Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1217 PM EDT Mon May 18 2020 Valid May 18/1200 UTC thru May 22/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tropical Storm Arthur ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: National Hurricane Center official track Best Model Proxy: 12Z GFS/00Z ECMWF blend Confidence: Slightly above average Tropical Storm Arthur as of 15Z is moving north-northeast and starting to pull away from the NC Outer Banks. Over the next 24 to 36 hours, the system is forecast to gradually turn off to the east and away from the East Coast. The 12Z NAM tends to be a tad slower than the global model consensus in advancing Arthur out to sea, with the 00Z UKMET the fastest of all the deterministic models. The better model clustering and consensus favors the 12Z GFS and 00Z ECMWF which as a blend is the best proxy to the latest NHC track. The 00Z CMC for its part does align itself well track wise with the GFS/ECMWF camp, but is seen as being a weak outlier. Please consult the latest NHC Forecast Advisories for more information on Arthur. Consolidation of Midwest/Gulf Coast troughs into cut-off upper low ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend...through 60 hours 06Z GEFS/00Z ECENS mean blend...after 60 hours Confidence: Slightly above average A pair of upper troughs and associated closed lows over the central U.S., one over Illinois and the other entering Alabama, will consolidate over the next day into a large cut-off low that settles over the Tennessee Valley Tuesday and Wednesday before then gradually weakening and lifting northward back into the Ohio Valley by Thursday. Model spread is minimal through Wednesday (60 hours), but by Thursday, the 12Z NAM and 12Z GFS are faster to lift the energy back north into the OH Valley versus the non-NCEP models. The NAM is also a tad stronger than all of the global models by the end of the period. The latest GEFS mean and ECENS mean are both a tad split by the end of the period too, with the GEFS mean farther north and the ECENS mean farther south with their respective closed low solutions. Will recommend a general model blend through 60 hours, with a 06Z GEFS mean and 00Z ECENS mean blend thereafter. Deep upper trough advancing across the Western U.S. by Tuesday Secondary upper trough over the Pacific Northwest by Thursday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend...through 60 hours 12Z GFS/00Z UKMET/OOZ ECMWF blend...after 60 hours Confidence: Slightly above average With the amplifying upper-level pattern across the continental U.S., a large upper-level trough and associated closed low is forecast to cross the West Coast today and advance inland across the Intermountain West by Tuesday. Given the larger scale omega block pattern evolving across the U.S., this upper low/trough across the West will slow down and should remain in place going through Wednesday. However, the arrival of another strong upper trough and associated closed low from the Gulf of Alaska and crossing the Pacific Northwest by Thursday should act to kick out the energy over the Intermountain West and largely back into southwest Canada, although there will be a trailing trough axis that advances east across the Rockies and the northern High Plains by the end of the period. Model spread is fairly modest through most of the period with the mass field evolution across the West. However, the 12Z GFS and 12Z NAM are perhaps a little faster to lift the initial energy northward out of the Intermountain West versus the non-NCEP solutions, and by the end of the period, the NAM is seen as being a weak outlier with the low center over southwest Canada. Will prefer a general model blend through 60 hours, with a blend of the 12Z GFS, 00Z UKMET and 00Z ECMWF thereafter given overall better model clustering and ensemble support with both upper troughs impacting the West this period. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison