Extended Forecast Discussion NWS Weather Prediction Center College Park MD 138 AM EDT Thu Apr 09 2020 Valid 12Z Sun Apr 12 2020 - 12Z Thu Apr 16 2020 ...Heavy rain/flooding and severe weather threat for the South on Sunday, then Appalachians/Eastern Seaboard on Monday... ...Threat of heavy snow and cold across the portions of the central/southern Rockies and High Plains Sun-Mon... ...Overview and Guidance/Predictability Assessment... A broad upper-level ridge across the northeast Pacific is forecast to expand northward across Alaska then eastward into western Canada during this extended forecast period. This will promote a broad region of downstream cyclonic flow across much of the CONUS. A vigorous mid/upper-level shortwave (former southwestern upper low) initially across the southern High Plains is forecast to accelerate northeastward, eventually interacting with northern stream shortwave energy and carving out a broad trough across the central/eastern U.S. Models continue to show spread with respect to the speed at which the southern stream feature moves northeast, which are tied to differences in timing of the northern stream wave. The GFS continues to trend slower, but remains faster than all other guidance. The UKMET and especially the ECMWF have been more consistent with this system over recent days, and these solutions were preferred for handling of this feature. These solutions indicate quick deepening of a surface low across the Southern Plains Sun as the upper-level system moves east, with energy quickly transferring to development of another surface low. The new low is forecast to move rapidly northeastward across the Tennessee/Ohio Valleys later Sun and into the Great Lakes and eastern Canada on Mon as it deepens. The trailing cold front is forecast to sweep off the Eastern Seaboard Mon night into early Tue as the low continues to deepen across Canada. A second wave of low pressure is forecast to develop along the front near the coast of the Carolinas Tue into Wed before moving northeast and out to sea. Farther west, models continue to struggle with handling of energy dropping down the eastern side of the North Pacific ridge, moving from British Columbia southward into the western U.S., and further carving out the western side of a broad upper-level trough. A somewhat stronger signal is evident in among ensemble members as well as the deterministic ECMWF for the most significant of these shortwaves to dive south into the northwestern U.S. Tue night into Wed, reaching the Great Basin Wed night into Thu. Farther east next week, broad high pressure at the surface is expected to dominate, with broad northerly flow across central Canada, providing constant replenishment to polar air moving southward into the central U.S. Based on the above considerations, the WPC forecast was initially based heavily on a blend of the 12Z ECMWF/UKMET during days 3-4 (Sun-Mon). During days 5-7 (Tue-Thu), ensemble means (particularly the ECENS, and to a lesser extent the GEFS) were gradually increased through time to account for increasing spread and uncertainty. A minority component of the ECMWF was included through day 7 to add forecast detail, as its solution showed some degree of alignment with the ensemble means. ...Weather/Hazard Highlights... Widespread showers and thunderstorms are expected across portions of the lower Mississippi Valley, Southeast, and southern Mid-Atlantic on Sunday in association with the warm front ahead of the developing Southern Plains/western Gulf Coast low pressure system. Models/ensembles show an increasingly strong signal for potential heavy rain/flooding concerns from the lower Mississippi Valley to the Southeast and Southern Appalachians Sat-Sun. As low pressure moves northeast across Tennessee/Ohio Valleys and quickly deepens, gusty winds may also pose a threat. On the northwest side of the low pressure system, sufficient cold air may be in place to support precipitation falling as snow across portions of the Midwest and Upper Great Lakes on Sun, with some accumulations possible, along with gusty winds. Additionally, severe weather may also pose a threat across portions of the Gulf Coast and Southeast on Sun. As the deep low lifts through the Great Lakes and the overall system moves north and east Mon, widespread precipitation is expected across the Mid-Atlantic and Northeast. With the low pressure system expected to track farther west than prior systems, the setup appears warmer across the Northeast where the winter weather threat appears low. A second wave of low pressure along the trailing cold front Tue-Wed may bring another round of showers and thunderstorms to portions of the Southeast and Mid-Atlantic, with additional rain (and perhaps interior snow or rain/snow mix) for the Northeast. A strong/windy cold frontal passage will spread anomalously cold air southward from Canada over the weekend. High temperatures 20 to 30 deg F below average are forecast across much of the Rockies and central U.S. Sun-Mon. Additionally, the system is also expected to produce a threat for heavy snow, working its way southward across the central/southern Rockies and High Plains Sun-Tue. Some slight moderation is possible by the middle of next week, but the pattern should support continued upper troughing and below average temperatures, centered across the north central U.S. (and eventually spreading east Wed-Thu), through the end of the forecast period. 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