Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun May 21 2023 Valid 12Z Wed May 24 2023 - 12Z Sun May 28 2023 ...Watching coastal front/low for the possibility of bringing heavy rain to the Southeast/Florida... ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles remain in decent agreement that a warming upper ridge next week will shift slowly eastward over the Plains before nudging into the Mississippi Valley/Midwest, with the entire pattern then barely moving through later next week. Early-mid next week will also see an upstream upper trough form over the Northwest as a Northeast U.S. upper trough lifts into the Canadian Maritimes. Energies separating from both of these upper troughs may amplify the upper-air pattern toward the end of next week, with the possibility of a closed low developing near California, and a broader trough/closed low developing over the Southeast/Florida. Meanwhile, near and just off the Southeast and into Florida, ensemble means continue to support an upper trough/low to linger over the vicinity later week and into next weekend. Given warm sea-surface temperatures along the Gulf Stream and the presence of a lingering front, some of the deterministic guidance has occasionally spun up a tropical system off the coast of the southeastern U.S. and tracked it toward the coast by the Memorial Day weekend. There is also the possibility for another piece of vorticity to break off over the Northeast and then interact with what may be present from Florida to off the Southeast U.S. coast. Needless to say, uncertainty is too high to provide the specifics at this point. Nonetheless, there appears to be a higher than normal chance for the heavy rain associated with the system to push onshore from the east coast of Florida to the Southeast and possibility farther up the East Coast as we head closer to the Memorial Day weekend. Overall, the WPC medium range product suite was primarily derived from a composite blend of seemingly best clustered and reasonably run to run consistent guidance of the 18 UTC GFS/GEFS, 12 UTC ECMWF/UKMET and a compatible 01 UTC National Blend of Models (NBM) for mid-later this week in a pattern with seemingly near average predictability and decent WPC product continuity. Switched guidance preference to a blend of broadly compatible blend of the GEFS/ECMWF ensemble means and NBM by next weekend amid more rapidly growing forecast spread and uncertainty. ...Weather/Hazards Highlights... It remains the case that a slow-moving and southern stream separated upper trough over Southeast for much of next week will broadly favor periods of heavy maritime rains, but also onshore from mainly Atlantic coastal portions of the Southeast/Florida as surface frontal lows rotate in proximity to tap deepened pooled moisture. Details remain uncertain and the NHC is additionally monitoring potential for any non-frontal low developments with deeper tropical moisture. Strong onshore flow could be expected to set up north of any of these surface lows with sufficient development. Otherwise, the aforementioned main upper ridge position slow translation should support summertime temperatures with values upwards to 15-20 degrees above normal into later week that should spread slowly east with time across the north-central U.S. to the Midwest into later next week. The heat may be tempered locally by scattered and potentially strong thunderstorm activity both with ejection of impulses from the West and with interaction with northern stream system/frontal approaches from southern Canada. Locally heavy thunderstorm activity may also 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, with details/local focus dependent on the ultimate shape of the still quite uncertain later week upper trough position. 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 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