Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sat Jan 04 2020 Valid Jan 04/1200 UTC thru Jan 08/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper trough crossing from the Midwest to the East Coast... ...Surface low near New England today/tonight... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similarly with this system. No significant changes were noted with the 12Z ECMWF/UKMET/CMC compared with their previous 00Z cycles. ...Shortwave ejecting out across the Plains, reaching the Great Lakes by Sun night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z CMC blend Confidence: Above average There remain some minor placement differences with the mid-level shortwave as it nears the Great Lakes. The 12Z CMC remains farther south with the 500 mb trough axis but the 12Z UKMET adjusted northward enough to be included in the remaining deterministic consensus. The ensemble means support a non 12Z CMC blend. ...Split-flow trough near the Pacific Northwest by Sun... ...Clipper type low approaching the Great Lakes by Tue... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/12Z ECMWF blend Confidence: Slightly above average Timing and latitudinal differences show up with the northern portion of a mid-level trough amplifying through the central to eastern U.S. Monday night. The 12Z NAM and 12Z CMC are slower while the 12Z UKMET is displaced slightly northward with the vorticity maxima compared to the ensemble means and remaining deterministic guidance. The 12Z GFS and 12Z ECMWF are thought to be closer to the middle of the model spread and are preferred at this time, with the 12Z ECMWF shifting a little north of its previous 00Z cycle. ...Southern stream trough/low near the TN Valley by Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM and 12Z ECMWF Confidence: Average The southern portion of a broadening mid-level trough over the eastern two thirds of the nation Monday night into Tuesday is expected to coincide with a weak surface low to cross the Tennessee Valley. The 12Z GFS becomes significantly faster by Tuesday morning while the 12Z CMC shows slowest of the deterministic guidance. The 12Z NAM and 12Z ECMWF show closest to the ensemble means but the 12Z UKMET adjusted a bit faster with its 500 mb trough taking its surface low away from the favored consensus. ...Shortwave/fronts approaching the Pacific Northwest Tue... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non 12Z NAM blend Confidence: Average Moderate spread continues to exist with a shortwave trough to approach the Pacific Northwest late Monday into Tuesday. A warm front is forecast to reach western British Columbia and coastal Washington Monday morning as a low nears just north of Vancouver Island. There are some minor placement differences with the surface low, the 12Z UKMET is toward the southern side of the deterministic spread while the 12Z NAM is farther north. A blend toward the middle (less 12Z UKMET influence) will align best with the latest ensemble scatter low plots. However, larger differences show up due to the approaching shortwave to the west. With this feature, the 00Z ECMWF was faster but the 12Z ECMWF slowed down. The 12Z NAM was weakest with the trough and considered the least likely to verify. Timing adjustments from the 12Z ECMWF/UKMET/CMC compared with their previous 00Z cycles allow for their inclusion as part of the preference, excluding the 12Z NAM. Of the non 12Z NAM guidance, the 12Z CMC was slowest, but just within tolerance given the latest ensemble spread. Therefore, a non 12Z NAM blend will be preferred with this system given differences from the 12Z UKMET's surface low influence is felt farther north away from Washington. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto