Extended Forecast Discussion NWS Weather Prediction Center College Park MD 117 PM EDT Sat Apr 11 2020 Valid 12Z Tue Apr 14 2020 - 12Z Sat Apr 18 2020 ...Southeast U.S. heavy rain threat through midweek... ...Overview and Guidance/Predictability Assessment and Weather/Hazard Highlights... The longwave flow aloft will be highlighted at the beginning of the upcoming medium range forecast period by an amplified northeast Pacific/Alaska ridge and a downstream CONUS trough. This pattern will mainly remain in place into midweek and models and ensembles seem to have a good handle of the flow evolution through this period. WPC products valid in this period were mainly derived from the 06 UTC GFS and 00 UTC ECMWF that better represents details consistent with predictability. In this scenario, an impressive/rapidly deepening low pressure system with high winds/heavy precipitation will lift into eastern Canada by Tuesday as a wavy trailing front settles over the Southeast U.S. and acts to focus deeper moisture to fuel a lingering heavy rainfall threat through midweek. Cold Canadian air also will spread down across much of the lower 48 in the wake of this system and additional northern stream impulse digging will again support snows down through the Rockies. Broad troughing across the Central and Eastern U.S. will maintain a colder than average airmass for mid-April standards. The greatest temperature anomalies are expected across the Plains to the Midwest early next week where daytime highs could be 20+ degrees below normal. Cold anomalies east of the Rockies should moderate back closer to normal during the middle to latter parts of next week, while temperatures should trend warmer across parts of the West as upper ridging builds aloft late period. A main forecast issue by mid-later next week concerns the handling of shortwave energy undercutting the eastern Pacific ridge, potentially digging a closed low off the West Coast. Deterministic models show the feature but timing and intensity continue to vary widely. Ensemble means are more washed out with this feature given forecast spread and the blending process, but show a decent trough near California that eventually works toward the Four Corners region by day 7 within emerging southern stream flow. Northern stream upper troughing eases over the central and eastern states at lower latitudes later next week as heights gradually rise over the South in this pattern. This could eventually allow for the return flow of Gulf of Mexico moisture back inland into the South. Meanwhile, northern stream impulses will progress with some uncertainty over the n-central through northeastern CONUS and offer some wintry weather chances over the more deeply cooled northern tier. Overall, prefer a blend of the GEFS/ECENS means mid-later next week, but applied greater weighting to the ECENS mean to provide system definition and to better maintain WPC continuity. Santorelli/Schichtel 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