Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 251 PM EDT Sun Sep 16 2018 Valid Sep 16/1200 UTC thru Sep 20/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation with preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwaves along the US/Canadian border/Florence moving into the Mid-Atlantic/Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM/ECMWF/UKMET blend Confidence: Slightly above average 07z update: The 12z ECMWF trended a bit deeper and slower compared to the prior run showing a similar but still faster solution than the 12z GFS. While, the CMC still remains the weakest through the Northeast, there is closer clustering overall to support a general model blend or for a "cleaner" tighter blend continue with a 12z NAM/ECMWF/UKMET blend Current setup depicts compact wave skirting the Central US/Canadian border with associated surface wave in NDAK, slightly attached to a weak but shearing vort center in the lower MO River Valley.. The main upstream core of the wave is translating through the southern Canadian Rockies. The lead shortwave and surface low quickly shift eastward across SE Canada... the 00z UKMET is generally a bit north, deeper and therefore slower to the main consensus but does not appear to have much affect to the sensible weather. The main wave will also advance and over-top the ridging in front of the remnants of Florence over the Ohio Valley, and will help to elongate/stretch the mid-level of Florence across the Mid-Atlantic and Northeast. There is strong agreement with the placement of best moisture convergence/flux across the Northeast to have good confidence in a blend for QPF, but the mass fields are a bit more complicated as the 12z GFS has slowed relative the lead wave and loads the bulk of the sheared shortwave energy in the Mid-Atlantic (eventually spurring a surface wave south of Long Island. While plausible, this even even slower and south of the 00z ECMWF to have lower confidence in it compared to the remaining guidance. The CMC is the key outlier though as it is very quick to the lift out the wave, and is very flat/weak with the expected stronger, northern stream wave. As such would favor something closer to the 12z NAM and 00z ECMWF/UKMET for these 3 systems. ...Southwest US/Tropical EPAC... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average 07z update: While trends continue to pull a tropical system northward toward Baja California with moderate spread in timing/track, the NAM continues to be the main outlier, including the evolution of the moisture into the Southwest. A more typical alignment of timing of the features/moisture into the SW evolves with the GFS/UKMET a bit faster and the ECMWF/CMC a bit slower...a split in timing provides modest confidence in a non-NAM blend remaining preference. The 12z NAM is a clear outlier, especially in the Southwest where it is the only guidance member to develop and rapidly deepen an EPAC cyclone. While not affecting the CONUS by Day 3, directly, the 12z NAM takes the shortwave over the Big Bend of TX and the TC and tightens a stronger gradient flow and moisture flux into the Southwest. Here, a slower more subtle enhancement of the monsoon is preferred in a non-NAM blend. ...Longwave trof over West/Intermountain West... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly below average 07z update: Both the UKMET and ECMWF trended a bit slower and more amplified in the middle stream of the large scale trof, more in line with the 12z GFS, with the ECMWF amplifying the middle stream over the southern wave. There remains some timing differences in the northern stream, but with the ECMWF trending faster, it is catching up to the CMC/GFS, initial preference. The 12z CMC shows similar evolution as the 00z run to suggest a non-NAM blend. The uncertainty to the inner structure of the three "streams"/shortwave to the larger scale wave continues to bode a low, but slightly increased, confidence (slightly below average) in the blend forecast. The West Coast/Intermountain West will be dealing with a transitional period, with a complicated larger scale trof advancing ashore late Mon into Tuesday. The large scale trof is a combination a combination of three streams/shortwave features, and each model supports an evolution slightly different. While the larger scale trof is generally agreed upon, the internal features lead to low confidence preferences. The northern stream, just north of the Canadian border is generally a zonal/sheared axis between the Arctic cold/jet streak shifting over Central Canada; embedded are small compact waves, with a consolidating closed low feature off Vancouver island by Wed/Thurs. The mid-stream is favored by the 12z GFS/00z CMC which are generally slower across OR/N CA into the Snake River Valley/WY Rockies... compared to a faster but similar depth UKMET/NAM. The 00z ECMWF is very weak through this portion of the larger scale wave, favoring a much stronger base through S CA. The orientation of the waves internally leads to a flattening/negative tilt with the NAM/UKMET which is not favored in the longer term cycle to cycle ensemble spaghetti analysis, a more latitudinal stacked/phased wave with the GFS/CMC and a flat washed out wave in the ECMWF. The ECMWF, is generally faster and much more flat than the bulk of 00z ECENS members, suggesting a bit more uncertainty and lower preference. As such a 12z GFS and 00z CMC/ECENS mean solution is favored in the West/Rockies but a low confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina