Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 134 PM EST Sat Mar 02 2019 Valid Mar 02/1200 UTC thru Mar 06/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Preference and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Surface low offshore New England and Nova Scotia tonight/early Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The 12z NAM remains on the western edge of the tightly clustered low center track from east of Nantucket Island to a position east of the northern tip of Nova Scotia through 03/12z. However, the differences in strength and positioning are no longer too large to keep the 12z NAM out of the general model blend. Otherwise, there is very good model agreement with the track and strength of the surface low, so a general model blend is preferred, with above average confidence. ...Phasing short waves and resulting surface low tracking from the Mid Atlantic to Newfoundland... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend (with less weight toward the 12z ECMWF) Confidence: Average The 12z NAM has shifted to the east with its surface low track (when compared to its previous two model cycles), a result of better agreement concerning the timing of the phasing of the northern and southern stream short wave into a broad cyclonic flow over the northern part of the US. The 12z GFS has remained consistent with its previous two model cycles in keeping the surface low further east. Though it has been trending slowly eastward with its depiction of the surface low track, the 12z ECMWF remains the westernmost member of the guidance envelope (joined with the 00z ECMWF ensemble mean). The general model trend has been to phase the northern and southern stream a bit later, resulting on a more easterly surface low track. Given this trend, a general model blend is preferred, though at this point less weight should be given the 12z ECMWF because of its surface low track (and precipitation swath). Because there is still some variability concerning when the phasing occurs, and considering the sensitivity of the forecast to that variability, confidence remains average. ...Closed mid-level low near WA coast Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The 12z NAM is west of the consensus with the closed mid level low that crosses Vancouver Island by 04/00z, then wobbles over northwest WA through the end of the period. The 12z GFS is a bit further southeast than the 12z ECMWF/00z ECMWF ensemble mean with the placement of the closed mid level low by 06/00z. Given that the system remains more or less in place after crossing Vancouver Island, a general model blend is preferred. However, the model spread results in an average forecast confidence. ...Trough approaching the CA coast Tue... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The 12z NAM remains close to the model consensus early it takes a closed low from 32N 142W at 04/12z to a position near 30N 133W by 05/12z. After that tine, it becomes faster than the consensus (mirroring the timing of the 00z UKMET). By contrast, opens the closed low at the bottom of the low wave trough more slowly, which is more consistent with the 00z UKMET/00z CMC and the GEFS and ECMWF ensemble mean positions. The 12z GFS stands out as a western outlier, as it is slower than the consensus with the bottom of the trough. The timing issues will have an effect on how quickly tropical moisture is picked up and transported to the CA coast by the end of the period. At this point, a general model consensus is preferred, though the spread in the guidance results in only average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hayes