Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1226 AM EDT Tue May 19 2020 Valid May 19/0000 UTC thru May 22/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Model Evaluation Including Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tropical Storm Arthur ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: National Hurricane Center official track Best Model Proxy: 00Z GFS, 12Z ECMWF blend Confidence: Slightly above average T.S. Arthur continues to pull away from the east coast with its north-northeast motion. Over the next 24 hours, the system will gradually turn to the east. The latest guidance still exhibits some differences, with the 12Z CMC considerably slow while the 12Z UKMET is on the faster side of the model spread. The NAM/GFS do trend slower than the ECMWF beyond 24 hours but the best model blend proxy to the official NHC track is a compromise blend of the GFS/ECMWF. Please consult the latest NHC Forecast Advisories for more information on Arthur. Cut-Off Low Over Lower Ohio and TN River Valleys ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend...through 60 hours ECMWF/UKMET/GEFS blend thereafter Confidence: Slightly above average Large anomalous closed upper level will further remove itself from the northern stream flow and become cut-off in the next 12-24 hours over the lower Ohio Valley, eventually settling over the TN River Valley by day 2/3. Through 60 hours, the model agreement is quite high and the latest guidance shows very good predictability and consistency from the past several runs. Beyond 60 hours, the CMC becomes an outlier and unusable as it drifts the low too far to the east and too fast. The 00Z GFS shows an weakening/opening up wave earlier than the rest of the models and allow the feature to lift north toward the Great Lakes by the end of the period, which appears to be too fast looking at the ensemble. The ECMWF/UKMET show better run to run consistency and are better aligned with the ensemble means (including the GEFS) such that will be the recommended model blend preference at this time. 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 GFS/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. Through 60 hours, the model guidance shows pretty good predictability but beyond that, with the approach of the stronger secondary system, how quickly this energy ejects eastward is different in the various deterministic solutions. The NAM and CMC appear to be too fast and too deep, so for now, will prefer a ECMWF/GFS blend after 60 hours which aligns better with the ensemble means. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor