Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 AM EDT Tue May 19 2020 Valid 12Z Fri May 22 2020 - 12Z Tue May 26 2020 ...Overview and Weather/Hazard Highlights... The large scale flow pattern across the CONUS during the medium range period will finally be transitioning from a blocky/amplified pattern to one more typical of summer as a ridge builds across the southern tier and northern stream energies shift east and north towards the U.S. Canadian border. A cutoff mid/upper-level low, and associated surface frontal wave, will initially be in place across the lower Ohio Valley on Friday, but should gradually continue to weaken as it lifts into the Mid-Atlantic on Saturday and eventually out to sea. Much of the heavy rainfall associated with this system across the Mid-Atlantic/Appalachians should be in the short range period, though scattered showers and storms may linger across the region into Saturday before the system moves east. Temperatures across the Mid-Atlantic states may begin slightly below normal underneath of the upper low, though should trend back towards normal the rest of the period and stretching up and down the Eastern Seaboard. Behind this, upper level ridging will build back in across the eastern third of the country with temperatures near or above normal across much of the Midwest and into parts of the interior Northeast. Farther west, a broad and slow moving upper-level trough is forecast to cross the West and Rockies Fri-Sat, accompanied by rain and high elevation snows and below average temperatures. A trailing frontal boundary from the system farther east is expected to linger across the Lower Mississippi Valley into the Southern Plains, which could provide some focus for development of convection across those areas. As the trough axis shifts east and eventually settles across the Plains states Sun-Tues, showers and thunderstorms are expected to become more widespread across the Central U.S.. Areas of heavy rain and thunderstorms are possible, especially across parts of the southern/central Plains early next week as some models indicate a broad upper shortwave may linger across the region day 6-7. At this point though, exact impacts/specific areas affected are hard to pin down given substantial model uncertainty in the details at this time frame. ...Guidance/Predictability Assessment... Model agreement is pretty good across the board day 3 into day 5. After this, uncertainty increases especially with regards to evolution of amplified troughing into the Plains states late in the period. Model run to run consistency remains poor, especially in the details and so an ensemble based approach is preferred. The forecast for this cycle of the WPC progs was based largely on a blend of the 12z/May 18 ECMWF with the 18z/May 18 GFS during days 3-4. After this, the GFS was dropped due to divergence from consensus with the system in the east. Day 5 onward, the blend was based mostly on the ensemble means, although did continue some minor use of the ECMWF through the period since it was the deterministic solution closest to that of the means. Santorelli 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