Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Mon Apr 27 2020 Valid Apr 27/0000 UTC thru Apr 30/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation Including Final Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 7Z Update: After the arrival of all the 00Z Guidance, the CMC is slightly weaker with the nor'easter off the coast of New England and the GFS is still a little bit faster with the shortwave trough, with same preferences adjusted for 00Z cycle. For the Midwest system, the GFS remains the quickest with mid-level height falls across the north-central U.S. and the CMC slightly weaker with the upper low, but overall good agreement in the models for an 84-hour forecast given the amplified pattern in place. For the Pacific Northwest, the ECMWF trended more in line with the other models regarding timing of the second shortwave on day 3. Developing Northeast coastal low through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z NAM/00Z ECMWF/00Z UKMET blend Confidence: Moderate-High The dual-surface low over the Mid-Atlantic region on Sunday is now consolidating into a primary low just off Cape Cod as a developing nor'easter. This is in response to a strong shortwave pivoting around a larger upper low over New England. A triple point low is likely to develop south of Nova Scotia by Monday night, and then evolve into the primary low by Tuesday. The models overall are in good agreement mass field wise, with the 00Z GFS a little faster with the 500mb trough axis by Tuesday morning and beyond, and the CMC slightly weaker with the 700mb low. Trough axis crossing Canadian Prairies, clipping Upper Midwest/Great Lakes through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: High A northern stream shortwave will track across Minnesota and over the northern Great Lakes through Monday, and will support the passage of a weak cold front across the northern Plains. There is enough model agreement to merit a general model blend. Low pressure tracking from the northern Plains to the Great Lakes ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/UKMET/12Z ECENS blend Confidence: Moderate-High Some of the shortwave energy from the large Pacific trough currently near British Columbia crosses the Canadian Rockies, and then amplifies as a downstream trough across the central and northern Plains by Tuesday morning. This will induce surface cyclogenesis across South Dakota and Nebraska, crosses the Midwest states on Tuesday, and then develops further as it reaches Michigan under the presence of a deepening upper trough and closed low. The models are in good agreement mass field wise through about 12Z Tuesday. The 00Z NAM appears a little faster and farther north with the mid level closed low, and faster with the cold front reaching the East Coast by the end of the forecast. The GFS is the fastest to bring height falls to the Midwest and Mid-South. The 12Z CMC is on the slower and weaker side of the guidance, and the UKMET is stronger. Taking these differences into account, a blend of the 12Z ECMWF/ECENS/UKMET works well as a starting point. Glancing effects (front) of lead shortwave late Tues/Wed in PacNW...Full bodied shortwave reaching PacNW by 00z Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 12Z Wednesday, then a 00Z UKMET/ECMWF/12Z ECENS blend Confidence: Moderate The next Pacific shortwave has trended a bit farther north and not as amplified. Given the older Gulf of Alaska upper level gyre will be eroding late Monday into Tuesday, the arrival of the upstream shortwave late Tuesday will reset the larger scale trough but displace it eastward. This will allow for strong southwesterly moisture transport over southwest British Columbia and clipping western Washington by late Tuesday and early Wednesday. A second Pacific shortwave will quickly approach on Wednesday, and with the lingering frontal zone directed toward the Washington coast, another surge of moisture will likely reach western Washington by Wednesday evening. A general model blend works through about 12Z Wednesday, after which the GFS trends faster with the second shortwave, and the ECMWF slower and farther south with the surface low. The 12Z UKMET and CMC are close to the ensemble means and can be used in conjunction with the ECENS for the day 3 period. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick