Extended Forecast Discussion NWS Weather Prediction Center College Park MD 357 PM EDT Mon May 11 2020 Valid 12Z Thu May 14 2020 - 12Z Mon May 18 2020 ...Active weather pattern expected to develop for portions of the central and east central U.S. later this week... ...Overview and Weather/Hazard Highlights... A Rex Block extending from the North Pacific to the Arctic which has been persistent in recent days appears likely to continue, but become somewhat reoriented during the medium range. The block should weaken somewhat as the area of negative height anomalies in the North Pacific shifts east into the western U.S. Another area of persistent negative height anomalies looks to become established across the northwest Pacific, changing the orientation of the block. The implications of this will be a trend toward at least a somewhat more amplified pattern across the CONUS, after beginning the forecast period a bit more zonal, as troughing becomes more persistent in the West and the subtropical ridge builds northward into the Southeast. Shortwave energy tending to eject from the broad western trough will interact with a wavy frontal boundary and then traverse the northern periphery of the southeastern U.S. ridge. This setup will keep areas from the Southern/Central Plains to the Ohio Valley/Great Lakes in a relatively active weather pattern with several rounds of showers and thunderstorms possible through the medium range. Rising heights over the Southeast will bring warming temperatures in contrast to the recent cool period. Highs are forecast to be 5 to 10 deg F above average Fri through the weekend from parts of the Lower Mississippi Valley/Tennessee Valley to the Mid-Atlantic. An upper-level shortwave and associated surface frontal wave are expected to bring showers and thunderstorms from the Mid-Mississippi Valley to the Great Lakes Thu-Fri, with areas of heavy rainfall possible. After that, as more pronounced shortwave energy moves from the West into the central U.S., the Central/Southern Plains are expected to become increasingly active Fri-Sat, with the potential for multiple waves of low pressure to move along a quasi-stationary front. Areas of heavy rainfall as well as severe weather will be possible, although the specific areas affected will likely depend on convection-scale processes with low predictability in the medium range. Farther west, heights should fall across the Northwest Sun-Mon as the aforementioned southern component of the Rex Block shifts eastward. A vigorous Pacific cold front should push inland along with the height falls, bringing dropping temperatures and widespread precipitation (rain and high elevation snows) to areas from central California and the Pacific Northwest eastward into the northern Rockies. High temperatures by Sun-Mon are forecast to be 5 to 15 deg below average across portions of the West Coast and interior Northwest. Elsewhere, the trailing end of a stationary frontal boundary is forecast to linger in the vicinity of South Florida and the Bahamas through late in the week, before an area of low pressure along the boundary forms and moves northeastward away from the state. This boundary is expected to focus showers and thunderstorms, with the potential for locally heavy rains across South Florida Thu and Fri, before activity begins to shift east of the state over the weekend. The National Hurricane Center and Tropical Analysis and Forecast Branch (TAFB) are monitoring this area for potential development as a sub-tropical or tropical system by this weekend. The late afternoon fronts update will reflect the forecast evolution of the system from the 17Z coordination call today. ...Guidance/Predictability Assessment... The 00Z ECMWF seemed to be the deterministic solution that fit best within the center of the ensemble spread during most of the extended period. The 00Z/06Z GFS and 00Z Canadian offered some additional details for the Thu-Fri period but were weighted much less or not all all, respectively, by the end of the period. Off the Southeast coast, a middle ground solution between the more aggressive GFS and less aggressive ECMWF solutions was preferred given the timing uncertainty in development. By next Sun/Mon, the 00Z ECENS and 06Z GEFS ensemble means were weighted more heavily as differences in the northern stream across Canada have increased in the past day or so, which at least tangentially affect the forecast progression along the northern tier. Overall, the longwave pattern remains in good agreement. Fracasso/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 precipitation across portions of California, Sat-Mon, May 16-May 18. - Heavy rain across portions of California and the Pacific Northwest, Sat-Mon, May 16-May 18. - Heavy rain across portions of the Central/Southern Plains, the Mississippi Valley, the Great Lakes, the Ohio Valley, and the Northeast, Thu-Sun, May 14-May 17. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Fri-Sat, May 15-May 16. - Heavy rain across portions of the Southeast, Thu-Fri, May 14-May 15. - Flooding occurring or imminent across portions of the Middle/Lower Mississippi Valley, the Great Lakes, and the Northern Plains. - Much above normal temperatures across portions of the Central/Southern Rockies and the Central/Southern Plains, Mon, May 18. 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