Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1144 PM EST Mon Feb 17 2020 Valid Feb 18/0000 UTC thru Feb 21/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Initial Model Evaluation with Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Pair of shortwaves crossing the Great Lakes/Northeast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC 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 main difference with the models at this point is a weaker amplitude with the CMC on the first shortwave trough, although all of the models are now similar on timing with the cold front passage. Broad trough axis tracking from the Rockies to the Eastern U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/18Z 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 CMC remains on the stronger side of the model guidance with the trough axis across the Rockies and northern Plains. The 12Z NAM becomes stronger as the system approaches the eastern U.S. and the GFS is a little more progressive. 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. Shortwave reaching the Pacific Northwest Thursday night ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/18Z 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 from the Gulf of AK 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: Non-CMC blend Confidence: Slightly above average The models show a strong split in the upper flow pattern by Thursday with a substantial southern stream trough and closed low emerging west of California as strong northern stream Pacific flow advances through western Canada, with the upper low expected to evolve Thursday afternoon and then slowly track westward towards the coast. The CMC appears displaced east of the model consensus, and all of the models are similar in the intensity of the upper low. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick