Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 321 PM EDT Wed Apr 22 2020 Valid Apr 22/1200 UTC thru Apr 26/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Southern stream trough exiting the Southern Plains, crossing the OH Valley Thursday and Mid-Atlantic Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z UKMET/ECMWF/ECENS with some 12Z GEFS/CMC blend Confidence: Average A positively tilted southern stream trough and associated surface low over the southern Plains this afternoon reaches the middle MS Valley tonight. This low pressure system will advance northeast and cross the OH Valley Thursday and push off the northern Mid-Atlantic coast Friday as an attendant cold front sweeps across the South. The 12Z GFS dramatically slowed this system and went from being one of the fastest solutions to the slowest while it also remains farther north with its surface low track over the OH Valley/Mid-Atlantic and is thus not preferred. The 12Z NAM remains the fastest solution with the trough and surface low and is thus also not preferred. The 12Z CMC is nearly identical to its previous run, remaining the weakest with the surface low, but remains in line with the 12Z ECMWF and UKMET for low track. Regarding timing and depth of the system, the 12Z UKMET/ECMWF have better ensemble support (12Z GEFS/00Z ECENS means). So, will favor a blend of the UKMET/ECMWF solutions and along with a preference for the 00Z ECENS ensemble mean along with some inclusion of the 12Z GEFS and 12Z CMC. ...Northern Stream Shortwave/Low center across the Northern Plains tonight/Thursday and Midwest Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z General Model Blend Confidence: Average Guidance brings a northern stream shortwave and associated low center down from Canada and across the eastern Dakotas tonight into Thursday. Most 12Z guidance then weakens the surface low by Friday in favor of the southern stream low over the TN Valley. The 12Z CMC and NAM hold onto the surface low the longest over the Midwest into Friday, but given the focus of the forcing/energy/moisture to the system to the south consistent among the models, none are removed and a general model blend is preferred. ...Shortwave trough digging across the Intermountain West... ...Energy amplifying into the Lower MS Valley Friday night... ...Associated deepening surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z CMC/ECMWF and 00Z ECENS mean Confidence: Slightly below average A shortwave trough currently shifting east to the Pacific Northwest coast is forecast to take a southeast turn as it crosses the northern Rockies Thursday night and amplify across the southern Plains on Friday before crossing the lower MS Valley by Friday night and pushing into the Midwest Saturday. Guidance is in agreement on the trough becoming negatively tilted Friday night, driving another deepening surface low center. Despite the 12Z GFS slowing from the 06Z run, it still becomes a fast outlier by Friday night and probably takes its energy a bit too far north and is the easiest to remove from preference. The 12Z NAM and UKMET are closer to the farther south and slower surface low track of the CMC/ECMWF than before, but are still distinct enough to not be preferred. The CMC/ECMWF camp is supported by the 00Z ECENS mean and somewhat by the 06Z GEFS. The preference will remain a blend of the slower CMC/ECMWF and ECENS mean. However, given some of the uncertainties over how strong the nearby northern stream energy and related interaction will be, confidence is a bit limited in the Friday/Saturday time frame. ...Trough reaching British Columbia on Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM Blend Confidence: Average A trough rounding a Gulf of Alaska low and associated cold front reaches the BC/Pacific Northwest Coast on Saturday. There is good agreement on depth and timing of the trough except for the 12Z NAM which is faster and a bit farther north. Therefore a non-NAM general model blend is preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Jackson