Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Sat Apr 21 2018 Valid Apr 21/1200 UTC thru Apr 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Lingering trough over the Northeast US, with progressive shortwave moving it through the Northeast by Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Models show very good agreement with the mass fields for this system and a general model blend is preferred with high confidence. ...Strong closed low over the southern Plains, pushing into the Southeast by Monday and then opening into a trough... ...Inverted surface trough in the western Gulf Coast region, with weak surface low developing in the central Gulf Coast states Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS; 12Z ECMWF Confidence: Average ---18Z UPDATE--- No change to the preliminary preference as the GFS and ECMWF remain fairly similar with their mass fields. ---PREVIOUS DISCUSSION--- There is fairly good agreement with the mass fields through 23.12Z (Monday morning), with increasing differences as the closed low opens into a trough. The 12Z NAM and GFS, and 00Z ECMWF, show the trough beginning to lift northeast and gain latitude as the closed low opens up, while the 00Z UKMET and CMC keep the base of the trough a bit further south. The NAM does lift the trough faster than the ECMWF and GFS. The reasonable similarity between the GFS and ECMWF, and their proximity to the ensemble means (GEFS, ECMWF, NAEFS bias-corrected) suggests this may be the most likely scenario, and the forecast preference is weighted in that direction. ...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 and northern Plains Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z NAM; 12Z CMC; 00Z ECMWF Ensemble Mean Confidence: Slightly below average ---18Z UPDATE--- Spread has been reduced for this system on the 12Z runs, but the ECMWF has now joined the UKMET in lagging the trough a bit slower. Given that this is a change from its previous run and the preliminary preference, will maintain the preference of the NAM, CMC, and add the ECMWF Ensemble Mean. This will provide an intermediate timing and placement of the sensible weather impacts. ---PREVIOUS DISCUSSION--- The preference for this system will be largely to maintain the previous preference from overnight, which was a blend of the NAM, CMC, and ECMWF. The 12Z GFS shows the vort max aloft to generally be concentrated further northeast as the wave arrives in the Plains, and even starts to close off a low at 500mb. Other models keep the mid-upper level vorticity max generally concentrated in the base of the trough further southwest. This seems to be a reasonable scenario. The 00Z UKMET shows a very different structure to the trough, with a stronger and larger northern portion of the wave along the Canadian border, and a much more elongated southern portion that lags the timing of the other models. This leads to much more widespread precipitation than depicted by other models. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers