Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 136 PM EST Sat Feb 10 2018 Valid Feb 10/1200 UTC thru Feb 14/0000 UTC ...See NOUS42 KWNO (ADMNFD) for status of the upper air ingest... ...12Z Model Evaluation with Preliminary Preferences/Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwave Moving Through The Central Rockies Today and Getting Sheared Out Over Mid-Mississippi Valley Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM Blend Confidence: Above Average ---18Z UPDATE--- No change to the preliminary preference. Models generally remain in good agreement. ---PREV. DISCUSSION--- The 12Z NAM continues to eject the shortwave a little faster than the other models, which was also the case yesterday. Other models have come into much better agreement and offer very little difference in the timing or amplitude of the shortwave, as well as the position or intensity of the associated surface low over northern New Mexico. Therefore, the preference is for a general model blend that excludes the 12Z NAM. ...Extensive Front Stretching from the Southern US northeastward into New England and Broad Southwesterly Flow Aloft... ...Weak Surface Low Development Along Front into Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Slightly Above Average ---18Z UPDATE--- No change to the preliminary preference. Models remain in good agreement. ---PREV. DISCUSSION--- A subtle wave ejecting in the broad southwesterly flow appears to be somewhat stronger than models were anticipating several model cycles ago. This was the wave that was expected to contribute to slow and weak surface cyclogenesis along the elongated front from the central Appalachians into the Northeast. Models have now trended toward a more pronounced (albeit still fairly weak) surface low, developing by this evening over Middle Tennessee, and then shifting northeast to the St. Lawrence Valley by tomorrow evening. In addition to this trend, model spread has been substantially reduced, and for this reason a general model blend is preferred. The mass fields from all the global models and the NAM look reasonably similar overall. ...Shortwave Digging into the Pacific Northwest on Sunday... ...Positive Tilt Trough Building Down the West Coast Monday... ...Closed Low Developing near the California Coast Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, NAM and ECMWF Confidence: Average ---18Z UPDATE--- Although the 12Z UKMET trended toward developing a closed low along the California coast on Tuesday, it still has a more elongated trough than most other models, and higher heights at the core of the trough. The 12Z CMC still has higher heights near the West Coast. Therefore, the preliminary preference of a blend of the GFS, NAM, and ECMWF is maintained. ---PREV. DISCUSSION--- The most notable model trend over the past several model cycles is that there is good agreement now on digging the shortwave more sharply along the West Coast after it finishes undercutting a northward extension of the East Pacific ridge. Most models now bring the core of the trough into the Oregon Coast around 44N, which is much further south than 3-4 model cycles ago. Despite the sharp digging of the trough, the 00Z CMC depicts a trough that is much broader than other models by 12/00Z, and has higher heights through much of the western CONUS ahead of the trough. Although it eventually converges to be closer to the other global models by 13/00Z, the initial differences are substantial enough to exclude it from the preference at this time. The other model that shows more substantial differences is the 00Z UKMET. Most models now have a closed low near the California coast by Tuesday, and in fact even the GEFS and ECMWF ENS means have a closed low. However, the UKMET shows a narrower and more elongated trough and never closes off a low at 500mb. Given the overall trend toward a closed low and the sharp digging of the trough along the West Coast, the UKMET is also excluded from the preference. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers