Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1245 PM EDT Tue Apr 21 2020 Valid Apr 21/1200 UTC thru Apr 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Developing long wave trough over New England with an associated surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The 12z NAM and GFS continue to remain on track/consistent with the timing and depth of the shortwave helping to swing the larger scale trof negative tilt through the Northeast Wed. The resultant triple point will track near the Gulf of Maine and quickly into Southeast Canada. As such a general model blend will continue to be preferred at above average confidence. ...Mid level trough tracking from across the Southwest and southern Plains to the Mid Atlantic... ...Surface low associated with the mid level trough... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z ECMWF/ECENS and 06z GEFS blend (with some low 12z NAM thru 60hrs) Confidence: Average to slightly below average Medium-sized shortwave is starting to take better shape across the Southwest today, supporting lee-cyclogenesis across eastern NM. There is solid model agreement initially, though the CMC is a tad south. This small difference with the 00z CMC grows greater through the Mid-MS Valley Thursday and is clearly lagging. The wave is weakening slightly through the Ohio Valley into increasingly confluent flow...this leads to increased spread in timing of the surface wave tracking through the Ohio Valley toward the East Coast into Friday. Like the 00z CMC, the 00z UKMET will lag too, eventually becoming less favorable. The 12z NAM continues a trend of stronger but also faster into this confluent flow, while the GFS is a bit slower but equally compact and favoring stronger flux convergence into the developing deformation zone, over stronger convection; eventually slowing its progress through the Ohio Valley into the East. These are known negative biases of the GFS would favor the 06z GFS timing/influence over the operational 12z GFS. The NAM appears too fast, given its strength relative to the ensemble suite, so a bit counter-intuitive, still it may not be too out of phase for some inclusion, especially through the Ohio Valley. The 00z ECMWF is timed well initially with the GFS and NAM and also favors stronger convective development south, opposite to the GFS, and more in line with typical spring (recent) pattern. This helps to elongate/shear the larger scale shortwave, making it generally weaker through the East Coast, perhaps a tad too much given inter-comparison with ECENS members. As such a ECMWF heavy blend with the 06z GEFS and 12z NAM and 00z ECENS mean. Confidence is slightly below average in this blend given the lower predictability given importance of convective scale influence on the higher level mass fields. ...Short wave and surface low tracking from western Canada to the Northern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly above average A subtle shortwave/speed max in NW flow out of NW Canada will drop into the Northern Plains by early Wed with solid model agreement. The 12z NAM/GFS continue strong agreement with the evolution of the this wave before it weakens across IA into the Great Lakes, under confluent flow regime and approaching stronger shortwave/cyclone from the southern Plains. The 00z ECMWF/UKMET are also in good agreement with the mass fields, and much like the GFS and NAM can tap some of the overrunning moisture from the Pacific/Gulf moisture to produce a solid/narrow swath of QPF across the southwest-southern Great Lakes region into Thursday. The CMC mass fields are a bit off with the surface low track but the placement of the frontogenesis/QPF may be tolerable within a blend; yet its removal for mass fields would likely present a better blend. Confidence is slightly above average The 00z NAM/GFS are close to the consensus tracking short wave energy from western Canada in the very fast mid level flow across the Northern Plains before 23/00z. The 00z NAM becomes slower than the 00z GFS after this time, eventually placing the trough over western ND (as does the 00z CMC, though not as far west as the 00z NAM). The 00z GFS is close to the 00z ECMWF/UKMET with the trough position extending from ND south through NE by 24/12z. There is more spread with the placement of the resulting surface low from ND into NE. Since there has been some consistency with the 00z GFS/00z ECMWF/00z UKMET with this feature, these models comprise the preferred blend. However, due to the slower solutions of the 00z NAM/00z CMC and the spread in the placement of the surface low at the end of the period, forecast confidence is only average. ...Short wave energy tracking from the Pacific Northwest to the Southern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 24.00z Non-NAM thereafter Confidence: Average A strong anticyclonically curved 250mb Pacific jet will help to tamp down the larger scale ridge in the Eastern Pacific and usher smaller scale embedded shortwaves across the Pacific Northwest starting late Wed, that will progress through the Rockies into more baggy, flatter/broad troughing. This will allow the lead wave to start to amplify across the Bitterroots before stretching vertically along the lee of the Rockies, supporting a lee trough and surface low in the High Plains, that builds south by 24.00z, with a stronger/deeper surface wave in the TX panhandle. The deep southerly return flow will support active convection throughout the Plains into the MS Valley. Overall, this is a difficult pattern to lock down the precise placement of these convective clusters, though given this the model spread is fairly consistent to have moderate confidence in a general model blend through about 72hrs (24.12z). By this time, the 12z NAM is very strong and fast; typical of day 3 negative bias. The 12z is a tad faster, in line with bias but not terribly so. The 06z GEFS is better timed...and for a more solid blend, perhaps should be used on day 3 over the operational GFS. However, WPC preference at this time, is a general model blend through 24.00z, shifting more non-NAM thereafter. Confidence is average given small spread but lower predictability/ flow regime. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina