Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1125 AM EST Wed Dec 18 2019 Valid Dec 18/1200 UTC thru Dec 22/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preference and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed mid-level low crossing New England on Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models are in very good agreement with this system, so a general model blend will be preferred. ...Southern stream trough crossing the Southwest through Thurs... ...Crossing the southern Plains/Gulf Coast states Fri/Sat... ...Cyclogenesis over the Gulf of Mexico... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z ECMWF/ECENS mean and 06Z GEFS mean Confidence: Average A southern stream trough advancing into central CA today will advance east-southeast and cross the Southwest through Thursday. The energy is then expected to cross the southern Plains and Gulf Coast states through Saturday while driving cyclogenesis over the central Gulf of Mexico. The guidance is in rather good agreement at least mass field wise with the system through 36 hours, however, thereafter the 12Z NAM begins to drop the energy farther south and to the south of the global model consensus. By the end the period, the NAM suggests a closed mid-level low over the northwest Gulf of Mexico which is well south of the global model ensemble suites. The NAM reflects more backing of the mid/upper-level flow downstream over the Gulf Coast which drives its Gulf of Mexico surface cyclogenesis farther north than the global models and thus a wetter solution with stronger moisture transport as a result. Overall, the 12Z GFS appears to be a tad too progressive as the energy begins to cross the Gulf Coast states. The 00Z ECMWF/ECENS mean and 06Z GEFS mean are all slower than the GFS. The 00Z CMC becomes gradually the slowest solution. Meanwhile, the 00Z UKMET mass fields by the end of the period are out of tolerance with all of the models with its depiction of a closed mid-level low over the TN Valley. A general model blend should suffice through 36 hours, but thereafter, a blend of the ECMWF/ECENS mean and GEFS mean will be preferred. ...Northern stream trough crossing the northern Plains/Midwest... ...Energy arriving across the Great Lakes/Northeast by Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Slightly above average A northern stream shortwave ejecting across the Pacific Northwest and the northern Rockies through Thursday will advance downstream across the northern Plains and Midwest through Friday. Thereafter the energy will progressively cross the Great Lakes and Northeast. Model spread is minimal with this system, but by the end of the period, the 00Z UKMET was seen as being a little slower than the remaining guidance. A non-UKMET blend will be preferred. ...Deep upper trough offshore the West Coast... ...Atmospheric river impacting the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend...through 60 hours 00Z ECMWF/ECENS and 06Z GEFS mean...after 60 hours Confidence: Slightly above average Onshore flow into the Pacific Northwest on Thursday begins to alter as a large mid-upper level trough upstream over the eastern Pacific begins to amplify and encroach on the West Coast. Persistent low-level southwest flow will be impacting the coastal ranges of Washington/Oregon with a tendency for the flow to gradually back in nature with time as the trough amplifies, but also as multiple waves of low pressure ride northeast up along an attendant front. One frontal wave will approach Vancouver Island by Thursday evening, with a second 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 details of these waves, but there is excellent agreement out of the guidance in bringing some very strong onshore flow into the coastal ranges as a southwest low-level jet of as much as 70+ kts is expected to arriving just ahead of the front and these surface waves Thursday night and 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 00Z UKMET Saturday appears to be a little too slow with the wave activity and it associated frontal timing, whereas the 12Z GFS may be a tad too fast. A general model blend should suffice with the details of all of this energy going through about 60 hours, but thereafter, a blend of the 00Z ECMWF, and the ensemble means (00Z ECENS mean and 06Z GEFS mean) will be preferred given better overall clustering. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison