Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM 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... Blocky flow at the higher latitudes will yield quasi-progressive flow across the mid-latitudes/CONUS during the medium range. Both northern and southern stream systems will flow across the lower 48 with the potential for some merging/phasing near New England late this weekend and over the central Plains next week. This may lead to bouts of heavy rain over the Southeast Saturday and possible coastal New England Sunday, with additional rain/snow into the West Sunday-Monday. Temperatures will be variable during the period, modulated by frontal passages. ...Guidance/Predictability Assessment and Preferences... Through the 12Z/18Z (20th) guidance, the deterministic guidance offered a reasonably-clustered starting point with differences noted in the pace of the eastern system this weekend and, increasingly, the speed of the trough pushing through the West. In the East, a blended solution sufficed as multi-day trends have been stable in the ECMWF ensembles (quicker trend in the GEFS after mishandling the lead-in trough through the South). To the West, the 12Z ECMWF was notable slower with the trough through California compared to much of the rest of the guidance, though this may have been an odd run with the guidance due to a steady narrowing of the guidance the past few days. Opted to maintain the previous preference which is mostly around the 12Z ECMWF ensemble mean, but with a minority weighting toward the quicker guidance. By next Wed, differences actually narrowed as the system lifts through the central states and develops surface low pressure in the Plains to the Upper Midwest. ...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. A little snow at highest elevations could be possible depending on exactly how the system evolves. 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 any 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. Fracasso/Rausch 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