Extended Forecast Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Mon Apr 19 2021 Valid 12Z Thu Apr 22 2021 - 12Z Mon Apr 26 2021 ...Record cold likely for parts of the central U.S. Thursday morning... ...Heavy Rainfall/Flooding Threat across the South into the weekend... ...Overview... Blocky upper ridging forecast over Alaska and Greenland at high latitudes and over the Greater Antilles in lower latitudes favors upper troughing over much of the CONUS. As an upper low south of Alaska splits off its southern portion toward the West Coast this weekend, additional troughing will sink southward out of Canada through the Great Lakes. Ridging will build in between these two systems next week across the Plains. The pattern is rather chilly for late April with precipitation chances focused over the South, East Coast, and the West. ...Guidance/Predictability Assessment and Preferences... Model solutions seem better clustered today with the mid-larger scale flow evolution days 3-7 despite a pattern over the lower 48 states with multiple streams and the surrounding blocky flow. System timing differences remain evident, but seem mitigated by a favored composite blend of the 06 UTC GFS, 00 UTC ECMWF/UKMET/Canadian and the 13 UTC National Blend of Models. This blend is consistent with ensembles/newer 12 UTC models and maintains good WPC product continuity. ...Weather/Hazards Highlights... A southern stream system ejecting from the Southwest and southern Rockies Fri will work over the Southern Plains where rainfall will expand along and ahead of a warm front with return of Gulf of Mexico moisture and instability. Heavy rainfall is likely from the southern Plains through the lower MS Valley and Southeast Fri-Sat. Runoff and flooding issues are expected, especially given portions of the region have received much above normal rainfall in April. The system may then lift up the East Coast under influence of northern stream energy from the Great Lakes to bring enhanced rainfall across the Mid-Atlantic and windy New England Sun/Mon. Meanwhile well upstream, an amplifying Pacific upper trough/low and surface front will work into the West Coast over the weekend into early next week, eventually bringing a lower elevation rain/mountain snow focus into CA, mainly the Sierra. Areas east of the Rockies will see below normal temperatures Thursday, perhaps by 10-20 degrees which would support record cold low temperatures from Texas northeastward toward the Corn Belt/Midwest. That cold air mass will push eastward but moderate into the Northeast/Mid-Atlantic. With northern stream troughing nosing southward out of Canada, areas along the border will see below normal temperatures as well. This should support moderately heavy snow over the north-central Rockies Thu into Fri with upslope. An area of above normal temperatures will be over parts of the West/Great Basin ahead of the upper trough/low while offshore. As that trough works inland into early next week, the milder air should shift eastward across the Rockies and into the Plains. Fracasso/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Northeast, Thu, Apr 22. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Fri, Apr 23. - Heavy rain across portions of the Southeast, the Mid-Atlantic, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Sat, Apr 24. - Heavy snow across portions of the Northern Rockies, Thu, Apr 22. - Severe weather across portions of the Southern Plains, Fri, Apr 23. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Great Lakes, the Mid-Atlantic, the Northeast, the Central Appalachians, and the Ohio Valley, Thu-Fri, Apr 22-Apr 23. - High winds across portions of the Northeast, Mon, Apr 26. - Much below normal temperatures across portions of the Central Plains, the Middle/Lower Mississippi Valley, the Tennessee Valley, the Southern Plains, and the Ohio Valley, Thu, Apr 22. 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