Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Thu Aug 22 2019 Valid 12Z Sun Aug 25 2019 - 12Z Thu Aug 29 2019 ...Overview and Guidance/Predictability Assessment... The large scale pattern will likely feature a deepening upper trough over the north central US into southern Canada from Sunday through Tuesday. Its forward progression is expected to slow early next week thanks to building heights and ridging over the eastern/northeastern Pacific. A surface low is expected to form and deepen as it tracks from near the US/Canada border toward Hudson Bay. Overall, while the large scale pattern is in relatively good agreement with the latest model guidance, there are some timing and strength differences noted. The latest GFS is faster and a bit deeper compared to the rest of the guidance, while the ECMWF/CMC offer fairly similar solutions. Run-to-run variability and spread at the surface and aloft will likely continue for a while. The latest forecast for this region was mostly a blend of deterministic/operational models for Day 3-5 followed higher weights toward the ECENS mean with some inclusion of the GEFS mean. Over the Northeast U.S., trends still suggest a closed low developing off New England from energy trailing part of a short range upper trough. There is fairly good agreement with the guidance supporting this idea; however, there are still timing differences with the GFS much faster for the low to depart. The ECMWF/CMC/UKMET all hold back the energy longer. With more ensemble members supporting this idea, the slower progression was favored at this time. Finally, over the southern latitudes, ridging will persist from the southern Plains westward toward the central West Coast. Farther east off the southeast U.S., an area of disturbed weather will gradually lift northwest. There remains quite a bit of uncertainty in the potential development of this system with both spatial and temporal model spread quite high by Day 5-7. ...Weather Highlights/Threats... Areas from the eastern half of the northern-central Plains into the western/upper Great Lakes will see the potential for one or more episodes of heavy rainfall and strong to severe convection during the period. During Sun-Tue there is decent guidance agreement that strengthening dynamics aloft/Canadian low pressure will push a cold front across this region while a leading warm front may provide an additional convective focus. The trailing part of the cold front may stall for a time over the south-central Plains. However frontal details and in turn precipitation distribution/intensity at any particular time become increasingly uncertain Tue onward in light of guidance spread/variability for upstream shortwaves aloft. The combination of moist flow ahead of weak upper level energy reaching the Lower Mississippi Valley at the start of the week and weakening surface front over the extreme Southeast may promote some areas of heavy rainfall from the central Gulf Coast through the southeastern states. Pockets of significant rain are also possible with diurnal convection over the Florida Peninsula. The eventual track/evolution of the surface trough now over the Bahamas will have to be monitored as well. The upper low forecast over/near New England early in the period may produce some scattered diurnally-favored rainfall in a surface pattern that would otherwise normally provide dry weather. Highest temperature anomalies of at least plus 10-15F are most likely over the southern Rockies/High Plains Sun-Mon with some record highs possible, while similar anomalies are possible over the central West Coast/Pacific Northwest by Tue-Thu as the Pacific ridge aloft approaches/reaches the area. The southern Plains should see a steady cooling trend closer to normal after Mon. On the other hand the northern into central Plains will likely see multiple days of well below normal highs with decent coverage of at least minus 10-15F anomalies Mon-Thu. Over the East expect moderately cool readings on Sun to trend gradually warmer with time, reaching modestly above normal levels by midweek. 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