Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Wed Mar 20 2019 Valid Mar 20/1200 UTC thru Mar 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Trough(s) and Developing Low in the East... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend (weight to ECMWF/UKMET) Confidence: Average 19z update: The 12z UKMET shifted faster (having weakened the first digging shortwave through the Great Lakes) and has come into much better agreement with the initial preference. The 12z CMC while trending west with the surface cyclone, it remains well east of the cluster and is shunted northeast faster as well by Sat. The 12z ECMWF trended stronger and as such favored a slightly eastward surface cyclone track...keeping it quite close to the initial preference. The tightening agreement of the UKMET with the ECMWF provides slightly increased confidence in a non-CMC blend though weighted more ECMWF/UKMET over the GFS followed by the lowest weighting in the NAM. ---Prior Discussion--- The synoptic variability is starting to reduce given the main pieces of energy that combine to develop the East Coast cyclone in the late week have reached the more data rich network for assimilation. Complexity is driven by two phasing events: (1) on Thursday with a southern stream shortwave and another wave digging sharply through the Great Lakes, and (2) on Friday when more substantial cyclogenesis will begin over the Northeast. At that point the initial shortwave will be ejecting in a negatively tilted fashion up the coast and will phase with yet another sharply digging shortwave from the Great Lakes. The 00z CMC is the lone deterministic guidance member (along with a bulk of CMCE members) that is slow with the northern stream digging wave through the Great Lakes, allowing for the southern stream wave to not shear and support surface cyclogenesis well off shore, starting it off away from the trends/growing consensus. The 00z UKMET, while more like the remaining guidance, is also very strong with the northern stream digging, delaying the deepening upper low's track and sfc development well south of the overall cluster. As the upstream shortwave digging across the Great Lakes the southern wave is kicked a bit further east and as they both phase across New England into Saturday, the UKMET is well east and then shifts northward. Ensemble solutions from the GEFS/ECENS mean have been coming into better agreement with traditional biases manifesting in the day 2/3 time period, ie GEFS/GFS fast, and the ECMWF/ECENS mean slow. The 00z ECMWF however, continued the overall EC trend faster, but has sped up to be faster than the bulk of ECENS members and is about mid-point to the the 06z GEFS mean and 00z ECENS mean...which may be a good thing in a blend that has issues mainly with timing. The 12z NAM/GFS and 00z ECMWF, depict a very similar evolution to both phasing systems. However, perhaps the ECMWF a bit stronger/concentric with the lead wave (surface cyclone developing wave), reducing shearing and leading to a broader weaker upper low on Sat into Sun relative to the NAM/GFS. The 12z NAM continues to be very strong and deep but also tad faster ejecting Northeast by the end of day 3, which seems less likely given the magnitude of expected binary interaction (ie, synoptic wobble/surface redevelopment expected in New England/Gulf of Maine). All in all, a blend of the 12z NAM/GFS and 00z ECMWF blend is preferred but weighting toward the ECMWF is suggested. Confidence is average. ...Remainder of the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 60hrs (23/00z) Trending toward 12z UKMET by 84hrs (24/00z) Confidence: Above Average up to 23/00z, average after 19z update: The 12z ECMWF/CMC both trended a bit slower but more importantly more weighting toward the base of the elongating trof similar to the 12z NAM across the Plains on day 3. The UKMET still has a stronger small scale vort center but has shifted a bit faster than the GFS, putting the trof axis in line with the NAM/ECMWF/CMC and leaving the GFS uncharacteristically slow. The timing/orientation of the base of the trof through the Southern Plains provides the most uncertainty while the remainder of the trof and coastal shortwave are agreeable enough to support a general model blend though would favor something close to the UKMET further south across OK/TX (esp. QPF). Confidence remains average after 23/00z. ---Prior Discussion--- A strong southeast to northwest ridge dominates the Plains early in the forecast period. The elongated trof along the West Coast will split, sending a bulk of energy into the Southwest consolidating into a closed low over UT by Fri with very strong model agreement. Model spread starts to increase as the wave reaches the weakening ridge as well as, the spine of the Central Rockies. Additionally a secondary mid-range wavelength trof enters the Pacific Coast more toward the Northwest. The combination of all 3 factors, continues to stretch the wave from the Southern Plains to Snake River Plain, with vortical stretching in Eastern CO/NM. The 12z NAM and 00z ECMWF are a bit more progressive and favor weakening the wave northward and back toward WY, falling into line with the last few runs of the ensemble suite. However, the trend has been toward slowing but also depicting a highly elongated trof. The 00z UKMET and 12z GFS both trended a bit slower supporting a a compact shortwave at the highest diffluence to the more W-E 250mb jet. This helps to tap increased moisture/instability out of the western Gulf. In reality the spread is small compared to average/normal but there is modest spread in the QPF axis/magnitude. So while, not highly confident, will follow the overall ensemble trends and favor a 12z GFS/00z UKMET blend at average confidence. This will also suffice for the blend of the second broader coastal trof (which is a bit further north) compared to the ECMWF/NAM. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina