Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1245 PM EDT Sat May 2 2020 Valid May 2/1200 UTC thru May 6/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Initial Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Shortwave and surface low tracking from Great Lakes to the Northeast through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/EC mean/NAM blend Confidence: Moderate A positively tilted shortwave trough in an active northern stream mid-level flow will cross Ontario and Quebec through the weekend and sustain a surface low and cold front crossing the Great Lakes, Ohio Valley, and then reaching the East Coast by Sunday night. The models are in excellent agreement through Sunday morning, after which the 00Z UKMET becomes faster with the upper level feature. The GFS and its ensemble mean are faster bringing the cold front across the East Coast and the arrival of height falls across the Northeast, and the CMC is not quite as amplified as the other deterministic guidance aloft. A blend of the ECMWF/NAM and some of the EC mean would work well as a starting point and maintains previous overnight model preference. Trough tracking from Pacific Northwest to the Great Lakes through Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/CMC/EC mean blend Confidence: Low-Moderate Shortwave energy pivoting around the Gulf of Alaska upper level gyre breaks off to the east and tracks near the U.S./Canada border, briefly becoming a closed upper low over Alberta and Saskatchewan on Monday before evolving into an open wave again. This will sustain a cold front crossing the Intermountain West, Rockies, and then the western High Plains by Monday. Similar to previous runs, the GFS and to a lesser extent the GEFS mean are still more amplified with the trough crossing the northern Rockies and northern Plains, and the UKMET likely too weak with the upper trough as it reaches the Great Lakes based on ensemble spread. By the end of the forecast period, the NAM becomes more amplified and is a little faster than the other models, and also loses ensemble support. Next storm system approaching Pacific Northwest early in the week ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/12Z NAM/00Z EC mean blend Confidence: Moderate The third area of interest will be across the northeast Pacific and the Pacific Northwest by late Monday and into Tuesday as a pair of shortwaves pivot around the broader upper level low over the Gulf of Alaska, with a couple of relatively weak cold fronts approaching the region. The 00Z UKMET is a bit stronger with the first shortwave/front approaching the region, however it is still within the ensemble spread and the other models are relatively close in agreement. With the second shortwave, the NAM and GFS are a little faster, the UKMET stronger, and the GEFS mean slower and well behind the operational GFS. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick