Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1130 AM EST Tue Dec 17 2019 Valid Dec 17/1200 UTC thru Dec 21/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Evaluation with Preference and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Eastern CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average ...07Z update... The models continue to be strongly aligned across with the surface wave and highly elongated vorticity stripe and warm conveyor belt remaining along the East Coast for the remainder of the day. The very strong/compact cold air descending across Ontario into the Great Lakes tomorrow and through New England Thurs is also fairly well agreed upon. The 12z GFS remains a tad weaker and therefore a bit faster with the associated shortwave...but not substantially so for a late Day 2 into Day 3 spread in the CONUS. As the surface cyclone develops along the SE Canadian Maritimes, the GFS being fast is north with the developing wave; while the slightly colder solutions of the CMC,UKMET and 12z NAM are slightly slower. The 12z GFS is correcting relative to the 00z/06z but still should be removed if forecasting for Canadian Maritime region. For the CONUS and US waters, however, will continue supporting a general model blend for these systems at above average confidence. ...Trough off West Coast splits through to north-central and southern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Exceptions: Non-NAM in northern stream on late Day 3 in Northern Plains Confidence: Slightly above average GOES-WV suite depict an elongated trof along/just west of 130W and appears to be sharpening into a more consolidated base. This would favor the evolution of the UKMET/CMC and ECMWF (ECENS/CMCE means) compared to the 12z NAM/GFS that still have stronger energy (and surface reflection) slightly north, but mostly in the lower tropospheric levels/surface However, as the wave enters the Western CONUS and strong Pacific jet adds speed energy/vigor to the troughing toward the Pacific NW by late Wed into Thurs. The guidance starts to align much better but starts splitting the flow. The northern stream energy evolves into a medium wavelength shortwave with some negative tilt orientation moving into the northern Plains by Friday. The 12z NAM shows some late-term over-amplification bias that typically does not manifest, though is along the faster fringe of the guidance suite as well. It is accompanied by the UKMET, but the 12z GFS which was partnered slowed a bit and trended toward the GEFS mean/timing. This generally matches the CMC/ECMWF/ECENS mean...so would favor a trend a Non-NAM solution in the northern stream. In the southern stream, the remaining shortwave energy remains consolidated through shrinks in size as it progresses through the Four Corners region on Thursday. The Guidance continues to tighten toward a common solution, anchored by consistent GEFS/ECENS means. The operational ECMWF shows some negative bias of very small but compact vorticity center that eventually digs south, slower across TX (away from the ECENS mean). The 12z GFS slowed substantially, but remains north along the Red River Valley at the end of Day 3...matching the UKMET/NAM and CMC along with the means. As such a general model blend will suffice across the southern portion of the trof. Many smaller internal shortwave/vorticity centers in a highly elongated/sheared environment moving through complex terrain is a fairly uncertain small scale forecast to lock down but overall confidence is slightly above average through the length of the trof, given the agreement overall in the mass fields. ...Persistent Onshore flow into the Pacific Northwest...... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z NAM, 00Z GFS, 12Z ECMWF, 00Z UKMET blend Confidence: Average In the wake of the elongated trof entering the Pacific Coast Wed... A strong closed low will park over the Gulf of Alaska and direct strong/persistent onshore moist flow into the Pacific Northwest as a strong Atmospheric River event. Timing of embedded shortwaves are tough to precisely lock down, but the large scale mass fields are fairly strongly aligned. There are some typical timing differences between the GFS/NAM/UKMET and the ECMWF/CMC mainly with respect to the arrival of the first surge of moisture, making shorter-term (less than 6hr) QPF forecasts a bit difficult but longer duration 12-24hr totals are likely to result in significant rainfall. With the 12z GFS slowing down significantly with the initial shortwave/nose of the jet...the timing is better overall and the GFS could be incorporated back into the preference. This results in a general model blend preference for the mass fields...please see WPC QPF forecast grids for preferences in timing...but confidence is increased to slightly above average given synoptic better agreement. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina