Extended Forecast Discussion NWS Weather Prediction Center College Park MD 342 PM EDT Tue Apr 13 2021 Valid 12Z Fri Apr 16 2021 - 12Z Tue Apr 20 2021 ...Central Rockies/High Plains Heavy Snow Threat Lingers Into This Upcoming Weekend... ...Heavy Rainfall Threat For Central Gulf Coast and Portions of Florida This Weekend and Early Next Week... ...Guidance/Predictability Assessment... A somewhat blocky pattern early on in the medium range period is expected to evolve over the period and by early to mid next week, much of the central/eastern U.S. could be within a troughing regime while some sort of upper ridging remains in place over the western U.S.. The exit of the closed low over the Northeast U.S. this weekend and the ridging anchored over southwestern Canada will allow for another strong shortwave to drop southward through the Plains/northern Rockies. This feature is expected to carve out a closed low over the Central Rockies and likely lead to a heavy snow upslope event. The deeper troughing over the central U.S. will keep a frontal boundary suppressed south into the Gulf Coast region and this stationary boundary interacting with waves of low pressure and moisture transport will be the focus for episodes of heavier rainfall. The latest guidance showed average to above average agreement and continuity from the previous cycle. The WPC blend was composed of the 00Z ECMWF, 06Z GFS, 00Z CMC, and 00Z UKMET initially then transitioned toward a blend favoring the 00Z ECENS/00Z GEFS means by day 6/7 where there was higher model spread. This resulted in a blend that trended well from continuity. ...Weather/Hazards Highlights... Closed low/trough dropping southward through the Rockies and its associated High pressure building across the Plains will favor below normal temperatures and a heavy upslope snow event for portions of the Central Rockies of WY/CO. As upstream ridging builds over the western U.S. and eastern Pacific, the troughing over the Rockies will remain in place with the threat of heavier snow sagging southward through this weekend. Daytime high temperature anomalies of 20F to perhaps 25F below normal are possible Friday and this weekend. Conversely, the upper ridging over the Pacific will lead to highs 15-20F above normal, particularly for northern California to Oregon and Washington. A frontal boundary stalling over the Gulf and portions of the Florida peninsula this weekend and early next week will be the focus for several rounds of potentially heavier rainfall. The greatest risk area appears to be from central/eastern Gulf Coast toward the Florida peninsula and much of this area has seen above normal precipitation the last 14 days (200-300 percent of normal) and this additional rainfall could worsen or initiate flooding in the area. Taylor 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 rain across portions of the Southeast and the Lower Mississippi Valley, Fri-Sat, Apr 16-Apr 17. - Heavy rain across portions of the Southeast, Sat-Tue, Apr 17-Apr 20. - Flooding possible across portions of the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Middle Mississippi Valley and the Lower Mississippi Valley. - Flooding likely across portions of the Middle Mississippi Valley. - High winds across portions of the Northeast, Fri-Sat, Apr 16-Apr 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