Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 155 AM EST Mon Feb 17 2020 Valid Feb 17/0000 UTC thru Feb 20/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Final Model evaluation with preferences & confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Upper trough crossing the Northeast on Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Broad cyclonic flow aloft over the north-central U.S. will support the passage of a few shortwave perturbations through the beginning of the work week. The lead impulse will track eastward across the Northeast on Monday, with an attendant cold front crossing New England by Monday night. There is enough mass field agreement in the models to merit a general model blend for this feature. Shortwave impulse and surface low across Upper Midwest Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS/00Z ECMWF blend Confidence: Slightly above average By Monday evening the upper level flow pattern is expected to become more separated with a northern and southern stream component. Two separate shortwaves in quick succession are progged to cross the Upper Midwest and northern Great Lakes region, with the first one tracking eastward across the Upper Midwest and the Dakotas on Monday, and then reaching the northern Great Lakes by early Tuesday. This will support a wave of low pressure along a cold front across this region. The 00Z CMC trended closer to the model consensus with the depth of the trough and the surface low position, although still a little weaker aloft. Similar to its 12Z run, the 00Z UKMET remains slightly slower with the progression of the cold front but better clustered with the low position over the Great Lakes region. There is enough agreement among the GFS and ECMWF to merit a blend of those two models as a starting point in the forecast. Second shortwave impulse reaching the Great Lakes Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z NAM/GFS/GEFS mean Confidence: Near average A mid level impulse will quickly follow in progressive northwest flow over northern Minnesota and northern Michigan on Tuesday, and eventually consolidate with the previous shortwave mentioned above. The GFS and NAM are now among the most amplified solutions whereas the non-NCEP guidance is a little weaker. The preference remains the same except to substitute the 00Z GEFS mean for the 18Z run. Broad upper level trough building across the Intermountain West ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS/00Z ECMWF/12Z EC mean Confidence: Slightly below average The southern stream component of the upper level flow is forecast to evolve into a broad trough over the Intermountain West by Monday night whilst an upper level ridge axis builds just west of the West Coast. Some of the shortwave energy may try to undercut the ridge and form a temporary blocking pattern. Most of the model differences become apparent after 60 hours, with the UKMET and NAM indicating the greatest degree of undercutting, and the NAM is also stronger across the northern Rockies and Great Basin by Wednesday night. Longwave trough approaching the West Coast by Thursday morning ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GEFS mean/00Z ECMWF/00Z CMC Confidence: Near average The upper level ridge holds over the West Coast region through Wednesday night, after which height falls ensue as the next storm system approaches from the eastern Pacific. This will likely be an elongated trough axis with a hint of a closed low developing. The NAM appears more amplified than the model consensus, and the UKMET slightly slower. The 00Z GEFS mean appears to have a good depiction of the evolution, and is supported by the CMC, ECMWF, and to a lesser degree the GFS. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick