Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 242 PM EDT Wed Sep 04 2019 Valid Sep 04/1200 UTC thru Sep 08/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation, Confidence, and Preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall Pattern Across the CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Exception: 12z GFS/ECMWF Northwest and Northern High Plains into Day 3. Confidence: Average to slightly above average 19z update: Likely, with better data regarding the upstream shortwave coming out of the Pacific Northwest and the nose of the jet across BC, the evolution of the northern stream appears to be coming into much stronger agreement, with timing, shape and only minor differences in strength of the jet core. So a general model blend can be supported for much of the Northern Tier/Great Lakes and Northeast. This also appears to positively affect differences with Dorian as well (see below). As for the west, there remains significant model spread with the high-latitudinal sharp trof approaching the West Coast at day three. The 12z UKMET looks similar to the 00z CMC which remained unflavored but the 12z CMC remains broad and slightly faster than the NAM/GFS/ECMWF which continue to show best agreement. As for the downstream wave, the affects of spread in the upstream are reduced, however, the ECMWF is much weaker coming ashore and looks odd compared to prior runs, this does not affect its timing or moisture transport to initiate convection/QPF across the High Plains on Day 3, except it may be a bit further south overall. The UKMET is better and may be included, but the CMC remains a bit off. As such will continue to prefer the 12z GFS/ECMWF for this wave. ---Prior Discussion--- The shortwave, increasingly positive tilt trof, crosses New England today. As such, the overall gradient tightens and a coupled polar/subtropical jet core develops over the Northeast, fed by the stronger Polar but also the approach/outflow for Dorian. The difference in guidance here, is mainly in the placement and orientation of the southern branch across the Mid-Atlantic and even into the eastern slopes of the Appalachians. Stronger, better anticyclonically curved entrances to the jet continue with the 12z NAM/NAM CONEST supporting stronger draw of the QPF swath to the north and west across the Coastal NC/SE VA. the 12z GFS follows this, but continues to vary over the last 4-5 cycles on strength/placement and so the tight QPF gradient varies, but has sped up compared to the 00z run and is ever so slightly southeast of the 06z run. Upstream, guidance is coming into better alignment in strength and timing of the preceding shortwave as well as the nose of the developing northwest flow/Pacific Jet that enters the Great Lakes 00z Fri. the solid timing/shape also provides a similar evolution of the surface wave and QPF forecast through the northern tier into the Great Lakes. However, afterward, the UKMET is a bit flatter and weaker as it digs through the Great Lakes and presses Dorian off the coast. While the slightly more amplified and stronger solutions of the GFS/ECMWF and CMC phase with the circulation of Dorian. Thereafter the shortwave/left rotor of at the tail of the jet streak, will amplify and dig southeast into Quebec and turn Dorian left toward Nova Scotia and the UKMET is weak here as well...so start to shift preference away from the UKMET after 06/00z at slightly above average confidence. Broad closed low off the West Coast, will continue to lift northeast into the Pacific Northwest being kicked by a stronger/full latitude closed low/trof over Gulf of AK/Northern Pacific. As it lifts it will weaken into the larger scale ridge in the Rockies but maintain sufficiently to emerge into the Northern High Plains by early Sat, with surface wave in WY. This surface wave will weaken from the strong northern High building in. Here the UKMET and NAM are slightly too fast and become flat in the wake of increased troughing downstream. The 12z GFS slowed a bit, and is timed well with the CMC/ECMWF. However, the CMC is very out of phase with the upstream trof elongating along the coast by day 3 and has some subtle issues with wave crossing the Rockies as well. So given strong agreement/placement and timing think a blend of the 12z GFS and 00z ECMWF will work best for the Northern High Plains, Northern Rockies, Great Basin and Pacific Northwest with these two systems. The NAM and UKMET are not too far out of phase so confidence in a GFS/ECWMF blend is average to slightly above average. ...Hurricane Dorian... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Please refer to the official National Hurricane Center forecast track 19z update: ---Prior Discussion--- For the next day or so, the guidance is well packed around the official 15z forecast, but by 36hrs the 12z NAM and GFS start picking up pace and slide a shade north and east of the ECWMF/CMC and UKMET. As it parallels the Carolina coast late Thurs into Fri, the GFS and NAM continue to lift a bit faster than the remaining guidance also with greater baroclinic support (jet entrance see first section), QPF axis is north and west. As Dorian, accelerates up the coast, there is a general left turn in the guidance toward Nova Scotia given stronger binary interaction with northern stream global trof digging into NW Quebec from James Bay. Only the UKMET deviates well east, also the slower guidance does eventually catch up with the NAM/GFS. Overall, the best proxy to the 15z forecast continues to be the GFS, with the 12z GFS even better aligned than the 06z run. ...Tropical Storm Fernand.... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Please refer to the official National Hurricane Center forecast track 19z update: ---Prior Discussion--- The mid-level center of Fernand is tracking fairly well and tight through the Mexican Plateau with solid model agreement for the remainder of the official NHC track. The surface wave is currently along the coast of NE Mexico and likely to swirl into extinction as the mid-level circulation moves away. Best Proxy to the official NHC forecast is close to a general model blend through tomorrow...though both the 12z GFS and 00z ECMWF and to a lesser extent the 12z NAM show a strong remaining mid-level shortwave that will cross S Sonora/N Sinaloa under the building ridge in the US Southwest. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Gallina