Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 207 PM EST Sat Nov 30 2019 Valid Nov 30/1200 UTC thru Dec 04/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep closed low tracking east from the Plains to the East Coast... ...Secondary surface low development south of Long Island... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: near the 00Z ECMWF mean or a 3-way blend of the 12Z NAM/12Z GFS/ECMWF Confidence: Slightly above average ...19Z update... Previous thinking continues to hold true for this storm system, near the 00Z ECMWF mean, with the understanding that future model cycles will continue to make minor adjustments. A blended approach continues to appear best at this time. The 12Z UKMET/CMC remain different enough with their handling of the surface low and redevelopment off of the Northeast coastline Monday into Tuesday to exclude them from the WPC preference. The 12Z ECMWF trended a bit stronger and west with its triple point low redevelopment (tertiary low) Monday night toward the New England coastline. It now lies the west-most of the NAM/GFS/ECMWF combination and slightly west of the 00Z ECMWF mean. However, when incorporated into a blend with the 12Z NAM/GFS, the 12Z ECMWF is an acceptable blended component for this system. ...previous discussion follows... Model agreement is fair with a large closed low as it tracks east through Monday/Tuesday, but minor differences in a related surface low track translate into significant differences at the surface. While the 12Z GFS/NAM and 00Z ECMWF are in good agreement initially with the secondary surface low position south of Long Island, the 12Z GFS is warmer in the lower levels across inland locations Monday morning/evening. The 00Z UKMET/CMC differ in position from the otherwise good model agreement. As Tuesday approaches, the model differences grow concerning the surface low with the 00Z ECMWF east of its ensemble mean, with the 12Z NAM toward the western side of the model guidance. The 12Z GFS ends up with a significantly stronger surface low toward the south side of the model spread, while the 00Z UKMET/CMC are faster to exit the low-mid level low away from New England but the 00Z ECMWF mean shows a decent blend of the better clustered guidance between the 12Z NAM, 12Z GFS and 00Z ECMWF. ...Upper low off of the West Coast through Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-12Z CMC blend Confidence: Average ...19Z update... The 12Z UKMET adjusted toward the previous preference while the 12Z CMC remained farther east with closed low as it drops southward off of the West Coast Sunday into Monday. The 12Z ECMWF remained rather similar to its 00Z cycle, so the preference is for a non 12Z CMC blend with less weight on the 12Z GFS given details mentioned below. ...previous discussion follows... A pair of mid-level closed lows along and west of the West Coast are expected to consolidate into a single low on Sunday off of the Pacific Northwest. This closed low is then expected to retrograde west and south through Tuesday. Trends in the ensemble spaghetti plots indicate a broad range of longitudes for the center of the low, but the 12Z GFS is much closer to the eastern edge of the ensemble guidance. While the 12Z GFS ends up closer to the middle of the latest model spread by a middle ground on Tuesday and the verifying position may wind up between the eastern GFS and western ECMWF/NAM, more weight toward the western ECMWF/NAM idea is preferred given the latest ensemble distribution. Trends do not favor one way versus the other at this time. ...Shortwave approaching the Northern Plains/Upper Midwest Monday/Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average ...19Z update... The 12Z UKMET trended slightly more amplified at 500 mb which is toward the remaining guidance. Otherwise, any changes with the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles were minor with the latest 12Z model consensus showing within the tightly packed model/ensemble guidance. ...previous discussion follows... Despite some minor timing/amplitude differences, the models are in decent agreement with a northern stream shortwave forecast to track across the Northern Plains and Upper Midwest. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto