Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1108 AM EST Thu Dec 26 2019 Valid Dec 26/1200 UTC thru Dec 30/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with preferences/confidence intervals ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ System moving from the West into the Midwest Fri-Sun ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend hedging toward ECMWF/ECENS Confidence: Average Well defined closed low entering Southern California, will continue to track eastward through Friday, as the northern stream shortwave energy along the eastern side of sharpening/anticyclonic meridional jet from BC to the Great Basin continues to sharpen and draw the remaining closed wave energy north into the High Plains by Sat. The high-amplitude trof will begin to have internal binary action into a large closed low by 29.00z. There remains small timing differences that lead to longitudinal differences in the placement/evolution of the closed low through the Central Plains, with the slightly faster (broadly weaker) GFS/GEFS along with the UKMET/12z NAM further east while the ECMWF/ECENS mean and the CMC are slightly slower with the center and therefore west relative to the packing. Still, a blend between the two camps is still quite viable until the strong northern stream shortwave rotates through the base of the closed low and goes negative tilt through the Middle to Lower MS River Valley into Sunday. So the stronger/more concentric solutions of the ECMWF/CMC are slightly faster kicking the wave through the base while the GFS/NAM/UKMET show greater elongation through the negative tilt/pressing the frontal zone/warm conveyor through the TN Valley faster. Continuity goes to the ECMWF though it remains west of the ECENS solutions...so a general model blend is preferred but hedging toward the ECMWF/CMC particularly on Sunday, but the changes of the 12z GFS/NAM allow for greater weighting there as well in the TN valley. Confidence remains average given some of the important features remain off the dense observation network and timing of the interactions of the internal shortwave features will allow for continued run to run variations that are impactful. System clipping the Great Lakes into the Northeast Thu-Sat Southeast US, moisture fetch off the Southeast Atlantic ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Compact shortwave leaving N MN will continue to progress through Southern Canada with the trailing energy clipping through the Great Lakes into New England further enforced by a progressive Arctic stream shortwave that clips N Maine by late Sat. Their influences are weak, but the trailing warm front and broad southwesterly flow aloft (along with return moisture feed out of the Bahamas/Antilles island chain) will provide some broad ascent across the area to produced mixed wintry precip that has some impacts particularly through terrain into the Northeast... while keeping strong Atlantic fetch into the Southeast. Mass fields are solid enough to support a general model blend at this point. The uncertainty is in the activity along the Bahamian island fetch and strength of confluent/convergence either into SE FL or angling around the western periphery of the strong but retrograding Subtropical ridge in the Sargasso Sea. System approaching Pacific Northwest Sunday morning ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Average The sharp/amplifying ridge over the West Coast, will quickly be squeezed by the approach of the next strong system entering the Gulf of Alaska. While the leading shortwave/height-falls are expected to drive through Vancouver Island, the blocking flow,enhanced by the coastal ranges, will allow for rapid cyclogenesis for the trailing kicker shortwave by late Sat into early Sunday, just west of the Juan de Fuca Strait eventually deflecting southward. Ensembles, particularly the ECENS show a fairly large spread in the strength of the ridging/blocking in advance of the cyclone development suggesting higher uncertainty and lower predictability. However, the 12z GFS/NAM and 00z non-NCEP deterministic guidance is fairly well packed overall. The non-NCEP suite suggests the energy of the cyclone is a bit more compact and smaller in scale by the end of the short-range period, followed by the 12z NAM and then the GFS, which is stronger, and being a tad faster (with its known bias), is starting to elongate/stretch southward along the N CA coast. GEFS solutions would suggest it being further east and closer to the NAM/ECMWF/UKMET/CMC...so thinking a non-GFS solution seems appropriate which feeds well into the Medium Range forecast preferences (see WPC PMDEPD for more details). Given the large variance in the ensemble suite, confidence is only average and with the system still well out in the Pacific, run to run variation is expected. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina