Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 144 AM EST Tue Feb 18 2020 Valid Feb 18/0000 UTC thru Feb 21/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Final Model Evaluation with Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Pair of shortwaves crossing the Great Lakes and then the Northeast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average A shortwave and associated surface low will cross the Great Lakes region on Tuesday and then advance east-northeast and clip portions of northern New England through early Wednesday. In the wake of this initial shortwave, there will be another northern stream shortwave that will follow with about 12 hours worth of separation. All of this energy by late Wednesday will be exiting northern New England and lifting through eastern Canada. The 00Z run of the CMC has trended in line with the model consensus compared to its earlier run, and so a general deterministic model blend is now advisable. Broad trough axis tracking from the Rockies to the Eastern U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z UKMET/00Z GEFS mean Confidence: Average As a transient deep layer ridge moves into southwest Canada and across the Pacific Northwest by Wednesday, there will be a positively tilted and broad longwave trough downstream that will cross from the northern Rockies and northern Plains to the Midwest states. By Thursday, this troughing will cross the Great Lakes and Northeast. The 12Z NAM becomes stronger by Wednesday evening as the system approaches the Midwest and Great Lakes, and becomes even more so by Friday morning near the East Coast. The GFS remains a little more progressive across the Northeast, and the 00Z ECMWF slightly slower. Surface cyclogenesis from this shortwave is expected to occur well offshore from the Southeast U.S., with the NAM indicating the low position closer to the coast compared to the model consensus, and the 00Z CMC more progressive. The UKMET and the GEFS mean appear to be a reasonable middle ground for model choices. Shortwave reaching the Pacific Northwest Thursday night ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/00Z GEFS mean Confidence: Average A northern stream shortwave is forecast to reach the Pacific Northwest by Thursday night as the pattern transitions to very strong Pacific flow crossing the northeast Pacific to western and central Canada. This shortwave will begin breaking down the western U.S. ridge that is expected to be in place midweek. The NAM and GFS are indicating a greater degree of amplitude, in contrast to the flatter solutions of the non-NCEP guidance. The UKMET and CMC also appear to be more progressive with this feature crossing the Pacific Northwest. Upper trough/closed low offshore California by Thursday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The models show a strong split in the upper flow pattern by Thursday with a pronounced southern stream trough and closed low emerging west of California as strong northern stream Pacific flow advances across western Canada, with the upper low expected to evolve Thursday afternoon and then slowly track westward towards the coast. The CMC has trended closer to the model consensus compared to its earlier run, and a general model blend can be incorporated for now. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick