Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Mon Apr 27 2020 Valid Apr 27/1200 UTC thru May 01/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep trough/low center exiting the Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models gradually take the deep upper trough and associated closed low impacting the Northeast gradually away from the region through Tuesday. The guidance shows rather minimal model mass field spread as the system exits away, and so a general model blend will be preferred. ...Northern stream trough amplifying over the Midwest... ...System advancing into the Eastern U.S. by Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z UKMET/ECMWF and 12Z GEFS mean Confidence: Average A northern stream shortwave trough clipping the Pacific Northwest and southern British Columbia today is expected to amplify southeastward down across the Midwest by Wednesday in response to a deep layer ridge developing over the western U.S. and southwest Canada going through the middle of the week. The guidance shows a strong closed low evolving over the Midwest which will focus on the OH Valley and western Great Lakes region by Thursday as a deep layer trough envelops much of the eastern U.S. by that time period. All of the model show reasonably good agreement with the height fall evolution and surface cyclogenesis going through Wednesday. By Thursday, there is some spatial spread with the surface low evolution as the 12Z NAM/GFS solutions takes the surface low/energy a little more quickly north across the Great Lakes, with the 00Z non-NCEP solutions tending to keep low pressure confined more to the OH Valley, and with a bit better defined triple-point low over the Mid-Atlantic states at the end of the period given placement of stronger height falls focusing still a tad farther south. However, the 12Z non-NCEP solutions (notably the UKMET and ECMWF) did trend just a tad toward the NAM/GFS camp. Like its predecessor, the 12Z CMC still appears to be a bit too weak with its surface low reflection, but the CMC also has its energy still on the south side of the guidance at the end of the period. The 12Z CMCE and 00Z ECENS means still favor the farther south solution as well. The 12Z GEFS mean though is very close to the new ECMWF at this point, with the GFS still a tad north of this camp. Based on the latest trends and clustering, a blend of the 12Z UKMET/ECMWF and 12Z GEFS mean will be preferred. This best approximates the model consensus with the NAM/GFS solutions still overall on the north side of the guidance and the CMC on the south side. ...Weak shortwave clipping the Northwest on Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Above average A modest and rather progressive shortwave trough will come in off the Pacific Ocean and clip areas of the Pacific Northwest and southern British Columbia by late Wednesday. A weak area of low pressure an attendant front is expected to accompany this energy. Given the full slate of the 12Z model guidance, the GFS appears to be perhaps a tad too strong with it shortwave energy and resulting surface reflection. The GFS low track is also a tad north of the model consensus, and so based on the latest trends/clustering, a non-GFS blend will be preferred. ...Shortwave moving into the West on Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Above average The models bring another shortwave trough in off the Pacific Ocean on Thursday and in across portions of the West. The guidance reflects a relatively low-amplitude to this system at least through this period, but the GFS is tad stronger than the NAM and also the non-NCEP models at this point. The latest GEFS/ECENS means are not as robust as the deterministic GFS, and so will suggest leaning away from the relatively deeper GFS at this point. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison