Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1223 PM EDT Sat May 16 2020 Valid May 16/1200 UTC thru May 20/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Shortwave and surface low tracking across the northern tier states ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF/UKMET/ECENS Confidence: Limited-Moderate A shortwave trough currently crossing the northern Plains is expected to amplify and evolve into a closed low as it moves toward the Upper Midwest by Sunday, with the shortwave energy taking on a more negative tilt. In the increasing amplified pattern over the nation, owing largely to the developing upper ridge over the western High Plains, this low is forecast to become a cut-off low by the time it reaches the Ohio Valley early next week, and slow down considerably with the omega block pattern developing. This system has the greatest level of uncertainty across the nation going into early in the week and beyond. The 12Z NAM takes the upper low farthest southwest across Illinois and then western Tennessee, and on the western edge of the ensemble spread. An even greater difference exist with the 00Z CMC which has a much weaker system and displaced well to the northeast of the model consensus, and differences with the CMC emerge as early as 12Z Sunday. There is relatively good clustering with the GFS/ECMWF/UKMET, although some depth and detail differences still exist. One of the things limiting confidence are run-to-run continuity changes, with most trending farther west over the past couple of days. This will have implications for potential heavy rainfall over the Appalachians for early to mid-week. Deep upper level trough developing along the West Coast by Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: High A large closed mid-upper level low is forecast to move from the eastern Pacific toward northern California by Monday evening, and this should settle across the Intermountain West by the end of the forecast period Tuesday evening. This will become part of the developing omega block pattern that is forecast to become firmly established across the U.S. by the middle of the week. Given the highly amplified pattern, the model guidance shows above average agreement through Monday night, after which the CMC becomes slightly slower with the trough and a little more pronounced with the downstream ridge over the Plains. However, it is still within the ensemble spread here so therefore a general model blend can be used for the western U.S. for now. Southern stream trough tracking from Texas and along the Gulf Coast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/ECENS/UKMET Confidence: Moderate A relatively weak southern stream shortwave will move across southern Plains and then the Gulf Coast region through this weekend and into Monday, with a surface wave of low pressure at the surface. This disturbance will eventually merge with the closed low that settles in across the southern Appalachians and Ohio Valley. There is good agreement on the magnitude of this trough, but timing differences remain with the NCEP guidance faster than the non-NCEP guidance, with the CMC and ECENS mean holding some of the shortwave energy back farther to the west. Ensemble spaghetti plots suggest more support for the slower solution here, and therefore the preference leans to the ECMWF/ECENS mean/UKMET for this region. Developing low over the northern Bahamas ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 12Z Monday, then 12Z NAM/00Z ECMWF/00Z UKMET Confidence: Moderate The National Hurricane Center is continuing to monitor an area of low pressure over the northern Bahamas. This system is expected to develop further over the next couple of days as it lifts northward and tracks close to the southeast U.S. coastline. There has been some trend over the past couple of days for this low to track farther west, thus having more potential implications for coastal areas. This may be due to the upper level low settling in across the Ohio Valley and Appalachians. There is good overall model agreement in the track through about 12Z Monday, after which the 12Z GFS/6Z GEFS is a bit to the west of the other guidance and has the low crossing the Outer Banks of North Carolina, and the others showing an offshore track for that area. The 12Z GFS did track farther offshore compared to its 6Z run once it passes the DelMarVa Peninsula, and the CMC is faster with the low by Tuesday off the New England coast. Once the system becomes officially identified as a tropical depression by NHC, then their forecast will become the official preference. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick