Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1145 AM EST Thu Dec 19 2019 Valid Dec 19/1200 UTC thru Dec 23/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preference and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Southern stream trough crossing the Southwest today... ...Crossing the southern Plains/Gulf Coast states Fri/Sat... ...Cyclogenesis over the Gulf of Mexico... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM/00Z UKMET and 00Z ECMWF Confidence: Average A southern stream trough advancing across the Southwest today will cross the southern Plains and Gulf Coast states through Saturday while driving cyclogenesis over the central Gulf of Mexico. This system will move into the Southeast on Sunday. Generally speaking, and fitting in with its bias, the 12Z GFS and latest GEFS member suite again appear to be too progressive with the evolution of this system as it impacts the Gulf Coast and Southeast going through the weekend. The 00Z CMC is also an outlier with its closed low placement much farther north and east of the model consensus by the end of the period. The better ensemble (00Z ECENS suite) and deterministic clustering suggests a slower solution that is close to a blend of the 12Z NAM, 00Z UKMET and 00Z ECMWF at this point, and so a blend of these slower solutions will be preferred. ...Northern stream trough crossing the northern Plains/Midwest... ...Energy arriving across the Great Lakes/Northeast by Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Above average A northern stream shortwave will advance downstream across the northern Plains and Midwest through Friday. Thereafter the energy will progressively cross the Great Lakes and Northeast. The 12Z NAM appears to be a bit too slow and too deep as the system arrives across the Northeast, and so a non-NAM blend will be preferred with this system. ...Deep upper trough offshore the West Coast... ...Atmospheric river impacting the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z UKMET/ECMWF and 00Z ECENS/06Z GEFS mean blend Confidence: Slightly above average Persistent low-level southwest flow will be impacting the coastal ranges of Washington/Oregon as multiple waves of low pressure ride northeast up along an offshore frontal zone associated with an amplifying upper trough. One frontal wave will approach Vancouver Island by late this afternoon, with a more subtle wave approaching by Friday morning a bit farther south down toward coastal areas of northwest Oregon and southwest Washington. There is very modest spread with the timing/depth details of these waves, but regardless, the models all agree on bringing very strong onshore flow/atmospheric river conditions into the coastal ranges as a southwest low-level jet of as much as 70+ kts is expected to be arriving just ahead of the front and these surface waves going through Friday morning. Additional wave activity is forecast to lift up along the front Friday night and Saturday as the front settles slowly southeastward and begins to edge inland farther south down across northwest California. The models all bring the upstream trough bodily inland across the West Coast by Sunday. The 12Z GFS and perhaps to some extent the 12Z NAM appear to be a tad too progressive with the energy advancing inland (especially relative to California). Meanwhile, the 00Z CMC is likely a little too slow. The better model clustering would tend to favor the 00Z UKMET/ECMWF solutions which have good support from the 00Z ECENS mean and 06Z GEFS mean, and so a blend of these solutions will be preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison