Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Wed Mar 14 2018 Valid Mar 14/1200 UTC thru Mar 18/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Evolution of Deep Cyclone over the Northeast U.S. and Eastern Canada through Saturday... ...Several Shortwaves Digging from Hudson Bay Region and Wrapping into Southern Side of the Cyclone... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Slightly Above Average ---18Z UPDATE--- No change to the preliminary preference. Models have come into better agreement with the details of the low-amplitude shortwaves that will be digging into the Great Lakes and Northeast with the 12Z model cycle. ---PREVIOUS DISCUSSION--- Models are in excellent agreement over the first 24 hours of the forecast, but slight differences grow with the arrival of shortwaves digging south from around Hudson Bay into the west and southwest sides of the large upper level trough and cyclone. However, even beyond 24 hours the position and intensity of the cyclone (through the depth of the troposphere), as well as the height gradient on the southwest periphery of the trough, are not the subjects of great disagreement among the available models and ensemble means. The digging shortwaves do perturb the large scale flow enough to lead to slightly different scenarios by the end of the short term forecast period, however it does not appear to substantially affect the depiction of sensible weather impacts. Given the lack of confidence in selecting the most representative model for sharply digging, low-amplitude shortwaves, the preference is for a general model blend which should better account for these small-scale uncertainties. ...Amplified Trough Along the West Coast with Closed Low Pushing into the Northern Intermountain West by Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF, 12Z UKMET, 12Z GFS Confidence: Slightly Above Average ---18Z UPDATE--- The 12Z CMC has backed off slightly on its amplification of the trough late in the period over central and southern California, but still shows lower heights as compared to the other models. Additionally, by 18/00Z it has a trough axis to the west of other global models. The 12Z NAM now sits well to the east, as the other global models have moved into better alignment (and sit very close to the ensemble means). For that reason, the NAM and CMC are excluded from the preference for this system. ---PREVIOUS DISCUSSION--- Over the past several model cycles, the global models and ensemble members have come into much better agreement with the timing of the closed low, generally trending toward the slower scenario offered consistently by the ECMWF and many ECMWF ensemble members. This allows for slightly higher confidence, as the models are fairly similar. As the closed low becomes increasingly separated from the polar jet arcing well to the north near the Arctic Ocean, changes should be relatively slow to occur as the low drifts to the southeast, with several smaller shortwaves rotating around the larger circulation. To smooth out these detail differences (for which lower confidence exists on specifics) a general model blend is preferred at this point. The one exception is with the 00Z CMC, which takes a shortwave near the coast of British Columbia around 16/06Z and more aggressively digs it along the West Coast, eventually leading to greater trough amplification around central and southern California. ...Negative Tilt Shortwave Ejecting into the Central Plains by Friday and Deamplifying into Ohio Valley by Saturday... ...Associated Strong Plains Surface Cyclogenesis and Evolution... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF, 12Z UKMET, 12Z GEFS Confidence: Average ---18Z UPDATE--- The 12Z CMC did trend slightly slower with the trough, and is more in line with the preliminary preference, but its surface low is still furthest to the northwest. Otherwise, there continues to be very tight clustering of the surface lows from the 12Z ECMWF, 12Z UKMET, 12Z GEFS Mean, and 00Z ECMWF Ensemble. Therefore, the preliminary preference is unchanged. ---PREVIOUS DISCUSSION--- Models continue to converge toward a more similar forecast for the evolving low in the Plains, but one of the primary difference appears to be the speed of the ejecting wave and the associated northeastward progression. Almost all models develop a closed low aloft over the central Plains at some point during the cyclogenesis process, which makes sense given a strong negatively tilted ejecting shortwave and strong low-level cyclogenesis. In general, the progression of the low in these situations tends to be on the slower end of guidance, which is represented by the 00Z ECMWF, UKMET, and ECMWF Ensemble Mean in this case. The 12Z GFS, NAM, and 00Z CMC are faster, with the 06Z GEFS Mean in the middle. There is also a general bias for the GFS to be on the faster end of model spread, and the ECMWF on the slower end, which is the case for this system. The preference is to continue to lean in the direction of slower models (ECMWF, UKMET), given the rapid development of the closed low aloft -- these tend to be situations in which slower low progression is favored. However, the GEFS Mean is also included in the preference in the event that the ECMWF and UKMET are slightly too slow. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers