Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 236 PM EDT Mon Apr 16 2018 Valid Apr 16/1200 UTC thru Apr 20/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00z model evaluation and final preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep occluded cyclone tracking into Ontario/Quebec Mon/Tue, centering over New England Wed, exiting Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Overall, there is fairly tight consensus with the upper wave/surface low tracking through New England today into Tuesday...including the shortwave emerging from Ontario across the Eastern Great Lakes. It is after the main wave as past the US and starts to rotate back into Quebec late Tuesday into Wed. The 12z NAM was a bit stronger initially by this point and further deepens. This slows its westward (then southward) progression and in combination with shortwave from the Arctic stream shows greater phasing/deepening to get out of alignment (this also seems to negatively affect the upstream wave in next section) to make it less preferable Thursday across New England. The 00z CMC also has some of this delay issue looping back into Quebec, and while not as deep as the NAM...the delay leads to a different evolution/interaction with the Arctic wave over SW Quebec/E Ontario by Thursday. The 12z GFS remained fairly consistent as tight to the remaining 00z ECMWF/UKMET and a bulk of ECENS/GEFS members. The differences appear small enough to support a general model blend at slightly above average confidence or a tighter solution may be to blend the 12z GFS and 00z ECMWF/UKMET. 19z update: The 12z CMC trended a bit better but remains a bit slower than the consensus. Otherwise the 12z GEFS/ECMWF/UKMET showed similar evolution to their prior runs and earlier preference to continue a general model blend preference at slightly above average confidence. ...Shortwave exiting Central Rockies into Plains by late Tues/early Wed with rapidly developing/occluding Sfc Low into the Great Lakes by late Wed/early Thurs. Reaching Upper Mid-Atlantic by late Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The 12z NAM remains central to the cluster with the shortwave crossing into the central Plains but also shows typical know biases with stronger/faster wound up occlusion through the Mid-MS Valley but also shifting focus toward the triple point low compared to the other very tightly occluded solutions of the 00z ECMWF/UKMET. The 00z ECMWF shows its bias being a bit slower compared to the remaining guidance and is quicker/further south with the transition to the coastal redevelopment midday Thursday. This is likely due increased phasing with amplifying Arctic shortwave that drops into SE Ontario...while other guidance show increased spacing (having been faster). This increased spacing is best represented by the 12z GFS, which is fast...its typical bias. The UKMET is central but holds onto a deeper surface occluded low through the Great Lakes...a known bias of it. So all in all, biases rule the day. The good news is the clustering is fairly tight even with the biases. So to alleviate the situation a general model blend is preferred, but at slightly above average confidence. 19z update: The 12z UKMET trended a bit slower and in doing so showed a much tighter phasing with the Arctic amplifying shortwave...even delayed compared to the 12z ECMWF. The ECMWF trended a bit faster and north with the surface low, but not significantly so and remains mildly spread south of the 12z NAM/GFS and now 12z CMC by late Friday, likely as the Arctic trof digs further south through the Mid-Atlantic in comparison. This interaction will likely be very difficult to precisely predict given small differences in strength/timing lead to larger variation in outcome. Still the spread is modest and far enough into day 3 to be too confident going one camp over another. A nice split in location would serve best...so a general model blend is preferred. Confidence is slightly above average early transitioning to average by end of day 3. ...Next Pacific trough reaching the West Coast late Tues and closing off across Great Basin into Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average A compact shortwave will break southward from the northern stream toward Northern California by late Wed. There is remarkable agreement in timing/shape and strength with the evolution of the wave even through 00z Friday. The 12z NAM and GFS both shifted ever so slightly south and west (NAM more than GFS) and a bit stronger compared to the 00z/06z deterministic and ensemble means. The shift was not significant and the overall timing/shape of evolution of the wave as it closes off across the southern Great Basin/Lower Colorado River Basin support continuing a general model blend at above average confidence. 19z update: The 12z Non-NCEP guidance and the GEFS mean all shifted toward the NAM/GFS solution a bit south and west and slightly deeper; the 12z CMC least so, and the UKMET more so, leading it to be faster by 00z Fri. Still these are minor differences to continue to support a general model blend at above average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina