Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1242 PM EDT Sat Sep 29 2018 Valid Sep 29/1200 UTC thru Oct 03/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Central and Eastern US... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS and 00z ECMWF/UKMET blend Confidence: Slightly above average Today into tomorrow, a fast moving shortwave and associated frontal zone will track across the northern tier before lifting across the Great Lakes leaving a draped frontal zone into the central and eastern lakes by Monday. It is around this time, that strong southwesterly quasi-zonal flow will isentropic ascend over the boundary ahead of the sheared remnants of the closed low currently entering N CA. This wave and associated jet streams will support a weak surface wave out of the Central Plains into the Great Lakes. The 00z CMC is a tad slow but also a tad too amplified into strong confluent flow to suggest its removal. Likewise, the 12z NAM is slow to shear/tamp down the wave, delaying its eastward progress. While both track very nicely with the remaining guidance, believe the 12z GFS and 00z ECMWF/UKMET appear most sensible given the overall tightly packed mid-level flow regime/pattern. Confidence is fairly strong in this evolution/blend given the run to run consistency and agreement in the ensembles to this GFS/ECMWF/UKMET blend ...Gulf Coast/Southeast.... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average South of the confluent stream, an elongated subtropical ridge remains over the Gulf into the Sargasso Sea... the lingering frontal zone that exists currently will remain across the region likely reinforced by natural airmass building (mT) in the Gulf...keeping the treat for coastal showers/thunderstorms annoyingly persistent but difficult to lock down precisely synoptically-speaking, in the day to day forecast. As such a general model blend would work in this region, but due to the storm scale nature, confidence is average (especially in the QPF fields). ...Western US/Northwest/Northern Rockies/Northeastern Pacific... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS and 00z ECMWF/CMC blend Confidence: Slightly below average A complex evolution will occur across the Eastern Pacific and Pacific Northwest with subtle timing differences leading to larger model spread and different evolutionary paths mainly by early Monday into Tuesday. The Rex block is nearly broken down and the closed low is advancing through N CA...this is fairly well agreed upon through the West and can be confidently blended with nearly any solution. Strong cross polar cold air continues to reinforce across North Central Canada, but along the Pacific coast and strong shortwave will descend and broaden across Vancouver Island by late Sunday into Monday...and at this point, there is good agreement. However, as the north Central Pacific compact closed low treks eastward to around 135W, models start to diverge on the magnitude and timing of binary interaction. The clear outlier remains the 00z UKMET, as it presents zero interaction and the broad shortwave in the NW translates eastward with a much faster/stronger upstream shortwave digging in its place by Tuesday. Most other guidance, supports this upstream wave, but not this aggressively; and so the UKMET is not preferred in any percentage in the blend. The other traditional fast guidance...the GFS and NAM, send this wave southward and show a binary iteration near 38N 130W on Tuesday. The 12z NAM shows the furthest interaction, leading to a broader dual spoke closed low. The 06z GFS showed a similar wider interaction but the 12z GFS trended a bit slower with the northern wave and evolves into a much tighter combination. The traditionally slower solutions also present a tighter combination of the two systems, though the ECMWF is much slower than the CMC and 12z GFS. Still, overall trends would suggest this slower evolution/tighter binary interaction to manifest...and so the preference is a 12z GFS and 00z ECMWF/CMC blend. Confidence is slightly below average given the small timing differences lead to larger spread. ...Tropical Cyclone Rosa (Southwest US)... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ See NHC forecast Best Proxy: 06z GFS or GEFS and 00z ECMWF/UKMET blend The 06z GFS indicated a bit more westerly shear and displaced the surface wave a but east of the official 15z forecast but by 84hrs it moves back west to be almost identical to the NHC forecast. The 06z GEFS mean is very similar to the operational 06z GFS but perhaps a bit further west, closer to official forecast...as such either the GFS or GEFS solutions are good potential proxies. The 00z UKMET/ECMWF follow the official track quite well but around 48hr start to lag, eventually being about 6hrs slower than the official forecast by 84hrs. The 12z GFS and 12z NAM both trended slower to match the timing of the UKMET/ECMWF but both eventually are further east of the official track. As such a blend of the 06z GFS or GEFS with the 00z ECMWF/UKMET seems to present the best proxy to the official forecast. The 00z CMC, tracks well but remains the overall weakest solution nearly sheared out before reaching Baja California. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina