Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1157 AM EST Mon Feb 17 2020 Valid Feb 17/1200 UTC thru Feb 21/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Pair of shortwaves crossing the Great Lakes/Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average A shortwave and associated surface low will cross the Great Lakes region on Tuesday and then advance east-northeast and clip portions of northern New England through early Wednesday. In the wake of this initial shortwave, there will be another northern stream shortwave that will follow with about 12 hours worth of separation. All of this energy by late Wednesday will be exiting northern New England and lifting through eastern Canada. The models at this point show minimal mass field spread at the surface and aloft, so a general model blend will be preferred with both shortwaves. ...Broad longwave trough over the Plains/Midwest Wednesday... ...Energy reaching the Great Lakes/Northeast by Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Average As a transient deep layer ridge moves into southwest Canada and across the Pacific Northwest by Wednesday, there will be a positively tilted and broad longwave trough downstream that will cross the northern Plains and a large area of the Midwest. By Thursday, this troughing will cross the Great Lakes and Northeast. The 00Z CMC becomes a bit of a slower and deeper outlier with the trough as it moves into the eastern U.S. by Thursday. The 00Z ECMWF and 00Z ECENS mean are also somewhat on the slower side of the guidance, but for now, will suggest a non-CMC blend given that there is some support for the slower ECMWF-led guidance from the 06Z GEFS ensemble suite. ...Lingering energy over the central Rockies/High Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS and 00Z ECMWF Confidence: Average The guidance suggests a trailing tail of energy associated with the aforementioned longwave trough becoming detached and lingering over the central Rockies and High Plains late Wednesday into Thursday. The 12Z GFS, 00Z ECMWF and 00Z CMC agree in taking this energy more progressively downstream. However, the 12Z NAM and 00Z UKMET hold back a considerable amount of energy by comparison. On Wednesday, the NAM is the strongest solution with its energy over WY/CO, but then by Thursday the UKMET suggests a closed low pushing east across SD/NE. The better ensemble support and deterministic clustering would favor a solution toward the GFS and ECMWF, so a blend of those two solutions will be preferred. ...Shortwave clipping the Pacific Northwest Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM and 00Z ECMWF Confidence: Below average A northern stream shortwave may clip the Pacific Northwest on Thursday as the pattern transitions to very strong Pacific flow crossing from the Gulf of AK through much of western and central Canada. The guidance is a bit split on whether this shortwave will separate out from the strong northern stream flow. The 12Z GFS and 00Z CMC favor the energy digging south offshore the Pacific Northwest and attempting to close off. The 12Z NAM and especially the 00Z UKMET are more progressive and show the system remaining more closely dominated by the strong Pacific flow farther north. The 00Z ECMWF generally splits the difference between the slower and faster camps. The ensemble means are generally split as well, with the GEFS slower and more offshore with the energy, and the ECENS suite faster and close to a blend of the NAM and ECMWF. The 00Z CMCE suite is also very close to this camp as well, so a blend of the NAM and ECMWF will be preferred. Confidence is limited given the collective mass field spread with the guidance. ...Upper trough/closed low offshore California by Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly above average The models show a strong split in the upper flow pattern by Thursday with a substantial southern stream trough and closed low focusing offshore of California as strong northern stream Pacific flow advances through western Canada. Overall, there is decent model agreement with the southern stream energy as all of the models show a negative height anomaly associated with a closed low just offshore of central California by the end of the period. The 00Z CMC is a bit farther east with the placement of the closed low and is generally an outlier compared to the suite of deterministic and ensemble guidance. So, a non-CMC blend will be preferred with this southern stream troughing on Thursday. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison