Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1123 AM EST Thu Nov 21 2019 Valid Nov 21/1200 UTC thru Nov 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Weakening closed low crossing the Four Corners into the Plains Friday/Saturday, phasing with potent northern stream shortwave across Great Lakes Sat into Sun... ...Surface low moving from the lower Mississippi Valley into the central Appalachians Friday night into Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Average The cold closed low in the Southwest continues to break down and stretch across the Lower MS Valley by the end of the week, before a sharp northern stream shortwave aids in cyclogenesis through the Tennessee Valley into the Appalachians before transferring to coastal low by Sunday morning in the New York Bight. The guidance is coming into some agreement in the evolution. However, the 12z NAM has broken from this trend, leading to a slower, southward emergence of the southern stream shortwave and a difference in the northern stream by deflecting the energy north; this leads to a much weaker and further east surface evolution. While the pieces may be hinting at a legitimate possibility in evolution (ie greater vertical convection closer to the Gulf Coast, weakening the trof overall)...there is very little support elsewhere to include it in a blend at this point. Otherwise, the guidance is in a typical spread, GFS fast, perhaps a tad too much, with a more reasonable GEFS mean noted, while the CMC is slower. The ECMWF also shows some negative bias, with a very small but compact shortwave in the northern stream; it does not't seem to negatively affect the lower atmosphere mass fields/evolution but it clearly looks odd in mass fields aloft. Still, this is good to suggest a Non-NAM blend, hedging toward the GEFS mean and the UKMET/ECMWF. Confidence is rising, but moderate variation in timing still may lead to a different evolution of the system to have only average overall confidence. ...Embedded shortwaves within strong zonal northern Pacific Jet into the Northwest... ...Associated surface wave across the Northern Tier by 00z Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 24.00z Afterward: Non-NAM/ECMWF blend favoring GEFS/ECENS mean Confidence: Slightly below average after 24.00z A global-scale trof is evolving across the Bering Sea into W AK, but along the southern periphery a very strong jet with tight height packing is developing in the far northeast Pacific/southern Gulf of AK, tapping some solid moisture. This is to be directed into the coastal Ranges of BC with slow southward drift by early Sunday. An embedded shortwave/jet speed max will reach the Central BC coast, early Sat followed quickly by another wave by late Sat into early Sun. With its passage on Sunday, the frontal zone/moisture stream will drop south into the Pacific NW. Both waves will progress across the Canadian Rockies and amplify slightly on the eastern side of the larger scale ridge. As is typical with such very fast and low amplitude wave packs, the guidance is agreeable until it passes the ridge and so the small variations in timing lead to large downstream differences in track/intensity to the developing clipper surface wave. This is no different, though the vast majority of guidance favors a more northern track represented by the 12z GFS, 00z UKMET/CMC, GEFS and ECENS mean. While the ECMWF, presents a northern wave, it presents second shortwave is a bit stronger and slower and therefore a bit south. This would present more QPF, sensible weather for the northern tier states relative to the aforementioned packing. Given the ECENS mean is further north, near the GEFS...think trending toward the means in preference is favored after the spread rapidly increases (24.12z). The 12z NAM appears on track with the cyclone but again by the end of Day 3, looks a bit too amplified given the flow regime...so will hedge away from the NAM as well. As such, favor the GEFS/ECENS mean inside a non-NAM/ECMWF blend, but confidence in this blend is slightly below average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina