Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 235 PM EDT Thu Mar 15 2018 Valid Mar 15/1200 UTC thru Mar 19/0000 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 from Hudson Bay Region Wrapping Through South Side of the Cyclone with Gradual Shift to the East By Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Above Average ---18Z UPDATE--- The 12Z CMC has come into better alignment with other global models, and is now included in the preference. A general model blend is still preferred overall. ---PREVIOUS DISCUSSION--- Deep, occluded cyclone currently near the mouth of the St. Lawrence River will continue to gradually shift north to Labrador over the next day or so. At the same time, digging shortwaves will carve out and reinforce the base of the trough across the Northeast through Friday. Another low aloft in the polar regions of Canada will shift southeast from Nunavut on Friday, and begin to interact with the original cyclone by Saturday. Despite the number of component parts involved to the broader evolution of the trough over Eastern Canada and the Northeast U.S., the numerical models are in fairly good agreement. The relatively small spread that exists generally is not manifest in a systematic way, so the preference is for a general model blend to account for uncertainty in the specifics of individual waves. The one exception is the 00Z CMC, which has systematically higher heights across a significant portion of the region in the early hours of the forecast, and this allows some differences to grow over time. ...Amplified Trough along the West Coast with Closed Low Pushing into the Northern Intermountain West by Saturday and Breaking Down into Smaller Peripheral Waves by Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 12Z ECMWF, 12Z UKMET Confidence: Average ---18Z UPDATE--- The most notable shift on the 12Z model cycle appears to be a trend toward a more amplified trough kicking through the Southwest on Sunday. All the global models and GEFS mean show a more amplified trough, while the 00Z ECMWF Ensemble Mean (which was closer to the consensus on the 00Z cycle) is much less amplified than consensus now. The 12Z NAM still appears to be too fast and amplified relative to the other models (especially given the strength of the wave), so it will still be excluded from the preference at this time. The high height bias on the 12Z CMC is not quite as significant as the previous model cycle, but still notable enough along the West Coast to maintain the preliminary preference. ---PREVIOUS DISCUSSION--- As with the pattern over the Northeast, the 00Z CMC also seems to have consistently higher heights along the West Coast, and given this bias away from the global model consensus and ensemble means (with a lack of support) it is excluded from the preference for this system. Other models show good agreement through Saturday, with differences emerging from one of the more potent peripheral waves rotating around the weakening closed low. This peripheral wave will move from near coastal British Columbia tonight, to the California coast by Saturday, and kicking through the Four Corners on Sunday. This will lead to lee cyclogenesis later in the weekend over the central High Plains, and models are actually tightly clustered in showing this occurring near the CO/KS border. During the phase of the forecast during which the wave moves through the Southwest, the 12Z NAM shows a considerably deeper trough with the core of the strongest height falls displaced further to the south. This has limited model support at this time, so the preference is to also exclude the NAM from the blend, which also maintains continuity from the previous model diagnostic shift. ...Negative-Tilt Shortwave Ejecting into the Central Plains by Friday with Associated Strong Plains Surface Cyclogenesis... ...Deamplifying through Ohio Valley and Carolinas This Weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 12Z ECMWF, 12Z UKMET Confidence: Slightly Above Average ---18Z UPDATE--- The remainder of the 12Z model runs maintained continuity relatively well with their previous runs, albeit with slight shifts toward the growing model consensus. Although the 12Z CMC has shifted its position of the upper level low over the Plains southeast (closer to other global models), it still remains on the northwest side of model spread and eventually shows a more amplified and slower trough kicking east through the Ohio Valley. Given these continued differences, the preliminary preference is maintained. ---PREVIOUS DISCUSSION--- Models have come into much better agreement as compared to 24-36 hours ago with respect to the ejecting wave into the Plains, and the associated surface low. Through 17/00Z there is fairly tight clustering among available models, with the largest initial difference coming from the 00Z CMC. Most models develop a closed low aloft near the base of the trough, where the strongest PVA is likely to be concentrated. This more closely matches conceptual models. However, the 00Z CMC shows the center of the low aloft roughly 150-200km to the northwest (more over NE CO than NW KS at 16/12Z). Therefore, the preference is to exclude the CMC at this time. Further out in the forecast, spread begins to increase primarily as it relates to timing. The 12Z NAM is faster than the other models as the wave deamplifies and gets kicked to the east into the Ohio Valley and Carolinas. It's about 6 hours faster than the other global models. The 12Z GFS did trend a little slower with the wave -- it previously was closer to the NAM, but has moved in the direction of the 00Z ECMWF and UKMET. The preference is to lean toward the global model consensus and the slower scenario overall given the initial development of a closed low aloft. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers