Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1225 AM EDT Sun May 17 2020 Valid May 17/0000 UTC thru May 20/1200 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: 12Z ECMWF/UKMET blend Confidence: Average Tropical Storm Arthur formed late yesterday evening off the southeast Florida coast and is forecast to track northward toward the Outer Banks by early next week 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 and through about 18.00Z, guidance is in reasonable shape. Beyond that time frame, the models differ on the longitudinal direction with the latest GFS west of the consensus and takes Arthur just onshore Cape Hatteras by Monday morning. Meanwhile, the ECMWF/UKMET offer a slightly weaker solution that allows the system to pull eastward and is just to the right of consensus and official track. The best proxy to the official NHC track is the 12Z ECMWF with some inclusion of the 12Z UKMET acceptable through most of the periods. For the latest and official forecast information, see updates from the National Hurricane Center. Shortwave and surface low tracking across the northern tier states ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS, 12Z ECMWF, 12Z UKMET blend Confidence: Slightly below average Mid-level shortwave energy tracking through the North-Central Plains and Upper Midwest early this morning will deepen and close off as it slows over the lower Great Lakes and Ohio Valley early next week. Forecast guidance has struggled with the evolution of this system, typical of cut-off lows. All of the guidance has converged on an idea to take the low further south than east compared to previous model runs and now show the center of the low reaching the Tennessee River Valley by the end of the period. A compromise of the GFS/ECMWF/UKMET appears the most stable run-to-run as well as mirrors the ensemble means the most. The NAM showed a solution the furthest to the south, which while not out of the question, did not fit the model envelope spread as much. Given the larger than average model and run to run spread, forecast confidence is set to slightly below average. Deep upper level trough developing along the West Coast by Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Large, longwave troughing expected to move from the eastern Pacific onshore toward the interior western U.S. by the end of the forecast period. The model guidance continues to show above average consistency in the developing omega block pattern. As such, a general model blend should more than suffice for mass field purposes. Southern stream trough tracking from Texas and along the Gulf Coast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average Weak mid-level shortwave energy skirting across the southern Plains is forecast to reach the Southeast U.S. before becoming absorbed into the larger cut-off low discussed above. Until that happen, the model agreement is average and overall the latest guidance shows fairly good consistency such that a general model blend is preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor