Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Tue Mar 19 2019 Valid Mar 19/0000 UTC thru Mar 22/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation Including Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall Pattern Across the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z GFS, 12Z ECMWF Confidence: Average ---06Z UPDATE--- No change to the preliminary preference. At this time, prefer to maintain a blend of the 00Z GFS and 12Z ECMWF, rather than the new 00Z cycle of the ECMWF. The new run showed some notable changes, with the initial surface low developing over 150km inland over North Carolina, and then taking an interior track until it passes 40N. The 00Z GFS still has greater support from the GEFS (00Z run) and the 12Z ECMWF Ensemble Mean. The initial low development closer to the Gulf Stream also makes more conceptual sense. That being said, the 00Z NAM, CMC and UKMET are all much slower (varying positions relative to the coast) than even the 00Z ECMWF, so there is the potential for future model cycles to trend slower. ---PREVIOUS DISCUSSION--- The greatest amount of model spread over the next few days is related to the coastal low expected to develop off the Mid Atlantic and toward the Northeast. The model preference listed above was based predominately around that system, but the preferred blend is applicable elsewhere. The 12Z ECMWF and 00Z GFS are in reasonably good agreement across most of the CONUS, and a blend of the two should represent a consensus forecast close to ensemble means (with an average amount of model spread). A notable trend with the Northeast coastal low over the past several model cycles is toward (1) a stronger low and (2) a position closer to the coastline. This is not an uncommon trend several days in advance of nor'easters, and given that it is present across multiple models, it seems realistic. Ensemble sensitivity analysis of the low shows that the forecast is sensitive to the upstream shortwaves that will be phasing. In particular, the forecast is especially sensitive to the strength of the northern stream wave that will dig into the Great Lakes on Wednesday. Models that show a stronger northern stream wave (00Z NAM, 12Z ECMWF) tend to show lower surface pressures off the Northeast coast later in the week, and vice versa. The intensity of the low remains a source of some uncertainty, but the 00Z GFS and 12Z ECMWF show very similar low tracks. Their forecast low positions are nearly on top of each other through 22.12Z, and are also nearly on top of the 18Z GEFS Mean and 12Z ECMWF Ensemble Mean positions. A blend of these models is the preference. Although the ECMWF shows a stronger northern stream wave, the overall evolution is similar and consistent with expected model trends in these scenarios. A blend of the two will offer a compromise between a stronger scenario (ECMWF) and a moderate-strength low (GFS). Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers