Extended Forecast Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Wed Apr 21 2021 Valid 12Z Sat Apr 24 2021 - 12Z Wed Apr 28 2021 ...Heavy Rainfall across parts of the Southeast Saturday... ...Overview... Progressive upper level flow is expected to evolve over the course of the period with the main systems of interest being shortwave energy deepening over the Northeast/New England late this weekend and early next week while a deeper longwave trough pushes onshore the West Coast. As it reaches the Central Plains by day 7, a more significant storm system could bring a variety of weather hazards to the central U.S. by the middle of next week. Temperatures will be variable during the period, modulated by frontal passages. ...Guidance/Predictability Assessment and Preferences... The 21.00Z and 21.06Z guidance showed fairly good clustering at the starting point of the medium range. A multi-model blend of the deterministic guidance was sufficient for the low pressure system tracking from the mid-Mississippi Valley on day 3 /Saturday/ to New England on day 5 /Monday/. The 06Z GFS was less favored though, as it was showing a faster bias compared to the other deterministic global models and the 00Z GFS was more agreeable and therefore was included more in the blend. Elsewhere, the ECMWF continued to be a slower solution with the next trough coming through the West U.S. and the CMC (and to some degree the UKMET) offered a more consensus approach. Either way, once the trough ejects into the Plains and cyclogenesis takes place, all of the models actually come together for a reasonable solution for day 6/7 in what may evolve into a more amplified flow pattern. The ECENS/GEFS means were used more for day 6/7 and this resulted in a blend that trended well with continuity. ...Weather/Hazards Highlights... Low pressure in the Tennessee Valley early Saturday will promote shower and storms, possibly severe (see SPC), with locally heavy rain especially over parts of Georgia and through the Carolinas. The system should then lift up the East Coast as trailing northern stream energy from the Great Lakes region merges into the coastal system and possibly closes an upper low. Enhanced rainfall is possible from parts of the Mid-Atlantic into New England with windy conditions region-wide as the low deepens. In the West, an amplifying Pacific upper trough/low and surface front will work into the West during the weekend into early next week. This system will spread lower elevation rain/mountain snow focused from the Pacific Northwest and northern Rockies southward into California and the Great Basin/central Rockies. The highest totals with this system will likely be from the Pacific Northwest into the Sierra Nevada. The majority of precipitation should be light to moderate though parts of the central West Coast and Sierra Nevada could see a period of locally enhanced activity. Once the system moves to the Plains by Tuesday, potential will increase for convectively-driven rainfall over parts of the Plains and Mississippi Valley. See SPC for details on the potential severe weather threat. In the wake of the eastern system, cooler than normal temperatures will be prevalent over the Ohio Valley and southern Mid-Atlantic into the Northeast. An area of above normal temperatures over parts of the West/Great Basin ahead of the Pacific upper trough/low will shift eastward across the Rockies and into the Plains by early next week and into the Northeastern 1/4 of the CONUS by midweek. Best potential for one or more days with temperatures 10F or more above normal will extend from near the Great Basin through the central-southern Rockies into the Plains/Midwest where temperatures may climb well into the 80s/70s, respectively. Trailing the front that crosses the West, temperatures over the region will decline to 5-15F colder than normal by next Monday-Tuesday and the back to near or slightly above normal Wednesday. Taylor/Fracasso Hazards: - Heavy precipitation across portions of northern and central California, Sun, Apr 25. - Heavy rain across portions of the Southern Plains, the Lower and Middle Mississippi Valley, the Great Lakes, the Ohio Valley, the Tennessee Valley, the Southern Appalachians, and the Southeast, Tue-Wed, Apr 27-Apr 28. - Heavy rain across portions of the Middle and Upper Mississippi Valley, and the Great Lakes, Wed, Apr 28. - Heavy rain across portions of the Tennessee Valley, the Southeast, the Southern Appalachians, and the Mid-Atlantic, Sat, Apr 24. - Heavy rain across portions of Maine, Sun, Apr 25. - Severe weather across portions of the Central and Southern Plains, Tue, Apr 27. - Severe weather across portions of the Lower Mississippi Valley, the Tennessee Valley, the Southern Appalachians, and the Southeast, Sat, Apr 24. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Northeast, Sun-Mon, Apr 25-Apr 26. 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 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