Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 234 PM EDT Thu Apr 12 2018 Valid Apr 12/1200 UTC thru Apr 16/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Large-scale trough moving across the West today, amplifying across the Plains Friday and Saturday... ...Deep occluded cyclone evolving Friday into the weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF; 12Z UKMET; 12Z EC Mean; 12Z NAM Confidence: Average ---18Z UPDATE--- Generally, no change to the preliminary model preference. The 12Z models made slight adjustments, but overall remained relatively consistent. The ECMWF, UKMET, and NAM are the preferred models. Ensemble mean preference was shifted from the GEFS to the EC ENS Mean given better consistency of the EC ENS -- both run-to-run and with these deterministic models. ---PREVIOUS DISCUSSION--- There is very good agreement that a potent trough will emerge into the Plains and lead to a strong cyclone, which should occlude relatively quick over the central Plains. Therefore, from a synoptic standpoint there is relatively high confidence in the big picture and related sensible weather impacts. However, detail differences continue and that leads to lower confidence for the specific placement and exact timing of those impacts. In particular, convective timing along the trailing cold front in the south-central US is tricky, and does not entirely depend on the synoptic pattern aloft. For more details on those specifics, refer to the QPF Discussion. The 12Z GFS continues to be on the fast end of model spread with the trough aloft, and generally with the cold front progression through the southern US. There are a few ensemble members that are close to that timing or even a little faster, but the bulk of deterministic models, ensemble means, and ensemble members are slower. The preference, therefore is to lean toward the model timing with greater support. Reasonable consistency now exists with the ECMWF, UKMET, and NAM in terms of their mass fields aloft, as well as the GEFS Mean which is considerably slower than the operational GFS. Although these models show differences in the exact evolution of the occluding low, the preference is for a blend of those four models to better capture the uncertainties with how that process will unfold. ...Low amplitude northern stream wave crossing the Great Lakes/Northeast through tonight... ...Associated weak surface low following a similar path... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Above average Models continue to be in excellent agreement overall with this system, with the 00Z CMC being the exception -- showing a much less amplified wave by 13.12Z (Friday morning). Thus, the preference is for a non-CMC blend. ...Deep trough and upper level low slowly dropping into the Pacific Northwest this weekend... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend; lean toward the 12Z ECMWF Confidence: Slightly above average Overall, models agree on the scale and amplitude of the approaching trough this weekend along the West Coast. The primary difference appears to be with timing, as the NCEP models (GFS, NAM, GEFS) are spreading height falls into the region a bit faster than the other models (CMC, UKMET, ECMWF). The preference is to trend towards a general blend of these options, but exclude the UKMET which is considerably slower than other models. The greatest weight will be placed on the operational ECMWF which is slightly slower than the consensus, which seems most likely in a situation with an amplified trough and associated closed low. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers