Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1242 PM EDT Wed Apr 15 2020 Valid Apr 15/1200 UTC thru Apr 19/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Last Shortwave in Larger scale global trof exiting New England by Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend (through New England) Offshore (Fri):12z NAM and 00z ECMWF/UKMET (06z GEFS/00z ECENS) Confidence: Slightly above average GOES-WV denotes a subtle shortwave crossing the Great Lakes with another upstream shortwave over NW Ontario. The combination of these features appears the be the last vestiges of the larger global trof that has dominated the Eastern portion of the continent over the last half-week. The Great Lakes wave will sharpen a bit as it slides through the Northeast with the upstream Canadian wave kicking it out by early Friday. The spread through the Northeast is small enough that a general model blend can be afforded, though marine interests near the Canadian Maritime may be best suited by a 12z NAM and 00z ECMWF/UKMET blend that depict a more amplified (but not too amplified wave) and supports solid clustering of the surface low within the highest concentrated areas within the ensemble cluster. Confidence is slightly above average for this blend. ...Northern Shortwave Dropping through Rockies and Moving Across Midwest/Ohio Valley Fri/Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 17.00z 00z ECENS/UKMET blend with 00z CMC/ECMWF at lower weighting after Confidence: Slightly below average GOES-WV suite shows a sharpening shortwave across (just west of) the Rockies. This wave's base will settle across the UT/CO Rockies by 00z Friday though the top/lead edge will shear/elongate into the Central Plains and support a surface trof that will eventually break into a surface wave through the Mid-MS Valley into the Ohio/S Great Lakes by midday Fri. The spread in guidance will already start by this time, with the 12z NAM and GFS both trending fast, with the NAM in a typical stronger bias, both tracking well north of the remaining guidance, including the 06z GEFS/09z SREF solutions. Given this is a typical negative bias, will continue to remove both solutions from the blend. The 00z ECMWF interestingly, trended slower and therefore more compact emerging out of the Rockies, even compared to the majority of ECENS members; again like the GFS/NAM...typical of a negative bias and should be reduced in preference compared to its mean, but not fully rejected as it has been leading the longer-term trend signal. The 00z UKMET has been consistent, and while initially stronger/more strung out through the Ohio Valley, it trends nicely with the ensemble trends over the last few cycles. This is true of the 00z CMC as well; and while it remains elongated as it emerges off the East Coast presenting a weaker surface reflection that slides to the east of the best clustering. Could afford some increased inclusion of that model. So while day 1-2 across the Rockies is fairly solid and a general model blend is preferred...will trend to a ECENS mean/UKMET and CMC blend thereafter...with lower weighting to the CMC toward the end of Day 3 along the East Coast. Given the overall trends and increasing spread/uncertainty through the Ohio Valley through the East Coast, confidence is slightly below average. ...Shortwave(s) Along West Coast through Fri and across the Southwest Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend thru 18.00z 06z GEFS and 00z ECMWF/UKMET/CMC/ECENS blend thereafter Confidence: Slightly above average Closed low appears to be moving through the apex of a stationary large scale ridge, with the main body still solidly intact. Guidance is in solid with the timing and general strength of the lead shortwave/closed low as it descends along the CA coast enough to support a general model blend...the uncertainty remains as it enters the terrain of the Southwest by early Sat. The 12z NAM and GFS both continue to be compact/stronger, though differing on the track. The NAM shifts north and faster, trending with the weakening ECMWF/UKMET/CMC in timing; while the GFS slows/sharpens across NM at the end of the forecast period. So would prefer a Non-NAM solution through the forecast period, mass-wise, but the moisture return/QPF signal by the end of day 3, suggests keeping it from the preference on Day 3. The 06z GEFS, however, is more comparable and would serve as a good replacement. As such a general model blend is supported through 18.00z becoming a 06z GEFS and 00z ECMWF/UKMET/CMC/ECENS mean blend thereafter. Confidence is slightly above average in this solution. ...South-Central Canada Shortwave Sat, with attendant cold front across the Northern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/00z ECMWF blend Confidence: Average Strong northwesterly flow will dominate NW Canada early in the forecast period, though with broad cyclonic flow and an elongated trof across the Central US, the pattern will fold leading to increased ridging then trof as stronger jet energy slides through northern AK Friday. So in response, initial cold air will spill southward across Central Canada and supports a cold front to reach the northern Tier at the end of Day 3, with a stronger surface cyclone across N Ontario. The UKMET/CMC send the energy eastward and faster than the otherwise clustering solutions, and while the difference across the CONUS is smaller...these solutions would not be favored at this time. The 00z ECMWF and ECENS mean continue to have solid continuity but slightly outpace the GFS/GEFS solutions, which is atypical, but still a good agreement overall. The 12z NAM is initially clustered well here, but by the end of Day 3, likely due to higher resolved colder air, it slows and shows greater cyclogenesis and delays the cold front just a bit too much. As such a 12z GFS/00z ECMWF blend is preferred but only at average confidence; while the clustering is good, the importance of the downstream effective blocking pattern and strength/depth of the cold air leaves the predictability of the environment a bit too high for higher confidence at this time. I ...Subtle shortwave amplifying along BC coast into PacNW late Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS and 00z UKMET/ECMWF blend Confidence: Average The static ridge over the eastern Pacific will maintain northward across the Gulf of AK even as the shortwave (two sections above) cuts through it today. As such strong jet/shortwave energy is directed north across AK. At the tail end of a trof over-topping the ridge on Thursday, favorable conditions on the northeast side of the ridge will allow for amplification as it tracks, semi-phased, with the main trof across Canada. However, the Canadian Rockies will separate the wave and eventually become a compact vorticity center along Vancouver Island, able to tap some limited moisture for some QPF on Sat. By this time, the 00z CMC and 12z NAM are slower/deeper than a more progressive ECMWF/GFS/UKMET, which continue to match the ensemble suite well. This generally jives with the preference of the phased northern stream portion to give some confidence in this blend. Still, this is a low predictability situation given the position to the ridge and affects of the Rockies on severing this connection fully. As such, there is average confidence in a 12z GFS and 00z UKMET/ECMWF blend. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina