Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 234 PM EDT Sun May 17 2020 Valid May 17/1200 UTC thru May 21/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 18Z Update: There were no major changes noted with the arrival of the 12Z UKMET, and the CMC was also consistent and now more in line across the Rockies and slightly to the west of its previous run with the upper low dropping south across Illinois. The ECMWF trended a little faster in bringing the closed low southward to western Tennessee by midday Tuesday, and close to its previous position by 84 hours. The 12Z NAM and a few of the 12Z GEFS members remain well to the west of the model consensus regarding the track of Arthur once it passes Cape Hatteras, NC, but otherwise good model continuity. Out west, the 12Z CMC can now be used as part of the model blend with the building trough over the Intermountain West. Tropical Storm Arthur ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: National Hurricane Center official track Best Model Proxy: 12Z ECMWF/12Z GFS blend Confidence: Moderate Tropical Storm Arthur formed late yesterday evening off the east coast of Florida, and is forecast to track northward toward the Outer Banks by early Monday before turning northeastward away from the Mid-Atlantic. The interaction or lack thereof with an approaching mid-level closed low will help determine the northward or northeastward steering motion of Arthur. The 12Z NAM begins to diverge from the consensus by Monday morning, with a westward track over the Outer Banks of NC, and this difference becomes more noticeable by Tuesday with the storm lingering off the New Jersey coast, whereas the other models are in decent agreement that a sustained track to the northeast will take place. There are also a couple of GEFS members that indicate a similar path and even bring the storm back to the west again towards New Jersey, perhaps owing to the building ridge to the north and the upper low to the west. The ECMWF/EC mean continue to favor a track to the southeast of the GFS/GEFS mean, and this is similar to yesterday. The best proxy to the official NHC track is about halfway between the GFS and ECMWF, close to the model consensus. For the latest and official forecast information, see updates from the National Hurricane Center. Consolidation of Midwest and Gulf Coast shortwaves into cut-off upper low ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z ECMWF, 12Z UKMET bland Confidence: Moderate This feature will be the most impactful in terms of sensible weather across the nation for the short range period. Two distinct shortwaves, one near the Gulf Coast, and another currently over Iowa, are expected to merge and evolve into a pronounced and cut-off upper level low. The northern shortwave will be the dominant feature and grow as it settles towards the southeast across Illinois and then the Mid-South by Tuesday night. Model forecast guidance over the past several cycles has struggled with the evolution of this system, and this is not unusual given cut-off low evolution. It now appears to be in better agreement on the main features with the arrival of the 12Z GFS and NAM. There has been a trend to the west to compared to previous model runs, and now show the center of the low reaching Tennessee by the end of the period, with the GFS slightly to the east. To account for these trends, a blend of the farther west ECMWF, NAM, and UKMET would serve well as a starting point in the forecast process. Deep upper level trough building in across the western U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Moderate-High With the amplifying upper level pattern across the continental U.S., an amplified upper level trough is forecast to become established over the West Coast and then the Intermountain West by the middle of the week as a big upper low from the northeast Pacific tracks southeastward. This will also be in response to a building downstream ridge axis across the Plains as an omega block pattern develops. The NCEP guidance is a little weaker with the downstream ridge, and the NAM becomes slightly stronger by Tuesday morning with the upper low. Towards the end of the forecast period, the CMC is slower in rotating shortwave energy from the Desert Southwest to the central Rockies compared to the other guidance, and also southwest of the model consensus with the developing surface low across Montana by Tuesday night. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick