Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Fri Mar 20 2020 Valid Mar 20/1200 UTC thru Mar 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00z Model Evaluation with Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Exiting Northeast Low pressure and associated front today... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The surface low and attendant cold front progressing through the Lower Great Lakes continues to progress eastward with high model agreement and solid run to run continuity to have above average confidence in a general model blend including the lingering frontal zone through the Carolinas. ...Phasing streams across Plains Sunday,broadens from Great Lakes to South Sun-Mon; East Coast late Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend (weight to 12z GFS/00z ECMWF and ECENS/GEFS means) Confidence: Slightly below average A complex evolution is starting to take shape with shortwave energy from northern and southern streams as well as multiple streams of moisture from the subtropical Pacific and the Gulf of Mexico will generally phase from the Great Lakes to the South. However, these are the low confidence evolution situations where small differences in timing, strength of either stream or increased latent heat release supporting under-forecast upscale enhancement to the upper jet structure/pattern leads to small but significant run to run adjustments. Cycles of the ECMWF have been trending a bit faster with the southern stream shortwave emerging from the southwest Sunday, but still generally lags the bulk of ECENS members and therefore the ECENS mean. However, it is paired with the 00z CMC suggesting the strength of the southern stream wave is associated with the Great Basin closed low inner core, rather than the fast moving undercutting shortwave currently well SW of Central CA (per Goes-W). The GFS/NAM and UKMET all suggest the undercutting wave is the dominant and is faster. This allows for greater phasing with the northern stream as it amplifies through the Great Lakes Monday; though the UKMET is slightly faster in to be less phased with increased binary interaction and northward drift to the developing coastal low at 00z Tuesday (again relative to other guidance). The 12z NAM unlike the GFS/UKMET shows typical late day 3 over-amplification bias, with increased QPF/latent heat release building a stronger anticyclonically curved 250mb jet and stronger northward surface development. Given alignment with typical bias, believe the NAM is untenable in the blend even as early as 22/18z. So will support a non-NAM blend at this time, trending toward higher weighting of the central solutions of the 12z GFS and 00z ECENS mean by 00z Tuesday along the east coast. As stated before, all guidance will have a tendency to struggle with phasing/upscale enhancement in transition season...so while the clustering is fairly tight confidence is slightly below average in a non-NAM blend. ...Closed mid level low approaching Central California Sunday, shearing into Four Corners late Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 23.00z; non-CMC thereafter Confidence: Slightly above average becoming average Strong northern stream closed low between 130-140W will continue to dig southward under influence of the larger scale gyre centered off the Central CA coast at this time. As the Great Basin and faster preceding shortwave either shear or accelerate eastward, respectfully, this closed low will continue to start to press eastward early Sunday promoting increased moisture flux into Southern CA eventually overspreading into the Great Basin Monday, with typical timing differences starting to manifest: GFS/NAM and UKMET shearing quicker and ECMWF/CMC slower. While there becomes greater spread on late Monday, only the CMC looks too slow to the overall mass field spread... so will suggest a general model blend through 23.00z and non-CMC thereafter with confidence slightly above average becoming average. ...Amplifying northern stream shortwave into closed low entering Pac NW Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Above average Next well define compact closed low rolls down the BC coast Sun into Monday, eventually spreading moisture across the Pacific NW by Monday. The evolution within the overall model suite looks fairly strong/well agreed upon, though Monday. Though the 12z NAM starts increasing depth relative to the remaining suite but atypical of this evolution, slides a bit east of the otherwise well clustered solutions. As such a Non-NAM blend is supported at above average confidence for Day 3 mass field solution. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina