Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 135 PM EST Tue Jan 23 2018 Valid Jan 23/1200 UTC thru Jan 27/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Broad Occluded Cyclone and Associated Trough over the Northeast, Progressing to near Labrador by Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: General Model Blend CONFIDENCE: Above Average ---18Z Update--- No change to the preliminary preference. Models remain in good agreement overall. ---Previous Discussion--- Models are in excellent agreement with the evolution of this system over the next 24 hours as it moves out of the Northeast region. Available operational models show tight clustering with the low position and limited spread with many of the mass fields. Therefore, a general model blend is preferred with above average confidence. ...Northern Plains Shortwave and Associated Secondary Cold Front Moving Quickly East to the East Coast by Wednesday Night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: General Model Blend CONFIDENCE: Slightly Above Average ---18Z Update--- No change to the preliminary preference. Models remain in good agreement overall, and several of the new 12Z model runs (UKMET, ECMWF) initialized closer to the GFS. ---Previous Discussion--- There are some initial differences across the Northern Plains, but it appears to have very little impact on the model scenarios through the next 24-36 hours as the trough progresses quickly to the east. The 12Z GFS initialized with slightly lower heights across much of the Northern Plains than other models, but it still shows a very similar amplitude and timing of the wave by 25/06Z (Wednesday Night). It does show slightly faster timing of the colder air over the Mid Atlantic, but overall is reasonably close to other models. Therefore, a general model blend is preferred with this system. ...Progressive Trough Pushing onto the West Coast Wednesday, and Deamplifying as it Pushes East to Great Lakes by Friday... ...Associated Surface Low near the Canadian Border and Trailing Cold Front Pushing into the Central US... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: Blend of 12Z GFS, 00Z ECMWF Lean to ECMWF (65%) vs GFS (35%) CONFIDENCE: Average ---18Z Update--- No change to the preliminary preference. The 12Z UKMET and 12Z CMC have not trended enough from their 00Z runs to be included in the preference at this time. The UKMET in particular still shows a stronger surface low that is much further south of the other models and ensemble means over the north-central US. ---Previous Discussion--- The 12Z GFS and 00Z ECMWF offer fairly similar scenarios through the first three days of the forecast, with the most notable differences emerging late in the period over the Plains. Despite a similar low position by 27/00Z (Friday evening), the GFS has faster timing of the cold front in the Plains, while the ECMWF is slower, and in better agreement with most of the other models. Even the GEFS mean (06Z run) is slower than the operational GFS. Given the neutral or deamplifying trend of the trough through the period, do not anticipate an accelerating cold front down the Plains as we sometimes see in winter. Therefore, the blend preference is weighted more toward the ECMWF, particularly at later hours. The 00Z CMC was not included as it suppresses the height falls further west and offshore initially along the Pacific Northwest coast, which leads to a flatter wave, a weaker surface low that is situated on the southern end of model spread. The 00Z UKMET was also not included, as it shows a stronger surface low (position furthest south of any model), with noteworthy trough amplification over the Plains -- unsupported by any other model. And although the 12Z NAM is not significantly different with the timing or position of the trough or surface low, it does have a cold bias in the Northwest behind the cold front, which could lead to lower forecast snow levels than would be suggested by other models. Model Trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500MB Forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers