Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 153 PM EST Sun Jan 05 2020 Valid Jan 05/1200 UTC thru Jan 09/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwave trough tracking across the Great Lakes/Northeast through late Mon... ...Weak surface low moving through southern Canada... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similarly with this system. No significant changes were noted in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...Split-flow trough near the Pacific Northwest today... ...Energy reaching the Great Lakes/Northeast Tue/Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z ECMWF and 12Z UKMET blend Confidence: Slightly above average ...19Z update... While small timing changes were noted, no significant changes occurred in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...previous discussion follows... As shortwave energy makes its way east into the Great Lakes region on Tuesday, There are differences with the handling of the 500 mb trough. The 12Z GFS is displaced north with the core of the base of the trough in the vicinity of western Ontario noted mid-day Tuesday. The 00Z CMC is displaced slightly south and slower than the remaining consensus. Fairly good agreement in the ensembles exists for a blend near the 12Z NAM, 00Z ECMWF and 00 UKMET. ...Southern stream trough/low near the TN Valley by Tue... ...Surface low impacting the Mid-Atlantic/Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM and 12Z ECMWF Confidence: Average ...19Z update... There was a subtle trend for more separation between the northern and southern stream components of the eastward moving trough axis described below in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. The result is a surface low track a little farther south and offshore as the low tracks north off of New England. A blend of the 12Z NAM/CMC (more west) and 12Z ECMWF/UKMET (more east) places a low closest to the center of most ensemble scatter low plots valid 12Z/08. A blend of the 12Z NAM/ECMWF is sill preferred which represents a small shift from continuity. The 12Z GFS is still considered too fast. ...previous discussion follows... While model differences are rather minor with this system, a modest amount of spread remains in the ensembles and run to run consistency in the guidance has not been great. The 12Z GFS stands out first with a faster 500 mb trough progression toward the East Coast, perhaps due to more separation in the flow with the northern component of the larger scale trough axis tracking through southern Canada. Among the non 12Z GFS guidance, specifically concerning the 850 mb low, from late Tuesday into Wednesday, the 12Z NAM is on the south side of the deterministic guidance while the 00Z ECMWF/UKMET/CMC are north. The 12Z GFS is also south like the NAM, but again is faster than ideal. A blend between the 12Z NAM and 00Z ECMWF is considered best for this system at this time, but adjustments in future cycles appear possible given the changes seen leading to today's runs. ...Shortwave/fronts approaching the Pacific Northwest Mon and Tue... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non 12Z UKMET blend Confidence: Slightly above average ...19Z update... While the 12Z UKMET adjusted a little slower with its trough progression, it is still the fastest of the latest deterministic guidance with this system. In fact, a small shift to be slower was noted in the 12Z ECMWF/CMC, so a non 12Z UKMET blend will still be preferred at this time. ...previous discussion follows... Yet another in a series of shortwave troughs will arrive across the Pacific Northwest late Monday into Tuesday. Initially a warm front will cross the region ahead of this energy, with the main trough crossing by late Tuesday. Model agreement with the warm front is now good, but the 00Z UKMET is faster with the mid-level shortwave along the West Coast. The faster timing of the 00Z UKMET carries over downstream into interior portions of the West and is a notable faster solution in the 500 mb ensemble spaghetti plots through Wednesday. While not a complete outlier, prefer to stick nearer to the better ensemble/deterministic clustering away from the 00Z UKMET. Timing differences among the non 00Z UKMET guidance are present (00Z ECMWF a bit faster) but are relatively minor. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto