Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 233 AM EDT Mon Apr 20 2020 Valid Apr 20/0000 UTC thru Apr 23/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Surface low tracking off the Mid Atlantic coast...then southeast of Nova Scotia... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~ Preference: 18z GFS/00z ECMWF/12z ECMWF ensemble mean blend Confidence: Average The 00z GFS is faster than the bulk of the guidance lifting the short wave over the Southeast this morning off the Mid Atlantic coast, then southeast of Nova Scotia by 21/12z. As a result, it is much faster with the surface low that develops after leaving the Mid Atlantic coast. The 19/18z GFS, while also faster than the consensus, was closer to the 00z ECMWF/12z ECMWF ensemble mean, which for a fairly tight cluster with the low. The 12z UKMET remains slower/left with the surface track, as its short wave is slower and a bit deeper than the consensus (and the 00z CMC). Based on the above, an 19/18z GFS/00z ECWMF/12z ECMWF ensemble mean is preferred. However, given the spread in the track of the surface low, especially after 21/00z, forecast confidence is average. ...Developing long wave trough taking on a negative tilt over New England... ...Surface lows over Quebec/Maine... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~ Preference: 00z GFS/00z ECMWF/00z UKMET blend Confidence: Average Short wave energy diving out of central Canada in the fast mid level flow drops to the base of a developing long wave trough over the Upper Great Lakes by 21/12z. The 00z GFS/NAM are close to the consensus through this time, but as the long wave trough takes on a negative tilt, the 00z NAM slows the forward speed of the trough, becoming slower than the consensus after 22/12z. The 00z GFS remains close to the consensus through the negative tilt. Its depiction of the primary low over Quebec and an incipient secondary surface low formation over southern ME/coast water is close to the 00z ECMWF/00z UKMET blend, forming a fairly tight cluster. In contrast, the 12z CMC becomes slower and weaker with the primary surface low. Though much of the guidance is close with the evolution of the long wave trough, there is some spread concerning the placement/strength of the surface lows associated with it. Based on the above, a 00z GFS/00z ECMWF/12z ECMWF ensemble mean comprise the preferred blend. However, due to the spread in the surface low details, forecast confidence is average. ...Mid level trough tracking from CA to the Mid MS Valley... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~ Preference: 00z GFS/00z ECMWF blend Confidence: Slightly below average The 00z NAM/GFS are close to the consensus taking the short wave energy from southern NV across the central and southern Rockies through 22/12z. After that time, the 00z NAM becomes deeper/slower with the elongating trough, though not nearly as deep and slow with surface low as the 00z UKMET. The 00z GFS remains close to the 00z ECMWF with the trough position as it approaches the Mid MS Valley, with the 00z CMC still a bit flatter and weaker with trough. The 00z GFS/00z ECMWF blend has general support of the ensemble means, so this blend is preferred. Since there is some model spread concerning how the pieces of short wave energy coalesce in the mid level trough, forecast confidence is slightly below average. ...Surface low tracking from western Canada into the Northern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The 00z NAM/GFS are close to the consensus taking the surface low out of Alberta and tracking it across the Northern Plains toward the end of the period. Though there are some subtle differences in the timing of the short wave energy supporting the surface low in the very fast mid level flow, there is enough clustering for a general model blend with this feature. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hayes