Extended Forecast Discussion NWS Weather Prediction Center College Park MD 357 PM EDT Fri May 19 2023 Valid 12Z Mon May 22 2023 - 12Z Fri May 26 2023 ...Heavy rain threat possible late next week from northeastern Florida to portions of the southeastern U.S... ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles offer reasonably good agreement for an upper ridge to edge ever so slightly east into the High Plains next week as an upper trough along the East Coast lifts and retreats into the Canadian Maritimes. As the trough lifts, ensemble means from the GEFS, EC mean, and CMC generally show good agreement that a piece of the vorticity associated with the trough will break off to the west or northwest of Florida as the vorticity interacts with the slow-moving front across Florida. The 00Z ECMWF agrees well with its own ensemble means regarding this interaction, while keeping the entire system moving rather slowly near/over Florida through late next week. On the other hand, the GFS has not settled into a stable solution for this system so far. The 12Z run did attempt to push the system toward the Carolinas by next Friday while establishing a stronger high pressure ridge to the north and northeast in better agreement with the ECMWF. The GEFS has continued to predict a pattern in agreement with the ECMWF and so has the CMC and CMC means. Meanwhile, ensemble means continue to support a slow-moving upper trough near the West Coast and into the Pacific Northwest with uncertainty in the timing and amplitude of the possible formation of a closed low toward the latter part of next week. A composite blend was used to compose the WPC forecast package based on 40% from the 00Z ECMWF/EC mean, 20% from the 00Z CMC/CMC mean, with the remaining portions based mainly on the 06Z GEFS mean. Only minimal GFS was used in this forecast cycle due to incompatibility with the rest of the guidance from day 5 onward. This overall solution maintains broadly good WPC product continuity. ...Weather/Hazards Highlights... Given the consensus among latest model guidance in support of a slow-moving/separated upper trough over southeast U.S./Florida late next week, heavy rain is proposed late next week from northeastern Florida to portions of the southeastern U.S. with a surface low forecast to be centered near the east coast of Florida, possibly losing frontal characteristics slowly with time. Strong easterly onshore flow can be expected to set up north of the surface low under such scenario. There could also be coastal wave developments farther up the East Coast but with plenty of uncertainty. Otherwise, the aforementioned upper ridge position should support summertime temperatures with values upwards to 15-20 degrees above normal through early next week should shift slowly east with time across the northern Plains and eventually the Midwest later next week as tempered locally by scattered and potentially strong thunderstorm activity both with ejection of impulses from the West and interaction with northern stream system/frontal approaches from southern Canada. Locally heavy thunderstorm activity may focus heavy local downpours down over the southern Plains as well with moisture/instability pooling with southern stream impulses. Above to much above normal overnight temperatures and a pattern favoring scattered to widespread showers and thunderstorms are also expected to linger all next week over much of interior West through the Rockies in moistened flow channeled between West Coast upper troughing and the downstream central U.S. upper ridge. Kong/Schichtel 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 - Heavy rain across portions of the Southeast, Tue, May 23 and Fri, May 26. - Heavy rain across portions of the Southeast, Tue, May 23. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Upper Mississippi Valley, the Southern Plains, the Northern Rockies, and the Northern Great Basin. - Flooding likely across portions of the Central Rockies, the Upper Mississippi Valley, the Central Great Basin, the Northern Rockies, and the Northern Great Basin. - Much above normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Great Basin, California, and the Pacific Northwest, Mon, May 22. - Much above normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Tue, May 23. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml