Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1235 AM EDT Sun Sep 30 2018 Valid Sep 30/0000 UTC thru Oct 03/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Central and Eastern US... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Within fast zonal flow aloft, a shortwave trough will track across the northern tier US and southern Canada through tonight, dropping a surface front across the lower Great Lakes by Monday. Upper level energy moving across the Intermountain West will generate a weak surface low that treks into the Upper Midwest Monday. 00z NAM/GFS fall in line reasonably well with previous guidance in the major synoptic features, perhaps on the southern side of the consensus with its low track by Monday night. By the end of Day 3 (12z Wed) there is some spread among the guidance in a weak shortwave across New England which is likely due to differences upstream with stronger energy coming into the Pacific NW. Overall, given the reasonable agreement for most of the major features in this area of the CONUS, a general model blend is preferred. ...Gulf Coast/Southeast.... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Much of the period will feature subtropical ridging across the Southeast US into the Texas Gulf Coast region. At the surface, lingering/stalling frontal boundaries in association with high moisture will likely generate mostly diurnally driven convection. So while mass fields are very similar and a general model blend is preferred, there is some spread in the QPF which is due more to mesoscale features more difficult to resolve in this time frame. ...Western US/Northwest/Northern Rockies/Northeastern Pacific... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS and 12z GEFS/ECENS Confidence: Slightly below average Several systems to consider across the West Coast/Pacific NW and in general there is below average confidence in the forecast. First, a shortwave trough will push into the Pacific NW today then trek eastward into the Northern Rockies. There is good agreement in the deterministic and ensemble guidance with this feature. Beyond that, the setup becomes more interesting and less agreeable as a piece of upper level energy from BC interacts with a closed low off the West Coast that then dips southward. Behind that, a stronger shortwave will swing across southwest Canada into the Pac NW and northern Rockies by Tuesday-Wednesday. The 12z ECMWF was slower compared to the 00z NAM/GFS and 12z CMC with the closed low off the West Coast by Tuesday and Wednesday while the 00z GFS was weaker with the shortwave energy moving into the Pacific NW. Finally the NAM was much stronger with the shortwave energy moving into Washington. Given the strength and spatial differences in the mass fields, confidence was slightly below average with a preferred blend of the 00z GFS and 12z ECENS/GEFS. ...Tropical Cyclone Rosa (Southwest US)... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ See NHC forecast Best Proxy: 00z GFS and 12z GEFS/ECENS blend Overall guidance is fairly clustered with the track of Rosa through landfall along the Baja California peninsula. The 00z GFS came in very closely to the current official NHC track and also lies closely with the 12z GEFS/ECENS. Differences creep up once the system weakens and becomes absorbed within developing upper troughing over the western US. The 12z CMC is by far the weakest of the deterministic guidance and dissolves Rosa before reaching the southwest US. The 12z UKMET meanwhile is fast compared to the 00z GFS/NAM. As such a blend of the 00z GFS and the 12z GEFS/ECENS seems to be a good proxy to the official NHC forecast. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor