Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 224 PM EDT Fri Sep 13 2019 Valid Sep 13/1200 UTC thru Sep 17/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...CONUS... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Exceptions: Non-UKMET in Pacific Northwest 12z GFS/ECWMF blend in Western/Central Gulf Coast Confidence: Above average 19z update: The 12z UKMET appears to have slowed a bit in the Pacific Northwest through Day 2, but still jumps a slight east of the increasing clustering (though ECMWF/CMC are a bit further south and slower than the GFS). Overall, a non-UKMET blend remains in place in this region. Elsewhere, similar evolution with the 12z UKMET/CMC/ECWMF/GEFS to support initial preference (with 12z ECMWF included along Western Gulf coast over the 00z run). ---Prior Discussion--- For the CONUS, the broad cyclone across the Upper Great Lakes continues to shift northeast into Canada with little model variation, the upstream shortwave tracking along the US/Canada boarder has come into good agreement as well, with some small depth, minor timing differences as it broadens the larger scale trof (with the precursory system) crossing New England into Sunday. Again, here the guidance is solid enough for a general model blend at above average confidence. Further west, the Gulf of Alaska broad closed low swings from a latitudinal orientation toward a more north-south one, stretching with a base shortwave entering N CA toward late Sunday early Monday. Only the UKMET being initially fast, favoring the northern portion of the trof offsets against the other guidance, including ensemble suite. This looks fairly straight forward with some continuity through the recent ensemble trends, so above average confidence can be had with this non-UKMET blend in the West. Convective complex over the Northern Sierra Madre Occidental on Friday night into Saturday, lifts slightly north as the elongated subtropical ridge at or along 32N breaks down. The 12z GFS continues to be the most robust with the wave, but by Sunday, other guidance supports the eventual upscale growth across the Southwest. While, the GFS will remain stronger/deeper through the forecast period, but given the approaching western trof, it also lifts a bit faster north with greater moisture flux into the Great Basin and maybe even the eastern Snake River Plain by late in the forecast period. Given the GFS is slightly faster with the western trof (in a typical timing difference with the ECMWF/CMC) and supported by the NAM; WPC will employ a typical blend to slow the GFS and speed up the ECMWF/CMC. While the UKMET, has issues in the northern stream, this wave is well timed and has similar strength to the ECWMF...to have some confidence in using it in the SW. ...Potential Tropical Cyclone Nine... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Official NHC Forecast Best Proxy: Non-NAM or 00z UKMET/ECMWF thru 15.00z, 12z CMC/GFS after Confidence: Below average 19z update: The 12z UKMET and ECMWF are steadfast waiting for convection to spur the surface wave, while convection is organizing better now, the delay in development slows the track north and turns back toward the SE further away from the official forecast track. The 12z CMC is on the flip-side not really developing the surface reflection until 15.00z NW of Grand Bahama and takes it north and along the official track the best, but is very weak overall, even compared to the GFS. So there is no best proxy to the NHC forecast, though it gets closest to the 12z CMC after 15.12z. ---Prior Discussion--- The appearance of PTC Nine continues to be highly tilted with a developing mid-level circulation further east of the Bahamian outer chain. The 00z UKMET/ECMWF continue to latch onto this MLC and utilize convective processes to stretch to the surface. This is initially in line with the 15z official NHC forecast. The 00z CMC continues to suggest the apex of the surface wave/surface low will track through the Central Bahamas closer to FL coast remaining weak. The 12z GFS shifted east from prior runs, and looks similar to the CMC...both west and further from the NHC forecast at least initially (through 15.12z) The 12z NAM, continues to be the most vertically titled, and shifted west through the southern FL peninsula well away from the official NHC forecast and is disregarded as a proxy solution. By 15.12z, northern influences from the lingering frontal zone come into play and the CMC/GFS both intensify from some baroclinic influences and while still left of the official track become much closer to it after 15.12z eventually turning eastward...with the GFS a tad stronger and faster than the CMC. The UKMET, ECMWF both start much deeper and more concentric...which given the shear environment would suggest a strong convective core. Given upwelling from Dorian, there is sufficient heat but may not be enough to be as strong as these guidance members require. So, a non-NAM blend is best proxy overall, but starting with the 00z ECMWF/UKMET until about 16.00z then shifting toward a 00z CMC/12z GFS afterward is likely to be a stronger proxy to the official forecast. Confidence remains slightly below average in this proxy to the NHC forecast track given the broad spread and contingency on persistent convective processes to develop the surface low. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Gallina