Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 307 PM EDT Fri Mar 23 2018 Valid Mar 23/1200 UTC thru Mar 27/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep trough; closed low offshore the Northwest today... ...Opening into a series of shortwaves moving into the Intermountain West this weekend... ...Shortwave currently near the Aleutians digging down the West Coast Sunday and leading to trough amplification and renewed Plains lee cyclogenesis by Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF, 12Z UKMET Confidence: Slightly Below Average ---18Z UPDATE--- No change to the preliminary preference. If anything, the non-GFS global models nudged the trough position slightly west on Sunday and Monday, which creates substantial height differences aloft by Monday over the Rockies and High Plains. For example, over New Mexico and southeast Colorado, the GFS has 500mb heights about 60-80 meters lower than the ECMWF, UKMET and CMC. ---PREVIOUS DISCUSSION--- Given the complexity of how the closed low near the Northwest opens up into a trough, with several embedded shortwaves in the larger circulation separating from the original circulation, confidence is slightly below average in the detail and specifics with the Western trough. Additionally, the shortwave that will play a big role in the further amplification from Sunday into Monday is currently over the Aleutians. For this reason, have used ensemble cluster analysis to evaluate the more likely trough configuration by Monday evening (27/00Z). The 12Z GFS shows a much broader trough than other models, with height falls spreading further out into the Plains than the other global models, which show the base of the trough lagging over the Desert Southwest. With a sharply digging shortwave into the base of the trough, the latter option seems to be more likely from a conceptual standpoint and this is also the configuration that is most popular among the GEFS and ECMWF ensemble clusters. The 12Z NAM has a narrower and sharper trough like the non-GFS global models, however its trough axis is displaced further to the east so it too spreads lower heights into the Plains. Therefore, the 00Z ECMWF, UKMET and CMC seem to have more robust support from available ensemble clusters. The 00Z CMC does show a high height bias in the first 24-36 hours over the east Pacific relative to all other models, and it is systematic from low latitudes to around 50-55N. The preference is thus for a blend of the 00Z ECMWF and UKMET. ...Shortwave traversing the Four Corners today and reaching the Plains by tonight with surface cyclogenesis... ...Becoming sheared out as it pushes through the Ohio Valley and Carolinas with surface low transferring to the coastal Carolinas... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC Blend Confidence: Slightly Above Average Models have moved into much better agreement with this system, and show fairly tight clustering of surface low and frontal positions as well as height fields aloft. There are some small-scale differences, but these are not systematic in a way that would rule out any one model. The exception may be the 00Z CMC which deamplifies the shortwave much quicker (6-12 hours) and as a result begins to weaken and lag the surface low more behind the other models. The preference is therefore for a non-CMC blend. ...Arctic shortwave digging sharply south through Quebec on Saturday with closed low developing off the Northeast by Sunday... ...Associated strong surface high settling into New England... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF, 12Z NAM Confidence: Average ---18Z UPDATE--- With the rest of the 12Z global model runs arriving, the 12Z GFS now is situated well east of the other models with the upper level low position by Monday evening over the Atlantic. It is also sitting east of the 12Z GEFS Mean. The 12Z UKMET also continues to look too strong with the digging wave, and the 12Z CMC is substantially weaker and further north with the upper low over the Atlantic late in the period. Therefore, the preference is to lean closest to the ECMWF, which has remained relatively consistent, and continue to include the NAM which is close to the ECMWF. ---PREVIOUS DISCUSSION--- The 00Z UKMET shows a much stronger shortwave digging south out of Quebec relative to the other models, and as a result ends up with a stronger upper level low over the Atlantic as it closes off on Sunday. This was the most notable difference among the models for this system, and most of the precipitation impacts should be offshore. The 00Z CMC also showed a high height bias over the Atlantic in the base of the trough, and that may affect its forecast somewhat. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers