Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sat May 02 2020 Valid May 02/1200 UTC thru May 06/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 19Z Update: The 12Z CMC has trended to a more amplified solution with the shortwave across the Upper Midwest and Ohio Valley in response to a stronger upstream ridge across the Rockies compared to its 00Z run, and slightly stronger (along with the 12Z UKMET) with the trough over the Northeast on Monday and slower with the second disturbance approaching the Pacific Northwest. There is also a slightly slower trend with the 12Z UKMET over the northeast Pacific and the ridge axis over the Intermountain West, and the ECMWF trended more amplified. Taking all of the models into account, the NAM remains faster and stronger with the shortwave crossing the northern Plains, and the CMC slightly stronger with the surface low crossing the mid-South with the more amplified trough crossing the Ohio Valley. The GFS remains slightly faster with the front reaching the East Coast, and the UKMET is stronger and farther south with the surface low approaching the Pacific Northwest by Tuesday evening, and more in line across the Great Lakes region compared to its faster 00Z run. Shortwave and surface low tracking from Great Lakes to the Northeast through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/UKMET/NAM blend Confidence: Moderate-High 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: 12Z ECMWF/UKMET/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. This system continues to have the most variability among the guidance, thus reducing overall confidence in details. Next storm system approaching Pacific Northwest early in the week ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/CMC/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