Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 145 PM EST Fri Jan 31 2020 Valid Jan 31/1200 UTC thru Feb 04/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Evolution of Trough over Eastern Half of the U.S. through Sunday... ...Coastal low tracking north offshore of New England late Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~ Preference: General model blend Confidence: Slightly above average ...19Z update... The 12Z ECMWF/UKMET/CMC adjusted toward the 12Z GFS idea of having a stronger/southward base within the northern stream portion of the mid-level trough over the Northeast Saturday night. The 12Z NAM now appears slightly weak with this feature but it is close enough to be included as part of a general model blend. Surface low agreement also increased offshore of New England, with differences not becoming more significant until Sunday morning offshore of Nova Scotia. ...previous discussion follows... An elongated mid-level trough axis with northern and southern stream components will track to the eastern U.S. through Saturday evening. Downstream of the trough axis, surface cyclogenesis is expected to occur off of the Mid-Atlantic coast on Saturday, tracking north offshore of New England through Sunday morning. The agreeable ensemble means were used as a baseline for where the deterministic models should lie. The base of the northern component of the trough axis shows some differences Saturday night near the northern Mid-Atlantic region. The 00Z UKMET is weaker and north with the base of the trough while the 00Z CMC is deeper and farther south. The remaining 12Z NAM, 00Z ECMWF and 12Z GFS are similar but the 12Z GFS is the most aggressive with the strength of the base of the trough axis. A blend of the 12Z GFS with the 12Z NAM and 00Z ECMWF are recommended since they fall within the middle of the latest spread and match similarly to the latest ensemble means. Regarding the offshore surface low, there is good agreement with track until the low is offshore of Nova Scotia, with a 12Z GFS and 00Z ECMWF blend likely closest to the middle of the minor spread until that point. Regarding strength, a blend of the 12Z NAM, 12Z GFS and 00Z ECMWF is reasonable, not including the weaker/outlier 00Z CMC. ...Pacific Northwest atmospheric river leading a longwave trough pushing into the West on Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of non 12Z GFS guidance Confidence: Slightly above average ...19Z update... No significant changes were noted with the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...previous discussion follows... There is reasonable agreement with the evolution of a longwave trough across the West Coast into the Great Basin into early next week. However, the 12Z GFS was noted to be a bit quicker than the consensus with the leading edge of the trough by early Sunday. The faster GFS also stands out when viewing ensemble spaghetti heights (540 dam at 500 mb), with a tightening of ensemble members toward the 00Z ECMWF mean over the past few cycles valid Monday evening but with the 12Z GFS ahead of 95% of the available ensemble members. A non 12Z GFS blend of guidance looks reasonable for the western U.S. trough since differences among the non-GFS deterministic guidance are minor. ...Leading shortwave tracking through southern Canada on Sunday with accompanying surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average ...19Z update... Only minor timing adjustments were noted with the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. A general model blend still looks reasonable for this portion of the country. ...previous discussion follows... A low amplitude shortwave at the leading edge of height falls across the western U.S. will track through southern Canada Sunday/Sunday night with a corresponding low at the surface. A warm front will extend southeastward from the low and impact the Great Lakes and New England with warm advection driven precipitation. While the 12Z GFS is slightly faster and 00Z ECMWF a bit slower, the differences are small enough that a general model blend looks appropriate for this system. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto