Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Sat May 20 2023 Valid 12Z Tue May 23 2023 - 12Z Sat May 27 2023 ...Heavy rain threat possible later next week for the Southeast/Florida... ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles now generally agree that a warming upper ridge next week will shift slowly eastward over the Plains before nudging into the Mississippi Valley/Midwest. 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 in response to the approach of a new digging east-central Canadian upper trough. Separated southern stream upper trough energies will meanwhile settle down near California and well downstream in this flow over the Southeast/Florida. 12 UTC ECMWF/UKMET solutions seem the most reasonable fit with the overall pattern across much of our fine nation and cluster best with GEFS/ECMWF ensembles for Tuesday into Wednesday morning. The newer 00 UTC ECMWF and UKMET runs remain in line and the 00 UTC Canadian has also come into the fold. However, recent Canadian runs have been producing scattered and run to run inconsistent excessive small scale QPF maxes. While these bullseyes may represent local potential given the slowed and amplified warm season flow pattern, predictability does not seem to warrant such precision, especially into longer time frames. Discounted past GFS runs that have been offering less likely upper trough/low stream phasing scenarios over the Canadian Plains and too slow subsequent downstream progressions through early-mid next week. However, the 18 UTC GFS and now especially the latest 00 UTC GFS has backed off from this less likely scenario, bolstering forecast confidence. Later, the 12 UTC ECMWF model then becomes a less likely outlier bringing much more ample upper trough energy eastward from the Northwest to the Plains later next week than the full envelope of ensemble and model solutions that instead dig height falls more into a western U.S. upper trough position. The latest 00 UTC ECMWF model has trended strongly away from the prior 12 UTC ECMWF run. Meanwhile, the ECMWF ensemble mean western U.S. upper trough by then is much deeper than the ECMWF model, but not quite as deep as the GEFS mean. Prefer a GEFS/ECMWF ensemble mean blended solution in this later time frame. Otherwise over the country, opted to apply less blend weighting to the GEFS mean to add emphasis to the ECMWF ensemble mean solution that both offers a seemingly favorable faster progression of leading frontal rains/cooling Thursday into New England and also shows a deeper lingering Southeast U.S. upper trough best consistent with earlier week trends. ...Weather/Hazards Highlights... A slow-moving and southern stream separated upper trough over southeast U.S. next week seems to broadly favor periods of heavy maritime rains but also onshore from mainly Atlantic coastal portions of the Southeast/Florida as series of surface frontal lows rotate in proximity to tap deepened pooled moisture, but details remain uncertain. Strong onshore flow can be expected to set up north of these surface lows under such scenario, but no experimental WPC Day 4/5 ERO threat areas are planned to be issued at this time. There are also some guidance indicated scenarios that show less likely coastal wave developments farther up/off the East Coast, but with plenty of uncertainty. Otherwise, the aforementioned main upper ridge position slow translation should support summertime temperatures with values upwards to 15-20 degrees above normal into early-mid next week that should spread slowly east with time across the north-central U.S. to the Midwest into later next week, all only 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 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 uncertain later next 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