Extended Forecast Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Tue Mar 10 2020 Valid 12Z Fri Mar 13 2020 - 12Z Tue Mar 17 2020 ...Heavy rain threat over the Southern Plains to lower MS and TN Valleys later this week and weekend... ...Record cold likely for portions of the Northwest this 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. Model consensus on this feature was relatively good, and the 00Z ECMWF/GFS/UKMET/Canadian (and 06Z GFS) 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 eastward 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 and latitudinal variability has continued to occur among the guidance for this feature, but a consensus position seemed prudent. 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/GEFS mean) and an intermediate solution near the 00Z Canadian. Continued to favor a weighted compromise that leans approximately 2/3 toward the slower/farther west camp of solutions (ECMWF/Canadian). Upstream anomaly of 500mb heights may exceed +3 sigma per even the ensembles, which teleconnects to a farther west solution of lower heights than the GFS/GEFS. The WPC forecast was initially based on a multi-model blend including the 00Z GFS/ECMWF/UKMET/Canadian and 06Z GFS during days 3-4. Starting on day 5 (Sun) through day 7 (Tue), weight was increased on ensemble means (especially the 00Z ECENS mean), with some continued use of the deterministic 00Z ECMWF. ...Weather/Hazard Highlights... Showers along the the cold front will exit the Northeast/Mid-Atlantic early Friday with the potential for some locally heavy rains across portions of the Southeast as the front slows across Georgia. As the lead upper-level low over the Southwest on Fri 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 some thunderstorms) with areas of heavy rain starting on Fri across the Southern Plains. North of the system, in somewhat colder air, an area of wintry weather is possible on Fri across portions of the western 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 a separate area of snow possible near the MT/ND border. 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 heavy 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. Precipitation may expand across Arizona by Tuesday with snow in the mountains. Temperatures are expected to initially be well above average along the Eastern Seaboard ahead of the cold front on Fri - with high temperatures 10-20 deg F above average for many areas. Cooler air should filter into much of the East Coast over the weekend as a strong polar high pressure area behind the cold front slips across New England. The Gulf Coast region, remaining south of the front at least through the weekend, should see above average temperatures and even record highs across Florida. 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. Both record low and record low maximum temperatures may be tied/broken especially over Washington state, despite the much larger anomalies over Montana. Fracasso/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 Hazards: - Heavy precipitation across portions of California and the Southwest, Fri, Mar 13 and Mon-Tue, Mar 16-Mar 17.. - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Southern Plains, the Middle Mississippi Valley, California, the Southern Appalachians, the Pacific Northwest, and the Ohio Valley, Fri-Sat, Mar 13-Mar 14. - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Sun-Mon, Mar 15-Mar 16. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Southern Plains, the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Tue, Mar 17. - Heavy snow across portions of the Pacific Northwest, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Fri-Sat, Mar 13-Mar 14. - Heavy snow across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, and the Southwest, Sat-Mon, Mar 14-Mar 16. - Heavy snow across portions of the Central Great Basin, Sun-Tue, Mar 15-Mar 17. - Heavy snow across portions of the Southern Rockies, the Central Rockies, the Central Great Basin, the Central Plains, and the Southwest, Fri, Mar 13. - Heavy snow across portions of the Central Rockies and the Central Plains, Tue, Mar 17. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Southeast, the Upper Mississippi Valley, and the Northern Plains. - Much below normal temperatures across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Sat-Tue, Mar 14-Mar 17. 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