Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1233 AM EDT Sun May 27 2018 Valid May 27/0000 UTC thru May 30/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation including final preferences and confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Subtropical Storm Alberto... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: See NHC Forecast Closest Guidance: 00z NAM/12z ECMWF blend over water 00z GFS/12z ECMWF blend over land (after 29/00z) There is a tighter cluster overall, but there remains some spread with respect to the dominant shortwave within the larger scale upper level trof. The western tracks, represented mainly by the 18z GFS/GEFS show the digging shortwave in the North Central Gulf becoming the dominant circulation in the pair. Opposed to this, is the 00z NAM/ECMWF and most dramatically the 12z UKMET. The UKMET continues with the broadest right hand hook toward the West Coast but has trended west over the past few runs; but also is now faster lifting north and rotating west into the pivot south of the western FL panhandle by Mon. The 00z GFS has flipped to become halfway between the UKMET and the ECMWF...and may be a good partner with the ECMWF after landfall compared to the NAM. The 12z CMC is uncharacteristically most flat showing least binary interaction between the northern stream shortwave and convection induced, near center vort. NHC initial forecast is a bit flatter like the CMC but quickly shifts toward timing of the 00z NAM and 12z ECMWF especially toward late Sun, through landfall and into the TN river valley...where the NAM is outpacing and the official forecast shifts more heavily toward the ECMWF. Overall, a 00z NAM and 12z ECMWF blend will likely match the forecast through the longest duration of the forecast for mass fields. ...Disturbances crossing the Great Lakes and Northeast U.S....developing surface low southeast of Long Island by early Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z NAM/GFS and 12z ECMWF (weight to EC). Confidence: Above average Shortwaves crossing the NW Great Lakes currently will continue to slide through the ridging across SW Quebec. With help from confluence out of the SE, a surface wave will develop late Sunday into Monday along the frontal boundary south of Long Island. Here guidance is trending toward a common solution that has been anchored with good continuity of the ECMWF. The 12z CMC remains slow, while the 12z UKMET is too far south. The 00z NAM may be a bit north showing some stronger depth than the 00z GFS...but similar to the ECMWF. As such a 00z GFS/NAM and 12z ECMWF blend is supported with greatest weight to the ECMWF at slightly above average confidence. ...Approaching northern stream shortwave and frontal zone across Northern Great Lakes into New England Mon to Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Northern stream shortwave and jet streak will progress quickly across Canada with a bit of deepening/amplification toward late Monday across NE Ontario/Quebec that will clip northern ME early Tuesday. An associated surface front will progress through as well. Models show good agreement, though there are some small depth/timing differences. The 12z ECMWF is a bit more aggressive/fast with some weak negative tilting toward ME, while the UKMET is a bit weaker flatter and the GFS/NAM uncharacteristically lag. To compensate with this mild spread, a general model blend is preferred at slightly above average confidence. ...Western U.S. closed low lifting into Northern Plains by Wed...associated lee cyclones/trof with eventual consolidation across MT/ND by Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Upper low over the Great Basin is already showing signs of breakdown into constituent peripheral shortwaves. Mesoscale convective upscale influence seems to be driving cycle to cycle, model to model difference in preference for one shortwave over another. For example, the 00z NAM and GFS prefer strengthening of a shortwave emerging from the eastern side of the upper low through the central Rockies toward SE MT on Monday enhancing the deformation zone toward the northern stream across the Dakotas; while the ECMWF is more reserved and loads the main upper low a bit more. This will lead to slight timing differences a the main upper low emerges with the NAM/GFS and UKMET stronger and further north by Tuesday. Overall, the pattern/timing and evolution of the wave and associated surface reflection are fairly agreeable with important but difficult to lock down mesoscale influences. By the end of the forecast period, the remaining upper low feature is well agreed upon, with only the CMC a bit deeper (closed feature). All in all, a general model blend will likely reduce some of these important convective clusters but for continuity and better overall forecast, a general model blend will serve us well through the forecast period. Confidence is above average for the larger scale timing/pattern but given the importance of convection upscale, overall confidence is only slightly above average in the blend as a while. ...Longwave trof amplifying south into Pacific Northwest by Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average Compact closed low in the NE Gulf of AK will quickly occlude and broad southwesterly flow regime will slide NE into the northern stream/N BC with the tail end of the trof sliding southward toward Vancouver Island by late Monday into Tuesday. This will support southeastward migration of the inner core which will allow for northern stream shortwave energy to rotate around the western side of the trof leading to amplification/negative tilting spilling over into the US Pacific NW by Wed, with a broader trof from the severed southwesterly subtropical southwesterly flow tailing across N CA. The 00z NAM is uncharacteristically broad further north but very slow with the trailing shortwave out of the subtropical Pacific. The ensemble trends toward a deeper solution are anchored by the ECMWF/UKMET and CMC and lesser so the 00z GFS. All considering, a non-NAM blend is preferred at 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