Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1131 AM EST Wed Nov 21 2018 Valid Nov 21/1200 UTC thru Nov 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall Pattern Across the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The current pattern across the CONUS features a deep layer ridge over the Intermountain West and a longwave trough downstream across the Eastern U.S. Meanwhile, a longwave trough is also positioned just off the West Coast with a separate southern stream shortwave traversing the Southwest. The latest guidance allows a digging shortwave trough and associated low pressure wave to cross the Northeast today and tonight before then lifting up across the Canadian maritimes. In its wake, an anomalously strong area of high pressure will nose across the Great Lakes, Northeast and Mid-Atlantic region for Thanksgiving and bring near record cold temperatures to the region. Across the West, the guidance is in excellent agreement depicting the deep layer ridge breaking down and the offshore trough advancing east and well inland through Thanksgiving. This will set the stage for a well-defined atmospheric river event to impact the Pacific Northwest and especially northern CA. A series of shortwave impulses upstream will follow suit and arrive going into Friday which will result in a multi-day heavy precipitation event. The lead trough axis will advance downstream across the Plains and MS Valley by Friday and reach the East Coast on Saturday as low pressure develops near the eastern Gulf Coast region and then offshore the Southeast coast. A complex interaction of wave activity is suggested by a couple of models like the 12Z NAM/GFS solutions which have a low center developing near the northern Bahamas near a front. The current southern stream shortwave over the Southwest advances downstream across FL and facilitates this development ahead of the larger scale northern stream trough. The NAM and GFS gradually allow for consolidating low centers near the Southeast coast, but the 00Z UKMET/CMC/ECMWF solutions are less emphatic on there being any separate low development near the Bahamas. As it is, the NAM appears too strong in general with the larger scale trough moving into the East. A compromise between the GFS and ECMWF will be favored for the time being. Meanwhile, by Saturday, shortwave energy traversing the West will be digging out across the Plains. This will reload a longwave trough across this region. The NAM and some extent the CMC are too deep with their height falls over the Plains. Better depth agreement is noted with the GFS, UKMET and ECMWF, but the UKMET appears way too slow. Based on the latest model spread, clustering and ensemble guidance, a blend of the GFS and ECMWF will be preferred for mass fields. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison