Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Sat Mar 24 2018 Valid Mar 24/1200 UTC thru Mar 28/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Pair of shortwaves moving into the Intermountain West this weekend, emanating from a devolving closed low... ...Further amplification of the Western U.S. trough via a wave digging sharply from the Aleutians, down the West Coast and into the base of the trough... ...Lee cyclogenesis over the Plains Sunday and Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF, 12Z UKMET, 12Z CMC Confidence: Average ---18Z UPDATE--- No change to the preliminary preference. The ECMWF, CMC, and UKMET continue to be slower with the trough than the GFS, although they have notably sped up slightly from their previous positions. This makes the 00Z ECMWF Ensemble Mean the slowest at this time, and given the trends on the other global models, that may speed up on its 12Z run. ---PREVIOUS DISCUSSION--- Models continue to show limited differences through Sunday Night, and diverging forecasts after that. In general, the same model spread that existed yesterday continues to show up today with the 12Z GFS developing a broader trough over the West and spilling height falls further to the east over the Plains than other models. At 500mb, there are still 60m height differences over the Southern Plains between the GFS and ECMWF. The 12Z NAM shows a slightly sharper trough than the GFS, but still has lower heights over the Plains (like the GFS), and amplifies the downstream ridge more than any other model over the Eastern CONUS. On the other end of the model spread, the 00Z operational ECMWF and ECMWF ensemble mean are the slowest, and hold the base of the trough back to the west the longest. The 00Z CMC and UKMET are closer to the ECMWF than the GFS, but do offer an intermediate solution. With an amplifying pattern and a shortwave digging sharply into the base of a positively tilted trough, the expectation would be for the trough to slow down -- a scenario better depicted by the ECMWF, CMC, and UKMET. This also represents continuity from the past several WPC model preferences, and is close to the consistent ECMWF ensemble mean. Therefore, the preference will be to lean toward a blend of those three global models. By 27/12Z on the 00Z model cycle, the GEFS members and ECMWF ensemble members 558-570 DM isoheights showed almost no overlap in the Southern Plains. In other words, the slowest GEFS member was just about faster than the fastest ECMWF ensemble member. ...Shortwave traversing the Midwest now and becoming sheared out through the Ohio Valley and Carolinas... ...Weakening associated surface low transferring to the Carolinas coastline by Sunday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC Blend Confidence: Above Average The 12Z CMC deamplifies the shortwave much faster than the other models, which seems to occur because its initial representation of the shortwave was weaker. This leads to a flatter wave, and a slightly weaker surface low that ends up slower and further south than model consensus. Otherwise, models are in very good agreement. The preference is for a broad consensus with this system with the exception of the CMC. ...Arctic shortwave digging sharply south through Quebec on Saturday with closed low developing off the Northeast Sunday and Monday... ...More accelerated development of the Carolinas coastal low well offshore on Monday and Tuesday... ...Associated strong surface high settling into New England... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z UKMET, 12Z ECMWF Confidence: Average ---18Z UPDATE--- The NAM, UKMET, and ECMWF continue to be most tightly clustered with the surface low over the Atlantic, and maintain reasonable continuity with their previous runs. Therefore, no change to the preliminary preference. ---PREVIOUS DISCUSSION--- Within the broader trough situated over the Northeast and eastern Canada, an embedded shortwave is situated over the North Atlantic around 40N/60W today. Most models are in good agreement with this shortwave, except the 00Z CMC which barely indicates a shortwave, with a much flatter height pattern aloft relative to the other models. This initial difference, along with the faster deamplification of the Ohio Valley shortwave (discussed above), affect the eventual forecast over the Northwest Atlantic, and as a result the CMC is not included in the preference. The 12Z NAM, as well as the 00Z UKMET and ECMWF, seems to be the most consistent with the position and intensity of the eventual upper level low. They all have very similar timing of the digging wave, while the 12Z GFS is still a little faster and kicks the trough and low a little further east. The preference is to remain closest to the more consistent models and this is also consistent with previous WPC model preferences. ...Shortwave rounding a Pacific ridge through the Gulf of Alaska on Monday and digging into the Northern Rockies Tuesday... ...Surface low developing over Alberta and Saskatchewan Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 00Z UKMET, 00Z ECMWF Confidence: Slightly above average ---18Z UPDATE--- No change to the preliminary preference. The GFS continues to be well south of all the other global models with the surface low position in southern Canada. ---PREVIOUS DISCUSSION--- The models are in fairly good agreement overall with this system, but there are subtle differences. This may be due to the variation in how they handle the (much larger scale) trough that will be gradually moving from the West out into the Plains. Most notably, the 12Z GFS is further south with the surface low by the end of the diagnostic period than any other model (even quite a bit further south as compared to the 06Z GEFS Mean). The 00Z CMC is also further south, but still slightly north of the GFS. Meanwhile, the 12Z NAM and 00Z ECMWF and UKMET are clustered fairly close to the ECMWF Ensemble Mean. The preference is to use a blend of those models, given the lean toward the ECMWF Ensemble Mean with the larger scale trough. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers