Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Wed Aug 21 2019 Valid 12Z Sat Aug 24 2019 - 12Z Wed Aug 28 2019 ...Overview and Guidance/Predictability Assessment... The latest model guidance continues to suggest the establishment of a broad mid/upper level trough over the northern CONUS, sandwiched between an eastern Pacific ridge that builds into the northeastern Pacific through the period and another ridge across eastern Canada. Individual, repeating shortwaves will drop through the mean flow across the northern tier of the U.S. during this time frame. Model guidance is in reasonably good agreement for Day 3 and 4 with above average predictability and less spread noted within the model envelope. Across the southern latitudes, ridging will hang on across the southwest U.S. while a weak shortwave will hover over the western Gulf into the Lower Mississippi River Valley. Model guidance diverges late in the weekend and early next week with respect to how the eastern U.S. upper level ridge evolves and whether a closed low forms off the New England coast. The past several runs of the ECMWF have been the most aggressive with this idea, forming the closed low closest to the coast. Meanwhile, the latest GFS and CMC have a similar idea, but further to the east and more progressive as upstream shortwaves approach. The preference was for more inclusion/weight toward the ECMWF and its ensemble mean, especially by Day 6-7. Additionally, models diverge on a potential low forming off the southeastern U.S. by Day 6-7 associated with an area of disturbed weather around the Bahamas. The updated forecast primarily used a deterministic blend (ECMWF/GFS/CMC/UKMET) for Day 3-4 with increasing weight toward the 00Z ECENS and 06Z GEFS means by Day 6-7 given some of the model uncertainty and spread. ...Weather Highlights/Threats... There is a continued threat for heavy rainfall over areas from the northern half of the Plains into the Great Lakes as upper troughing becomes established over the northern tier states. From the weekend into early next week the primary surface focus will be with a cold front that pushes into the northern Plains as well as a leading warm front. As upstream energy feeds into the mean trough aloft, the Plains to Great Lakes part of the front may stall for a period of time by next Tue-Wed and provide an opportunity for training convection or repeat activity. Some convection may be strong to severe during the period so check Storm Prediction Center outlooks as the details become more clear in future days. The area encompassing the central Gulf Coast through the Southeast and southern Appalachians/southern Mid-Atlantic will also see the potential for areas of heavy rainfall with a combination of ample moisture, one or more upper level impulses, and the wavy surface front settling over the Southeast. Diurnal convection over the Florida Peninsula may produce locally significant totals as well. While some of the specifics of flow aloft by the middle of next week have yet to be resolved, leading up to that point confidence is increasing for a significant cooling trend over the northern Plains where some locations may see multiple days with highs of 10-15F or so below normal. On the other hand areas from southern Oregon/northern-central California into the southern High Plains should see above normal readings for most of the period with pockets of plus 10-15F anomalies and possibly a few daily records. Cooler air may reach the southern Rockies/High Plains by next Wed. Near to below normal temperatures over the eastern half of the country during the weekend should trend toward near or slightly above normal readings by next Tue-Wed. Rausch/Taylor Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml