Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Aug 22 2019 Valid 12Z Sun Aug 25 2019 - 12Z Thu Aug 29 2019 ...Overview and Guidance/Predictability Assessment... Most guidance continues to show a deepening upper trough over the central part of southern Canada and northern CONUS during Sun-Tue with depth tending to stabilize thereafter. The overall trough should have a slow eastward drift with time as a strong eastern Pacific ridge pushes toward the Pacific Northwest/British Columbia Coast. As for specifics, clustering is fairly good for leading energy that sharpens/deepens Sun-Tue and surface low pressure that strengthens as it tracks from near the U.S.-Canadian border toward Hudson Bay. However to the west individual models and ensemble members still depict a wide variety of possibilities for how shortwave energy may round the top of the Pacific ridge and feed into the mean trough. Therefore confidence is not very high for resolving trough details and resulting surface front positions mid-late period. Run-to-run variability and spread at the surface and aloft will likely continue for a while. Overall a mostly operational model blend early in the period followed by a model/mean blend provides the best balance of detail versus reflecting forecast uncertainty. Note that the general sharpening trend of the upper pattern over the eastern Pacific seems to cast some doubt on the 18Z GFS and new 00Z run that bring the Pacific ridge farther eastward than the ensemble means or recent ECMWF runs. Over the Northeast remaining guidance has recently gravitated toward a number of recent ECMWF/UKMET runs that have been showing a New England closed low developing from energy within the trailing part of a short range upper trough. The ECMWF mean is the first ensemble mean to depict this closed low. The new 00Z GEFS mean is showing some trends in the direction of the operational model cluster. Across southern latitudes guidance has been fairly consistent in showing an upper ridge axis persisting over the southern Rockies and extending back to the central West Coast, with some connection to the eastern Pacific ridge. The same is true for an upper weakness forecast to evolve over the Lower Mississippi Valley and western Gulf Coast region Sun-Mon and then split apart in opposite directions thereafter. Farther east there are hints that slow development may occur from a surface trough currently over the Bahamas, though in much weaker form than depicted by the past couple UKMET runs. The manual forecast remains close to yesterday's WPC-NHC coordinated depiction off the southeastern coast. ...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 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