Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 203 PM EST Mon Jan 20 2020 Valid Jan 20/1200 UTC thru Jan 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ ...Amplifying shortwave crossing the Midwest through Monday... ...Closed low evolution over the Southeast by Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: non-NAM consensus Confidence: Near average A shortwave impulse currently over the Midwest is tracking southeastward on the east side of an upper level ridge over the Rockies. This is forecast to evolve into a closed low near the Southeast U.S. Coast by Tuesday morning and then track eastward out to sea. This will induce surface cyclogenesis along a frontal boundary situated well offshore. Except for some light rain near coastal areas, this should mainly be a marine impact. The 12Z NAM is considerably farther west than the other guidance, and also stronger with the surface low. This becomes apparent mainly after Tuesday afternoon. ...Southern stream shortwave crossing California by Monday... ...Energy crossing the Southwest/Plains Tuesday and Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: General model blend Confidence: Above average A southern stream upper level trough is currently approaching southern California and will quickly track eastward across the southern Rockies and then reach the south-central U.S. by Wednesday morning. The 12Z GFS/NAM appear slightly more progressive and more amplified with the shortwave trough compared to the non-NCEP guidance as it crosses the Southwest through Tuesday. There is now enough agreement among the guidance to merit a general blend of the deterministic guidance with this shortwave perturbation. ...Upper trough/closed low over the Pacific Northwest by Tuesday... ...Energy ejecting out across the northern Plains Wednesday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: Blend of the 12Z ECMWF/UKMET/GFS Confidence: Slightly below average A closed upper low currently over the Northeast Pacific is forecast to reach British Columbia Tuesday night and then reach the northern High Plains and south-central Canada by the middle of the week. The latitudinal differences are now minimal as it approaches the Pacific Northwest on Tuesday compared to earlier model runs. As the upper level energy emerges over the northern High Plains, the 12Z GFS and the 12Z GEFS mean are slightly farther south with the upper low center, and more spread exists by 84 hours on Thursday evening over the central-northern Plains, with the 12Z CMC indicating the most amplified upper level pattern by that time. ...Upstream mid-level trough offshore of the Pacific Northwest early Thursday along with accompanying surface cyclone... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Preference: 12Z GEFS mean/00Z EC mean Confidence: Average Timing differences appear to be the significant reason for model differences with an upstream, shallow amplitude longwave trough approaching the Pacific Northwest Wednesday night. The NAM appears flatter with this wave compared to the model consensus, and the GFS more progressive. There are modest differences reflected in the ensemble spread and given the source region for this system is an area of higher uncertainty, will prefer to go in the middle of the spread toward the ensemble means. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick