Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 345 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 Final Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Trough exiting New England late Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Slightly above average GOES-16 WV denotes shortwaves over MI and NW Ontario, western side of the remaining trough that has shifting east across North America over the past few days. 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 remains small (as expected for Day 1) and a general model blend is preferred. ...Northern Shortwave Dropping through Rockies and Moving Across Midwest/Ohio Valley Fri/Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z ECMWF/UKMET Blend with some 12z CMC/00z ECENS at lower weighting Confidence: Slightly below average GOES-17 WV denotes positively tilted shortwave trough across the northern Rockies to the northern Great Basin. The wave base will settle across the UT/CO Rockies by 00z Friday though the top/lead edge will shear/elongate into the north-central Plains and support a surface trough that will eventually break into a surface wave through the Mid-MS Valley into the Ohio Valley by midday Friday. Divergence 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 12z GEFS/09z SREF solutions. Given this is a typical negative bias, will continue to remove both solutions from the blend. The 12z ECMWF sped a bit from the 00z run and is now in excellent agreement with the 12z UKMET for both timing and latitude of the low level features now that the UKMET took a bit of a north shift. The 12z CMC took a greater north shift and is now between the NAM and UKMET/ECMWF, particularly for the northern edge of the Day 2 QPF in the Midwest and Day 3 QPF in the northern Mid-Atlantic to southern New England and will therefore reduce the CMC in the preference. Given the overall trends and continued spread/uncertainty through the Ohio Valley through the East Coast, confidence remains slightly below average. ...Shortwave(s) Along West Coast through Fri and across the Southwest Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/UKMET/CMC/ECENS Blend Confidence: Average Closed low off the OR coast this afternoon will remain closed as it swings south along the CA coast through Thursday night, opening into a wave as it ejects east across the Desert SW/Four Corners Friday through Saturday. Timing differences beginning on Day 3 are typical with the 12z NAM and GFS both continue to be compact/stronger and quicker, though differing on the track. The NAM shifts north and faster, while the GFS slows/sharpens across NM at the end of the forecast period. The 12Z ECMWF/UKMET/CMC all slowed compared the previous run and are in agreement, so preference goes to them. Given the shifting, confidence is average in this solution. ...South-Central Canada Shortwave Sat, with attendant cold front across the Northern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM 12Z General Model Blend Confidence: Average Strong northwesterly flow will dominate NW Canada early in the forecast period, though with broad cyclonic flow and an elongated trough across the Central US, the pattern will fold leading to increased ridging then trough 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 push across the breadth of the northern Plains Friday night/Saturday 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 12z ECMWF and 00z ECENS mean continue to slightly outpace the GFS/GEFS solutions, which is atypical, but still a good agreement overall and are in agreement with the 12Z CMC/UKMET. 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 the 12z NAM is not preferred with the remaining clustering good. The downstream blocking pattern and strength/depth of the cold air leaves the predictability of the environment a bit too low for higher than average confidence at this time. ...Shortwave amplifying from BC coast into PacNW Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM 12z General Model 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 trough 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 trough 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 Saturday. By this time, the 12z NAM is faster/deeper than the less progressive 12z ECMWF/GFS/UKMET/GFS, 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. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Jackson