Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 139 PM EST Thu Feb 21 2019 Valid Feb 21/1200 UTC thru Feb 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low ejecting out of Southwest tracking through Plains/Midwest/Great Lakes Sat/Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 19z Update: The largest adjustment was the 12z CMC shifting toward a slightly faster and more importantly, stronger solution; it is a shade slower than the 12z NAM/GFS. The 12z UKMET, like the NAM/GFS also depict an early, strong response to deep moisture/instability to rapidly deepen cyclone through MO into Midwest; its occlusion is the strongest leading to deeper surface low that is west of the main cluster. The 12z ECMWF showed positive adjustment with the trof in south central Canada, significantly reducing the westward shift of the track/upper level pattern on Day 3; yet the 12z run is also slower than the 00z run. Still, the overall spread has reduced in the mass fields to suggest that a general model blend may suffice, splitting the timing difference between the ECMWF/UKMET/CMC and the GFS/NAM. Confidence remains slightly above average in this blend. Please note: the QPF preference may incorporate the magnitude of the NAM, but the QPF axis appears clearly too far north, as WPC is favoring an ECMWF/GFS/UKMET axis for this hazard. ---Prior Discussion--- Closed low over the Southwest is fairly well agreed upon in the large scale, but typical timing biases start to manifest as it ejects into the Southern Plains by 12z Sat. The 12z GFS continues to show this slightly faster ejection shifting a bit faster north and east into Sunday but as it intersects with deeper moisture/instability late Sunday, the system receives some feedback to rapidly occlude, this slows the northeast timing and increase TROWAL/deformation QPF band to the NW of the surface cyclone, and leads the GFS to slow compared to prior runs and start matching in track/placement of the cyclone toward day 3 across the Midwest into the Great Lakes, providing some confidence in more of its inclusion to the preference. The 12z NAM has remained consistent and while slightly slower initially ejecting compared to the GFS, it also shows upscale enhancement on late Sunday for a strong/deep occlusion. The 00z UKMET also falls generally in line with this solution, but is a bit east and deeper/fast through the end of Day 3, typical of its negative known bias; some lower weighting/incorporation to the mass fields seems warranted. The 00z ECMWF, is a slow member of the 00z ECENS solutions suggesting some concern for falling into its known slow bias; however, it is fairly well timed with the NAM/UKMET through the Plains to have some confidence in its inclusion initially. A stronger northern stream shortwave/cold plunge out of the Arctic develops a southern Prairie trof that is clearly out of place in the ensemble suite, with only the CMC showing some vague similarity. As the cyclone occludes and the TROWAL enhances by Sunday, this trof has negative influences, maintaining the depth of the cyclone as well as drawing it back toward the West and delaying its weakening across the Great Lakes into Ontario by 00z Monday...as such the ECMWF is included in preference but eventually becomes eliminated/removed by Day 3. The 00z CMC, like the ECMWF is slow, but is also the weakest solution with very limited TROWAL enhancement and little upscale enhancement that is expected by late Sat into Sunday...as such it is not included in the preference. As such the overall preference is a non-CMC blend but initially no 12z GFS (until 24/00z) transitioning to no ECMWF after 24/12z with inclusion of the 12z GFS. The overall mass spread is fairly small to build slightly above average confidence; however, it is the higher impact, small scale details that show moderate spread in the Day 2/3 period that lead to the time specific inclusion/exclusion within this blend. ...Pacific Northwest Trof Sat/Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend weighting toward 12z ECMWF/UKMET Confidence: Average to slightly above average 19z update: The CMC trended a bit slower and does not shift the inner core of the wave east into the Rockies by the end of the forecast period, this leads to better agreement with the placement of the inner core, but the trof extending back into the Pacific is a bit faster/SE than the trend would suggest. This is grown more true as the ECMWF shows a slower southward shift of the western extent of the trof matching best with the 12z GFS. The UKMET slowed a bit too and continues to be similar to the initial preference if a shade faster than the GFS/ECMWF. The QPF/moisture axis in the ECMWF/UKMET/CMC continues to be south of the GFS leading to some uncertainty to the the QPF portion of the preference. Overall will support a non-NAM blend though will continue to weight the GFS and CMC a bit less than the ECMWF/UKMET overall. ---Prior Discussion--- A trof will be descending along the BC coast on the eastern side of an amplifying Gulf of AK ridge by the weekend. The initial lead wave will bleed through the North US/Southern Canadian Rockies Sat/Sun and act as a kicker wave to the developing deep cyclone described above, this appears fairly well handled with some depth concerns mainly after affecting the Northwest (so please refer to section above). As flow under-cuts the developing ridge, a weak Omega block pattern develops over the Northwest aiding slowing of the trof dropping south as well as tapping some subtropical moisture stream for developing an Atmospheric River toward Day 3. The 12z NAM and 00z CMC are clearer outliers with respect to the elongating/amplifying trof, as they both shift the shortwave energy onshore and press height-falls into the Northern Rockies. The 12z GFS has maintained a bit of increased enhancement to the trof which delays the southward press of the moisture stream into the Pacific Northwest, but given the overall setup (upstream ridge) and the uncertainty in the precise latitude of the under-cutting moisture, it still remains somewhat viable in the blend at the Day 3 time-period, though the UKMET/ECMWF are a bit better agreed upon and have some better ensemble support. At this point, a 00z ECMWF/UKMET preference with some inclusion of the 12z GFS is preferred at average to slightly above average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina