Extended Forecast Discussion NWS Weather Prediction Center College Park MD 553 PM 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 through Monday... ...Overview and Guidance/Predictability Assessment... Broad, upper-level ridging will nose into the northeast Pacific and Alaska through this coming week. As such, a broad region of downstream cyclonic flow will be present 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. The GFS evolution has been and continues to be more progressive and ultimately out of sync with other pieces of guidance. The CMC, UKMET along with the ECWMF/EC Ensemble have been a bit further west and more pronounced with each shortwave feature during the extended periods. A combination of those four models maintained a sense of continuity, favoring quick deepening of a surface low across the Southern Plains on Sunday as the upper-level system moves east, with energy quickly transferring to development of another surface low. This feature will then rapidly lift through the Tennessee and Ohio Valleys, Great Lakes and into eastern Canada on Monday. A trailing cold front will slowly makes it way through the Mid-Atlantic and Southeast, providing a focus for continued convection across the Southeast and Mid-Atlantic regions as it moves offshore. 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. The WPC forecast consisted of a compromise of the 00Z CMC/UKMET/ECWMF/EC Ensemble means through the extended periods. The weighting of the ensemble means were increased toward the end of the forecast. ...Weather/Hazard Highlights... Much of the Gulf Coast states, Southeast and southern portions of the Mid-Atlantic will have widespread showers and thunderstorms as a warm front ahead of the developing Southern Plains/western Gulf Coast low pressure system lifts north. The environment over much of this region will be conducive for periods of intense rainfall that may lead to rapid runoff, urban ponding or localized flash flooding. Additionally, these thunderstorms may become strong to severe, causing other hazardous conditions such as damaging winds or hail. As previously noted, the trend from the past few runs of model/ensemble guidance show an increasingly strong signal for potential heavy rain/flooding concerns from the lower Mississippi Valley to the Southeast and Southern Appalachians through Sunday. General consensus depicts a swath of 3 to 5 inches across from parts eastern Gulf Coast to the southern Mid-Atlantic with very isolated maximums possibly reaching or exceeding 6 inches. 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. 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. Campbell/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 rain across portions of the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Northeast, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Sun-Mon, Apr 12-Apr 13. - Heavy rain across portions of the Southeast, Wed, Apr 15. - Heavy snow across portions of the Central Plains, the Central Rockies, the Southern Plains, the Southern Rockies, the Upper Mississippi Valley, and the Great Lakes, Sun-Mon, Apr 12-Apr 13. - Heavy snow across portions of the Central Plains, the Middle Mississippi Valley, and the Upper Mississippi Valley, Sun, Apr 12. - Heavy snow across portions of the Central Appalachians, Wed-Thu, Apr 15-Apr 16. - Severe weather across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Sun, Apr 12. - Severe weather across portions of the Southeast and the Mid-Atlantic, Mon, Apr 13. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Northern Plains, the Tennessee Valley, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley. - High winds across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Northern Plains, the Northern Rockies, the Southern Rockies, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Southern Plains, Sun, Apr 12. - High winds across portions of the Central Plains, the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Northern Plains, the Tennessee Valley, the Middle Mississippi Valley, the Northeast, the Southern Appalachians, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Mon-Tue, Apr 13-Apr 14. - Much below normal temperatures across portions of the Central Plains, the Central Rockies, the Lower Mississippi Valley, the Central Great Basin, the Northern Plains, the Great Lakes, the Northern Rockies, the Southern Rockies, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley, Sun-Thu, Apr 12-Apr 16. 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