Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Tue Mar 10 2020 Valid 12Z Fri Mar 13 2020 - 12Z Tue Mar 17 2020 ...Southwest heavy rain threat to redevelop over the Southern Plains to lower MS Valley and TN Valley later this week and weekend... ...Guidance/Predictability Assessment... A split flow pattern is expected across the CONUS during the medium range with a progressive northern stream and less progressive southern stream. Model consensus through the medium range is relatively good at large scales, with expected increasing differences through time at smaller scales. A relatively strong low pressure system is forecast to cross eastern Canada Fri-Sat, with the trailing frontal boundary moving off the Eastern Seaboard, and the trailing front lingering for several days across the southern U.S. Models consensus on this feature was relatively good, and the ECMWF/GFS/UKMET all appeared to handle it reasonably well. Meanwhile, an upper-level low initially over the Southwest on Fri is forecast to weaken into a broader shortwave as it moves east into the Southern/Central Plains on Sat and the Mid-Atlantic on Sun. The shortwave is expected to produce a relatively weak wave of low pressure that rides along the aforementioned frontal boundary lingering from the Southern Plains to the Southeast Sat-Sun. Timing variability has continued to occur among the guidance for this feature, with the 12Z (Mon) guidance slowing it down some, and the 00Z GFS speeding up slightly again. Overall, a blend of the ECMWF and GFS should handle this feature reasonably well. Farther west, another vigorous shortwave is expected to dive southeastward from the Gulf of Alaska into the northwestern U.S. Fri-Sat as it deepens into a closed upper-level low. The general model/ensemble consensus is that this feature should slowly drift southward along the West Coast through the weekend and into early next week. The primary model differences surrounding this system are between solutions that keep the feature farther west through Tue (ECMWF/ECENS mean), and solutions that move the feature inland across the interior West more quickly (GFS/CMC/GEFS mean). At this time, favored a compromise that leans approximately 2/3 toward the slower/farther west camp of solutions. The WPC forecast was initially based on a multi-model blend including the 12Z ECMWF/UKMET/18Z GFS during days 3-4. Starting on day 5 (Sun) through day 7 (Tue), weight was increased on ensemble means (especially the ECENS mean), with some continued use of the deterministic ECMWF. ...Weather/Hazard Highlights... Showers and a few thunderstorms will accompany the cold front across the eastern third of the CONUS Thu/Thu night, with the potential for some locally heavy rains across portions of the Tennessee Valley and southern Appalachians, and accumulating snows possible for northern New England. The upper low crossing the Southwest will bring potentially heavy rains to portions of that region Thu into early Fri before the system moves east. As the upper-level system moves east into the Plains, a weak wave of low pressure expected to develop along the lingering surface front will help to focus increasingly widespread showers and thunderstorms with areas of heavy rain starting on Fri across the Southern Plains. North of the system, in somewhat colder air, an areas of winter weather is possible on Fri across portions of the central High Plains (see WPC medium range Winter Weather Outlooks). On Sat, shower and thunderstorm activity should shift a bit eastward toward the lower Mississippi and Tennessee Valleys. With the surface front expected to stall near the Gulf Coast early next week, and continued northward moisture advection crossing the boundary, showers and storms could linger from the Southern Plains into the Tennessee Valley into early next week. The potential for multiple days of at least locally heavy rains across the same areas could result in areas of runoff issues. The upper low and associated frontal system reaching the Northwest on Fri will spread rain and mountain snows inland across much of the western states through the weekend and into early next week. Activity should gradually shift southward/eastward through time, reaching California Sat-Sun and the Wasatch and central Rockies by Sun-Mon. Temperatures are expected to initially be well above average along the Eastern Seaboard ahead of the cold front on Fri - with high temperatures 10-18 deg F above average for many areas. Cooler air should much of the East Coast over the weekend as a strong polar high pressure area behind the cold front influences the region. The Gulf Coast region, remaining south of the front at least through the weekend, should remain above average. Strong arctic high pressure nosing down east of the Canadian Rockies will bring cold conditions to portions of the northern Rockies and northern High Plains Fri through the weekend, with high temperatures ranging from 20 to as much as 40 deg below average for some areas, and many areas remaining well below freezing through the weekend. Ryan 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