Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1138 PM EST Sun Dec 29 2019 Valid Dec 30/0000 UTC thru Jan 02/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation with preferences/confidence intervals ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Dominant Central Plains through Great Lakes and Northeast closed low/surface cyclone(s) including coastal Low Tues/Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Mature/vertically stacked low over the Upper Midwest currently will continue to pivot around the Great Lakes the next couple of days, with several impulses rounding through the low. Model agreement is very tight and well clustered through 48 hours with some differences beginning to creep up as the low picks up speed and moves east over the Northeast U.S. and New England. The GFS ends up being on the faster side of the model envelope by 48 hours, but the differences are fairly minor enough to go with a general model blend through the forecast period. ...Pacific shortwave diving along the West Coast Sun-Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 01.00Z; ECMWF/CMC blend thereafter Confidence: Slightly above average Through about 01.00Z, model agreement is very strong/high with the compact closed low that is currently just off the CA coast. This system will further drop south/southeast reaching the Baja region Tuesday. Beyond 01.00Z, there are some differences on how quickly and how much energy is sheared northeastward with a northern stream shortwave trough dropping through the central Plains. The ECMWF is the furthest south with the initial closed low and therefore keeps most of the energy removed from the northern stream. Meanwhile, the 00Z NAM and the GFS to some degree allow for more energy to phase and show a quicker/more open solution. The UKMET, while initially slow and suppressed, picks up its evolution by 02.00Z with a faster solution similar to the GFS. Finally, the CMC offers a middle ground solution between the ECMWF and GFS/NAM. For now, will side for a slower solution and prefer a ECMWF/CMC blend after 01.00Z through the end of the period. ...Atmospheric River sinking into Pac NW by Late Tues... ...Amplifying into broad trof across Northern Rockies/northern Plains by late Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly below average In the wake of the closed low near the coast currently, very vigorous Pacific flow, noted upstream in the GOES-West WV suite, becomes very zonal into the Pacific NW but is very strong with internal smaller scale waves rippling fast through it. Combine this with deep anomalous moisture tapped from the warm central Pacific aids a strong Atmospheric River that slowly drops south into the CONUS by early Tuesday. In the last 2-3 model cycles, agreement has trended higher and now most of the deterministic models offer similar large scale solutions well within the ensemble spread as well. This puts forecast confidence up to average/slightly above average. The ECMWF does show a slight fast bias with the surface reflection (and ~ 10 mb weaker) impacting the coast by 01.00Z, even ahead of the GFS and other deterministic models. But overall the differences are fairly minimal, so will go with a general model blend at this point. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor