Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 157 AM EST Fri Dec 13 2019 Valid Dec 13/0000 UTC thru Dec 16/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS Evaluation with Preliminary Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Outside of the two areas of the CONUS mentioned below, model agreement is good enough to support a general model blend. ...Eastern 1/3 of the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 00Z NAM blend Confidence: Slightly above average ...07Z update... The 00Z ECMWF/UKMET/CMC stayed similar to or adjusted slightly slower than than their 12Z cycles with the 500 mb trough axis progressing into the East through Saturday morning making the 00Z GFS stand out as a faster outlier of the deterministic camp. While the 00Z GFS's surface low is likely a bit too far east Saturday morning given stronger clustering back to the west, the 00Z ECMWF position appears a bit too far west. By Sunday morning, model differences are relatively minor. A general blend excluding the 00Z NAM appears best overall given the NAM's depiction of low level flow and its 925-850 mb circulation stands out from the remaining consensus. The 00Z GFS displacement should be resolved when blended with the non-NCEP guidance. ...previous discussion follows... Timing differences remain with a mid-level trough axis that is forecast to extend from the Ohio Valley to the Southeast Saturday morning. While the ensemble spaghetti height trends have been faster over their past 4 12/00Z cycles, the 00Z GFS still looks a tad too fast through Saturday morning. On the other side, the 12Z UKMET/CMC appear too slow given the latest ensemble distribution and the faster trends. The 00Z NAM is in the middle regarding timing but is not supported with its stronger 925-850 mb flow into the Mid-Atlantic region and its 925-850 mb low that develops Friday night. A strong surface low develops over the interior Northeast on Saturday, tracking into southern Quebec Sunday morning. Model agreement over the East gets better by Saturday evening, but a 00Z GFS/12Z ECMWF blend is preferred to keep consistent from start to end with the evolution of the East Coast storm system. ...Western 2/3 of the CONUS.... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/00Z UKMET blend Confidence: Average ...07Z update... The 00Z ECMWF adjusted slightly slower and more amplified with the upper trough reaching the Great Plains Monday morning. This adjustment is in a good direction but perhaps too far with greater ridging downstream. The 12Z ECMWF appears like a better fit when blended with the 00Z UKMET. The 00Z CMC is flatter and its lack of surface wave across Texas is not supported in the remaining guidance. ...previous discussion follows... Zonal flow at the onset of the short range transitions to more amplified by Monday morning with a ridge just off of the West Coast and troughing along and just east of the central Rockies. Some relatively minor differences develop with an amplifying shortwave off of the West Coast Saturday morning and with a related surface low off of the Oregon coastline later in the day. Ensemble scatter low plots have been consistent and suggest the 00Z NAM/GFS are too far south with the surface low valid 00Z/15. Better support exists for a 12Z ECMWF/UKMET blend here. The 12Z ECMWF/UKMET blend is also supported downstream as the trough axis translates east and a surface response develops over the Southern Plains. The faster 00Z NAM/GFS at 500 mb goes against the slowing trends observed in the ensembles and against conceptual thinking with an amplifying flow pattern over the eastern Pacific. Ensemble means support a blend of the 00Z ECMWF/UKMET with the frontal boundary extending from Kentucky/Tennessee back into Texas with the surface wave in the vicinity of the ArkLaTex. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto