Extended Forecast Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Mon May 18 2020 Valid 12Z Thu May 21 2020 - 12Z Mon May 25 2020 ...Heavy rains possible for the Mid-Atlantic/Appalachians through Thursday... ...Overview and Weather/Hazard Highlights... The large scale flow pattern across North America during the extended forecast period will evolve from one fairly typical of spring (amplified with cutoff features) to one more like summer (building ridge across the southern tier with strongest westerlies shifting north to the U.S./Canada border). A cutoff mid/upper-level low, with an associated surface frontal wave, are forecast to be in place across the Tennessee Valley/southern Appalachians on Thu, and expected to drift slowly northeastward across the Mid-Atlantic through Fri night/Sat as it gradually weakens, eventually moving out to sea Sat night/Sun. Instability associated with the upper low, along with low-level easterly upslope flow, will produce continued widespread showers/thunderstorms across portions of the Mid-Atlantic and central Appalachians Thu/Thu night, with at least locally heavy rain possible on already saturated ground (due to heavy rains forecast during the short range). Along with the clouds/precipitation, high temperatures are forecast to be 5 to 15 deg F below average on Thu. Scattered showers/storms may linger across the Mid-Atlantic through Fri before the system moves east of the region. Additionally, Tropical Storm Arthur is forecast by the National Hurricane Center to be post-tropical, and located just east of Bermuda Thu morning, before dissipating shortly thereafter. Farther west, a broad and slow-moving upper-level trough is forecast to cross the Northwest and northern Rockies Thu-Sat (accompanied by rain and high elevation snows and below average temperatures), before emerging into the central U.S. on Sun. Ahead of this, a leading shortwave and cold front will support areas of showers and thunderstorms across much of the Plains Thu-Sat. A trailing frontal boundary from the system farther east is expected to linger from the lower Mississippi Valley into the Southern Plains, which could provide some focus for development of convection across those areas. As the larger upper trough axis shifts east by Sun-Mon, showers and thunderstorms are expected to become more widespread across the central U.S. Throughout this time period, areas of heavy rain and severe thunderstorms are possible, but the specific areas affected will likely be determined by mesoscale/convective scale processes with low predictability in the medium range. ...Guidance/Predictability Assessment... A blend of the 00Z ECMWF/06Z GFS was used as a basis for the WPC forecast during days 3-4 (Thu-Fri). More weight was placed on the ECMWF relative to the GFS due to somewhat better alignment with the overall model/ensemble consensus. During days 5-7 (Sat-Mon), use of ECENS/GEFS ensemble means was increased, along with some continued minority use of the ECMWF through the end of the forecast period. Ryan 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 Hazards: - Heavy rain across portions of the Plains and the Mississippi Valley, Thu-Mon, May 21-May 25. - Heavy rain across portions of the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Thu, May 21. - Flooding possible across portions of the Central/Southern Appalachians, the Northern Rockies, the Middle Mississippi Valley, the Mid-Atlantic, the Northern Great Basin, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of the Mississippi Valley, the Great Lakes, and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley, the Northern Rockies, and the Northern Great Basin. 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