Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 236 PM EDT Sun Apr 22 2018 Valid Apr 22/1200 UTC thru Apr 26/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Strong closed low over the south-central US today, gradually opening into a trough and lifting into the Mid Atlantic and Northeast... ...Phasing with northern-stream shortwave digging through the Great Lakes Tuesday Night into the Northeast-... ...Weak surface low over the interior Southeast, redeveloping along the Atlantic coast by mid-week... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z NAM, 12Z GFS, 12Z CMC Confidence: Slightly below average ---18Z UPDATE--- The 12Z ECMWF continues to show a stronger shortwave digging into the Great Lakes, and actually never seems to quite phase it with the trough/wave lifting out of the Southeast. This continues to lead to a surface low further inland, and a mean trough position further west of the other models. The 12Z UKMET has trended in this direction, but not quite to the degree of the ECMWF. Meanwhile, the 12Z CMC has remained relatively consistent and similar to the GFS and NAM. Again, the ECMWF scenario can't be ruled out, but will maintain continuity with the preliminary preference and previous overnight preference. ---PREVIOUS DISCUSSION--- Models continue to show some run-to-run variability with this system, likely related to both the specifics of how the upper level low opens up into a trough and begins to lift out of the Southeast, as well as the interaction with the digging wave over the Northeast. That being said, the 12Z NAM and 12Z GFS both seem reasonably similar to the 00Z CMC and 00Z UKMET in terms of general trough amplitude and trough axis position by 26.00Z (Wednesday evening). The greatest difference continues to come from the 00Z ECMWF which shows a stronger digging wave that also digs more sharply into the Great Lakes, allowing the trough to become more amplified with the mid-level vort max further southwest as compared to other models. Ensemble cluster analysis from the GEFS+ECMWF seems to give more support to the non-ECMWF deterministic model consensus, although a couple clusters still appear similar to the ECMWF. Therefore, although the ECMWF scenario can't be ruled out, the preference will be to lean toward a non-ECMWF blend, which is consistent with the overnight preference as well. ...Shortwave pushing into the Pacific Northwest today... ...Amplifying into the northern Rockies through Monday Night, and then digging into the Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, 12Z ECMWF, 12Z CMC Confidence: Slightly above average ---18Z UPDATE--- No change to the preliminary preference. The 12Z UKMET remains slower with the trough, and the 12Z NAM continues to be more amplified than the global model consensus. ---PREVIOUS DISCUSSION--- Most of the global models show better agreement today as the wave reaches the Plains. The 12Z GFS is very similar to the 06Z GEFS Mean, 00Z ECMWF Ensemble Mean, 00Z ECMWF, and to a slightly lesser extent the 00Z CMC by Tuesday afternoon (25.00Z). This continues into Wednesday as well. The differences come from the 00Z UKMET and 12Z NAM which have more amplitude to the trough, an upstream ridge axis that is positioned to the west of the other global models, and are slower with the timing of the trough axis. The preference is to go with the consensus better supported by the ensembles. ...Northern stream portion of Gulf of Alaska trough breaking off and digging through the Canadian Rockies Tuesday... ...Continuing to dig into the northern Plains Wednesday, with associated surface cold front pushing southeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z CMC, 12Z UKMET, 12Z GEFS Mean Confidence: Slightly below average ---18Z UPDATE--- No change to the preliminary preference. ---PREVIOUS DISCUSSION--- The 00Z ECMWF also shows considerably more amplitude to this wave than the other models as the wave digs into the Northern Plains. That may be due to the synoptic pattern over the Western US and eastern Pacific, with the ECMWF furthest to the west with its position of the Pacific low, and also with the ridge axis in between. The other models are reasonably similar with this digging wave. The preference is to lean closest to the CMC, UKMET, and GEFS Mean which show reasonable similarity with all three components of the evolving synoptic pattern between 100W and 150W (digging shortwave, western ridge, and east Pac low). ...Southern stream portion of Gulf of Alaska trough breaking off and developing into a closed low around 35N/135W by Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z CMC, 12Z UKMET Confidence: Slightly below average ---18Z UPDATE--- No change to the preliminary preference. ---PREVIOUS DISCUSSION--- As stated for the previous system, the overall synoptic pattern over the eastern Pacific and western US will be linked, so the preference is largely the same as the previous system. The 00Z ECMWF and its ensemble mean are on the western end of model spread with the position of the upper level low, with the NCEP models (NAM, GEFS, GFS) on the eastern end. Given the preference of the CMC and UKMET for other components of the pattern over the western US, and their approximation of a median scenario for the low position over the Pacific, the preference here is to lean toward a blend of the CMC and UKMET. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers