Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 124 PM EST Fri Dec 27 2019 Valid Dec 27/1200 UTC thru Dec 31/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with preferences/confidence intervals ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Central CONUS Large closed low/surface cyclone... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM through 30.00z; 00z ECMWF/UKMET/CMC blend thereafter Confidence: Slightly above average 19z update: The 12z ECMWF trended favorably toward a more concentric solution initially nearly centralized in the 00z ECENS suite and with the 12z GEFS suite trending toward the ECMWF, it along with slight adjustments of the 12z CMC and UKMET provides increasing confidence in a non-NAM solution through 30.00z then shifting to a 12z ECMWF/UKMET/CMC blend (focused on the ECMWF). Confidence is raised to slightly above average. Consolidation of multiple systems across the Central Plains later today into tomorrow, closed low in the Southwest lifts northward, expanding a high-amplitude trof as northern stream/meridional jet carves out the western portion of the trof. Timing/shape of the short-wave features continues to lead to smaller scale but meteorologically important evolution differences to the deep surface cyclone that moves out of the Central Plains before pivoting across S MN/N IA by the end of the weekend. Here the, NAM is a bit slower with the polar jet coming north and the right entrance to the exiting jet across the Great Lakes/Ontario, is ideally oriented for dual jet enhancement and development that presses the energy too far north on a stronger than ensemble low level jet. The 12z GFS has the similar stronger LLJ relative to the CMC/ECMWF but does not go as far north as the NAM, but still is on the northern fringe of the overall ensemble suite. The 00z ECMWF is on the other side of the suite by the mature stage of the cyclone and is much more concentric. This has been stable through the longer term trends, thought is even west and south of the bulk of ECENS members suggesting a compromise, which is where the CMC/CMCE and UKMET are generally through 30.00z so a non-NAM solution is preferred. As the northern stream shortwave reaches the base Sunday, it sharpens a bit and swings negative tilt through the Mississippi Valley, supporting a stronger triple point surface cyclone that will blast through the Great Lakes Monday. The NAM being off-balance with the initial center hub is discounted a bit, but seems to have similar timing and placement in the stream (geographically) to the ECMWF/CMC/UKMET...but remains too fast and then north. The 12z GFS is initially slower bringing the shortwave out and so shifts a bit east by the time it rolls through the Great Lakes and appears as an eastern outlier on Day 3 in the ensemble suite though better than the 06z, so will hedge away from the GFS on day3. The UKMET appears to have a typical wet bias through the Tennessee Valley/Cumberland Plateau into the upper reaches of the Ohio Valley, so while the mass fields are strong, it may be highly prudent to temper the QPF fields as well. Run to run variance and the west v. east differences keep the confidence at average for this blend. ...Pacific shortwave diving along the West Coast Sun-Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average 19z update: The 12z UKMET trended favorably to the timing of the initial preference having stretched more of the shortwave energy into the southern stream vs over-topping the ridge, while it remains more compact/a tad deeper than say the more broad CMC...there is increasing confidence toward its inclusion the preference. So a Non-NAM blend is preferred overall at slightly above average confidence. A narrow, northeast angling ridge is building through the west/northwest through early Sunday morning, deflecting the initial strong shortwave impulses rolling through the Pacific into BC. The squeezing flow/blocking forces the upstream shortwave energy to elongate/stretch southward as it approaches the Pacific northwest late Sunday. Here the 12z NAM is out of sorts, showing a typical negative over-amplification bias noted in the ending hours of the cycle run. The UKMET is closer to this, and loses more energy over the top of the ridge relative to the remaining guidance suite, this makes the shortwave a bit smaller and then more compact as it curls into a closed low along the West Coast, and given strong flow, accelerates further south faster...so thinking it should have less influence in the blend as well. Otherwise, the ECMWF/GFS and CMC show fairly similar evolution even in a lower predictability flow regime...to have slightly above average confidence in a 12z GFS and 00z ECMWF/CMC blend. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina