Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 238 PM EDT Sat Mar 17 2018 Valid Mar 17/1200 UTC thru Mar 21/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Persistent W-NW Flow Aloft over Northeast US through Monday... ...Weak Surface Low tracking from Lake Superior Today to Long Island Sunday Associated with Digging Shortwave Aloft... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC Blend Confidence: Above Average Models are generally in excellent agreement with the mass fields from the Hudson Bay Region into the Northeast US, associated with continued W-NW flow on the southwest periphery of the broad upper level low over Eastern Canada. The most notable differences are with the 00Z CMC, which amplifies the upper level pattern more than other global models: a deeper trough along 60W and higher heights upstream in response along 80-90W. As a result, it also builds a slightly stronger surface ridge in faster across the Northeast. The preference is to stay closer to the global model consensus, and thus a non-CMC blend. ...Deamplifying Wave Becoming Increasingly Sheared Out as it moves from Ohio Valley into Carolinas This Weekend... ...Associated Weak Surface Low Tracking On Similar Path... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Above Average There is very little model spread with this system, both aloft and at the surface, and minor model differences don't seem to make a significant difference in their sensible weather impacts. Therefore, a general model blend is preferred with high confidence. ...Large Closed Low near Oregon Rapidly Breaking Down by Sunday into Small-Scale Distinct Waves... ...Several Distinct Shortwaves Becoming Increasingly Embedded in NW Flow over the Northwest and Northern Plains as Northern Stream Jet Sags South... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend, Weighted to 12Z GFS Confidence: Below Average As the upper level low begins to rapidly break down over the next 24 hours, the distribution, motion, and strength of the resulting waves should be somewhat chaotic. That being said, the models do seem to have a decent handle on several primary shortwaves that should emerge -- one lingering near the coast of Oregon and Washington through Monday before moving into the Intermountain west, and another near southern Alberta eventually moving into the Northern Plains. As would be expected in a complex flow pattern, models do offer somewhat different forecasts, and confidence is not high enough to select a particular model. However, the 12Z GFS does appear to be reasonably situated in the middle of the distribution in terms of timing of the waves as they begin to push east, so will place a slightly greater weight on the GFS. The messy breakdown of the upper level low will also likely contribute to uncertainty downstream and at later forecast hours elsewhere in the CONUS. ...Potent Shortwave at the Base of Devolving Western Closed Low... ...Strong Trough Ejecting into the Plains later on Sunday... ...Lee Cyclogenesis over the Southern Plains... ...Low Pressure System Reaching the Mid Atlantic Coast Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 65% GFS (12Z); 35% ECMWF (00Z) Confidence: Slightly Below Average Models continue to show a fair amount of uncertainty with this system, particularly as the trough reaches the Mississippi River on Monday Night and begins to enter a region of accelerating, confluent flow on the south side of the persistent upper level low over eastern Canada. Ensemble sensitivity analysis from the GEFS, CMC, and ECMWF ensembles all show that the forecast of the surface low off the Mid Atlantic coast on Tuesday is very sensitive to the ridge and associated upper level height patterns from the Upper Mississippi River Valley into western Ontario on Sunday and Monday. Higher heights (and thus a "faster erosion" of the upper level low over eastern Canada) tend to correspond to a slower system, and vice versa. The 00Z CMC most aggressively builds the heights in this ridge, notably more than any of the other global models. The 00Z UKMET also shows this signal, but to a lesser extent. Given that higher heights have been a bias occasionally observed in the CMC lately, the preference is to lean away from that scenario at this time. Both the UKMET and CMC have the surface low furthest to the west (slowest) by 21/00Z (Tuesday Evening). The 12Z NAM offers a surface low track that is further northwest and slower than most other models. As is its tendency, it has a stronger trough as it emerges into the Plains, and that has downstream implications over the next 24-36 hours with slightly more downstream ridging along the coast, more meridional flow downstream of the trough, and a stronger vorticity max. Given a lack of support from other models (it is situated northwest of most of the 00Z ensemble members), the preference is also to lean away from the NAM at this time. The 12Z GFS and 00Z ECMWF seem to be more plausible scenarios right now. The overall flow pattern (more zonal, with less blocking) would tend to favor flatter and more progressive low tracks. By 21/00Z, the GFS low position is situated almost exactly in the middle of the other operational models. It is slightly slower than its previous run, but over the past several runs has offered decent consistency on the track if not the exact timing. However, the 00Z ECMWF can also not be discounted. It is faster, but still situated well within the envelope of most of the ensemble members (GEFS and ECMWF). Therefore, the overall preference is to lean more toward the GFS, but incorporate a decent amount of the ECMWF as well. ...Low Amplitude Shortwave Reaching California on Sunday and Amplifying into Secondary Trough Mon-Tue over the Mid-South... ...Reaching the Southeast Atlantic Coast late Tuesday with Secondary Coastal Surface Cyclogenesis... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM Blend Confidence: Average The 12Z NAM has a different configuration to this trough as it begins to amplify across the Southern US, partially due to differences with how it handles the lead wave, and differences with how quickly it amplifies the wave over the Plains. The NAM does this earlier, which creates more substantial height differences aloft rapidly on Monday. The remaining models do show some differences, but are actually fairly similar in the position and strength of the upper level shortwave, vorticity max, and surface low position as cyclogenesis begins late on Tuesday. Therefore, the preference is for a Non-NAM blend. ...Closed Low Settling South in the Pacific to around 35N/145W by Sunday Afternoon and Slowly Drifting East through Tuesday... ...Increasing Southwesterly Flow and Possible Atmospheric River Approaching California by Tuesday Night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM Blend; Weighted to the 12Z GFS Confidence: Average Overall model differences with this system are not extremely large, as is typically the case with slow-moving closed lows that are initialized quite close. By Tuesday, some variance does emerge that may have an impact on the sensible weather. The 12Z NAM has a more amplified downstream ridge along the West Coast, more than the global models and it is excluded from the preference at this time as a result. Otherwise, the global models are reasonably close. The 12Z GFS is situated in the middle of the other models in terms of timing of the low position and strength. It is very close to the 00Z ECMWF Ensemble Mean, and sits between the 06Z GEFS Mean and 00Z operational ECMWF. Therefore, a general Non-NAM blend is preferred, with slightly greater weight placed on the GFS as the model closest to a consensus scenario at this time. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers