Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 208 PM EST Sun Dec 01 2019 Valid Dec 01/1200 UTC thru Dec 05/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep closed low crossing the Midwest/Mid-Atlantic regions... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average ...19Z update... Adjustments seen with the 12Z ECMWF/UKMET/CMC were generally toward the west/closer to the Northeast coast, most notably Monday night/Tuesday morning with the triple point surface low development. The 12Z GFS/ECMWF/UKMET are no on the western side of the ensemble scatter low plots. Despite these deterministic trends, ensemble scatter low plots have not trended much over their past 4 12/00Z cycles valid 12Z Tuesday. Also of note, the ECMWF has been flip flopping between a westward and eastward surface low position between its 12Z and 00Z cycles. For this reason, and given the relative good agreement among the latest guidance a general model blend is preferred, with the 12Z NAM/CMC helping to pull the surface low a little farther away from the coast compared to what the 12Z GFS/ECMWF/UKMET would show by themselves. ...previous discussion follows... Differences with this system are relatively minor aloft, regarding the 500 mb closed low position and strength. There are no major outliers but there are artifacts of the guidance which fall on the edges of the ensemble envelope, mainly with the surface to 850 mb low. The 00Z CMC departs the greatest from the remaining deterministic consensus regarding its surface low(s) track off of the New England coast with a position farther offshore and lies on the outer edges of the ensemble scatter low plots. The remaining non-00Z CMC guidance also shows some differences, with each having one or two forecast features which deviate from the favored middle ground. The 12Z NAM is perhaps the closest to the middle of the latest model spread concerning the surface low position Monday morning with the 12Z GFS farther west and 00Z ECMWF a bit farther east. Monday night, a triple point low develops offshore of Cape Cod with again the 12Z GFS farther west and the 12Z NAM farther east with the redeveloped low, while the 00Z ECMWF/UKMET lie somewhere in the middle. By Tuesday morning, the 12Z NAM surface low is perhaps too strong, the 12Z GFS is on top of Cape Cod with its surface low (too far west?) while the 00Z ECMWF/UKMET are somewhere between the 12Z NAM/GFS regarding position and strength. For Tuesday evening, the 12Z NAM, 12Z GFS, 00Z ECMWF and 00Z UKMET are in reasonable agreement as the low is pulling away from New England. In summary, a 4-way blend of the 12Z NAM, 12Z GFS, 00Z ECMWF and 00Z UKMET seems best to smooth out smaller scale issues contained within each model at various times and near the GEFS/ECMWF ensemble means. The problem with using the ensemble means 100% however, at certain times, they will blend the primary surface low with the secondary/triple-point low resulting in a washed out middle position. ...Upper low approaching the West Coast by Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z UKMET blend Confidence: Slightly above average ...19Z update... Except for the westward 12Z UKMET, adjustments from the latest non-NCEP guidance are in better agreement with the ensemble means and previous preference. Therefore, a non 12Z UKMET blend looks reasonable for this system. ...previous discussion follows... The ensemble spaghetti height plots at 500 mb continue to show west/east spread regarding a mid-level closed low as it drops southward off of the West Coast Tuesday into Wednesday. There haven't been any clear trends with the past 4 12/00Z cycles of the ensemble guidance as to which way to lean (west or east). For now, the preference is to go in the middle, toward the rather agreeable ensemble means and staying away from the edges of the ensemble spread. The 12Z GFS is closest to the ensemble means, with the 00Z CMC the second closest. The 00Z ECMWF/UKMET are farther offshore while the 12Z NAM is south of the GFS position. ...Shortwave approaching the Northern Plains/Upper Midwest Monday/Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average ...19Z update... No significant differences were noted with the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...previous discussion follows... The models bring a northern stream shortwave southeast across portions of the northern Plains and upper Midwest by Monday and then drop it down across the OH Valley through Tuesday. The guidance is in good agreement through Wednesday evening (the end of the short range period), with only minor timing/strength differences. A general model blend is therefore preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto