Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1248 PM EDT Mon May 21 2018 Valid May 21/1200 UTC thru May 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Weakening shortwave energy over the Midwest today sliding toward Northeast by Tues/early Wed with associated surface wave crossing the OH Valley/Lower Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Model agreement continues to tighten with respect to the weakening of the shortwave energy across the Midwest entering the confluence zone over the Great Lakes before rapidly shifting to the east. The 00z ECMWF/CMC are extremely tightly clustered but a bit slow but also a bit further north compared to the 12z GFS and NAM though the UKMET is closer to them than the latter, just a shade faster. The differences are minor with respect to this system but when setting the stage for the next system/interaction (see section below) into Wed there are greater issues. At this point, a general model blend can be supported at above average confidence but this will change for Wed, see below. ...Trough digging across the Northeast Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly above average Zonal flow across the northeast by early tomorrow will carry weakening shortwave out of the Midwest and set the frontal zone across the northeast (see above). A pair of shortwave features will amplify the flow across N New England by early Wed with the lead wave reaching northern ME by midday/early evening Wed. This will support coastal cyclone enhancement. Given the 12z GFS is already south of the increasing cluster, it also depicts typical fast bias and increased amplification of the wave especially the surface feature off Nova Scotia. This is likely to reduced spacing between the lead height falls and the secondary shortwave that is upstream. The remainder of the guidance including the 12z NAM is much broader with the spacing, the DCVA is a bit reduced and leads to a more sensible trof evolution. As such a non-12z GFS is supported at slightly above average confidence. ...Mid level trough/weakness lifting out of eastern Gulf across and thru Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Large scale agreement is fairly solid especially between the 12z GFS and 00z ECMWF, particularly with the vort center near the FL panhandle and the weak surface reflection initially and through tomorrow. The remainder of the warm conveyor is well placed with the inflection with the TROWAL/western branch of the cyclonic flow lifting through eastern NC by midday Tuesday. The GFS is a bit more amplified here than the slower 12z NAM or flatter ECMWF/UKMET but is generally matched to the 00z CMC. Overall convective processes and upscale enhancement will drive this pattern, either maintaining the inner core over AL/GA a bit longer into Wed as suggested by the ECMWF/UKMET...but a general model blend of the mass fields should suffice. Confidence is slightly above average given the influence of localized convection feed back, but all in all is reduced given the similar end results. ...Large scale trough/closed low impacting the West thru Tuesday before down-scaling entering the northern High Plains late Thurs/Fri with associated surface waves/lee trof... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z ECMWF/12z NAM blend Confidence: Average Goes-W WV depicts a fairly symmetric closed low over the southern Sierras with a branch of the subtropical jet providing good diffluence downstream across the central Rockies at this time. Models remain is solid agreement as the upper low shifts eastward into the central Colorado River Valley before weakening and lifting northward into Wed. The flow supports weak lee-troughing/cyclogenesis with upslope convection. Here the timing differences start to manifest which, in turn, leads to increased/decreased interaction with a stalled northern stream shortwave in the SW Canadian Prairies. The 12z GFS and NAM both suggest a stronger northern shortwave and increased speed with the weakening upper low feature. The 12z GFS is particularly fast and shows a stronger/faster jet rounding the base of the trof increasing the diffluence and upscale enhancement of the wave through N WY/MT from Wed into Thurs. The NAM supports this trend but does not go to the degree of the GFS. While the remaining guidance is much slower to lift north and in most cases depict a weaker Canadian influence. The 00z CMC/UKMET both suggest a more compact remnant close low feature enhancing at the diffluent portion of the upper level jet structure through the northern High Plains...and lead to a compact symmetric shortwave and sharper surface trof/low feature in N NDAK by 00z Friday. The ECMWF seems to split the difference of the GFS/NAM and the CMC/UKMET. This also appears to be more sensible within the run to run ensemble suite including the 00z and 06z GEFS members. As such will favor the 00z ECMWF but include some of the 12z NAM to hedge to the trend presented by the GFS/NAM and run to run ensemble suite. Confidence is average in this blend ...Approach of Pacific trough becoming closed low offshore the central CA late Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM/GFS and 00z ECMWF blend Confidence: Average Goes-W WV shows an elongated N-S trof over the East Central Pacific advancing toward the east. By late Wed into Thursday, the first pieces of guidance lead by the 00z ECMWF/CMC show the base of the trof beginning to close off followed by the GFS and NAM. The UKMET is much delayed but also broader with the cyclonic development. This leads the ECMWF/CMC to dig south compared to the NAM/GFS with the GFS a bit south but east of the NAM. The ECMWF is deeper and slower than the CMC while the CMC at least is match in longitude with the GFS. Run to Run ensemble spaghetti analysis does show a tightening of solutions at a common middle ground between the GFS/NAM and ECMWF with a trend toward slightly weaker solutions. The ensemble means depict this well and are inside (closer clustered) than their operational brethren. This provides some confidence a blend of the 12z NAM/GFS and 00z ECMWF will reach a compromise to the ensemble suite...otherwise utilize a mass field of the GEFS/ECENS mean if available. Confidence is quite good in a common location between the GFS/NAM and ECMWF but the spread between them and resultant blend is likely to be a bit too broad and lose some fidelity of signal to have high confidence in its blend (average to slightly above average) Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina