves on Sunday that Slowly Meander in a Lingering Shear Axis on Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Blend (0-48h); GEFS/EC ENS Means (48-84h) Confidence: Above Average early, trending to Below Average late Through around 18/12Z (48 hour forecast), the models are in quite good agreement overall -- even with the beginning stages of the breakdown of the upper level closed low. The global models and NAM all have similar positions and amplitudes of peripheral waves until that time, so the preference through Saturday Night is for a general model blend with relatively high confidence. After that point, the peripheral waves get caught in a residual shear axis as an upper level ridge rapidly builds into the Gulf of Alaska and British Columbia over the top of a strong closed low around 37N/145W in the Pacific, and shortwave ridging also builds into California and Nevada behind a departing trough (discussed below). The result will be relatively small-scale waves slowly meandering in weak flow and confidence in the forecast specifics is lower. Beyond 48 hours, the preference is to trend toward a blend of the GEFS Mean and ECMWF Ensemble Mean. As with the cyclone over Eastern Canada, the GFS and ECMWF are probably the operational models that most closely approximate these ensemble means, but the ensembles will be better able to capture the variety of possibilities. ...Stronger Peripheral Wave on the Western Periphery of the Closed Low moves into California on Saturday... ...Wave Ejects into the Plains on Sunday with Lee Cyclogenesis... ...Eastward Progress into the Mid-Mississippi Valley on Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 00Z ECMWF, 00Z CMC Confidence: Average As this strong wave kicks through the Southwest, all the models show strong lee cyclogenesis and all models also show a closed low forming aloft over the Plains (including the ensemble means). In general, with this sort of process, the preference would be to lean on the slower end of guidance as systems with a closed low aloft tend to progress more slowly. The 12Z NAM and 00Z UKMET tend to spread height falls into the Plains the fastest, and this may be due to them indicating considerably more (around 1 degree latitude) amplification to the trough as it reaches the lower Colorado River Valley early Sunday, as compared to other global models and ensemble means (which are tightly clustered). As the low closes off aloft by early Monday, the 12Z GFS is about 150km north of all other models, and appears to be even north of the GEFS mean. The GFS is, however, also on the slower end of the model spread. Therefore, the preference is to include the GFS to represent the potential for a slower progression to the wave, but the greater weight will be placed on the 00Z ECMWF and 00Z CMC which may have the low position at a more likely latitude. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers