Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1241 AM EDT Sat Apr 21 2018 Valid Apr 21/0000 UTC thru Apr 24/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model evaluation including preference and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Progressive shortwave moving through New England/N Mid-Atlantic late Sun...Mon ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z NAM/GFS and 12z ECMWF blend Confidence: Slightly above average Lingering troughing exists across the Northeast today into Sunday with little fanfare or impact. As the closed low over the South shifts eastward, a weak shortwave slides through south central Canada and sharpens slightly as it crosses the Northeast late Sunday into Monday. Only the 12z UKMET remains out of phase already showing signs of deeper and shifted south and west amplification of the trof crossing the Great Lakes Sunday which further deepens and lags the growing consensus, further northeast. The 12z CMC is also a bit slow at peak sharpening of the trof 12z Monday in line with its bias. As such will favor a 00z NAM/GFS and 00z ECMWF blend across the Northeast through the weekend and start of next week. Confidence is slightly above average given the pattern and fairly tight ensemble suite. ...Strong closed low in the Four Corners steadily pushing east into the Southeast by Monday... ...Inverted surface trough in the western Gulf Coast region, with weak surface low developing in the central Gulf Coast states Sunday...shifts to Southeast by Tues ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS and 12z ECMWF/CMC blend Confidence: Average Strong closed low appears centered near the Sangre de Cristo Range with numerous shortwave vorticity centers along the outer periphery. This pattern is expected to continue with slow eastward progression. The upper low will start to elongate a bit north to south, though the NAM continues to support a stronger northern portion of the upper low even as early as late Sat/Sun. Still the spread is not too bad even through midday Sunday into the Lower MS Valley, especially with timing of the convection and frontal zone. After this period however, there is fairly sizable departure. The 12z UKMET which presents a strong shortwave moving through the Great Lakes aids stronger ridging across the Central Ohio Valley and Upper TN Valley...directing the upper low further south and east. The NAM departs further north at this time with some strengthening/upscale feedback, while most guidance suggests slow weakening. The 00z GFS supported by the GEFS members significantly slows across MS/AL especially within the lowest troposphere levels...this is opposed by the ECMWF/CMC and their ensemble suites that press the surface low with the deepest convection/latent heat release along the SC coast by the end of the forecast period. The 7H pattern would suggest the ECMWF may be a bit too flat connecting the conveyor belt further north and west versus the GFS/CMC showing more of a comma head feature across the East Central Southern states. To compromise a blend of the three will be preferred...at average confidence (though confidence is higher earlier on Sat/Sun). ...Strong shortwave/attendant cold front nearing the Pacific Northwest currently before shifting north of the US border this weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Models are in very good agreement with the mass fields for this system and a general model blend is preferred with high confidence. ...Secondary shortwave reaching the Pacific Northwest early Sunday; amplifying and pushing into the Northern Rockies Monday. Potentially combining with northern stream energy by Tues into Northern High Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly below average A shortwave enters the Pacific northwest Sunday and quickly translates across the Northern Rockies. In this process slow phasing/connection starts to manifest across central MT with the trailing trof from the preceding strong deep wave over Central Canada. Concurrently, the wave is situated in a flow regime supporting increased amplification and cyclonic development even spurring a modest surface reflection across the Great Basin that translates to southeast WY/northern CO by late Mon...connecting to the frontal zone through the Dakotas on Tuesday. While there is good timing/shape to this at the surface...the upper level pattern is a bit more complex due to influences from other systems/moisture streams. The 00z NAM has significantly changed over the last few runs to come into a common evolution/solution presented by the 12z ECWMF/ECENS mean. Both are a bit faster and further south (well presented in the run to run ensemble suite...where ECMWF members dominate a faster/south trof by 12z Tues across the central Rockies, and therefore are less connected to the northern stream digging trof through southern Canada. The 00z GFS has picked up pace to be better timed with the ECMWF, however, it is further north and supports winding up the mid-level circulation through N WY into SD late Monday into Tuesday with stronger influence from the digging trof out of southern Canada. This evolution is very comparable to the 12z CMC and the CMCE. The 12z UKMET seems a middle ground solution between the two camps, though coming to this solution quite differently in the evolution. While the spread is moderate, guidance wants to come to a common solution reducing overall spread in impact/sensible weather. Confidence is not high (slightly below average), but a general model blend is supported to smooth out these uncertainties. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina