Extended Forecast Discussion NWS Weather Prediction Center College Park MD 425 PM 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 East Coast/Florida... ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles remain in decent agreement that a warm upper ridge will shift very slowly eastward over the Plains before nudging toward the Mississippi Valley/Midwest by the weekend. Meanwhile, fairly amplified troughs will be anchored along both the West and the East Coasts, resulting in little overall motion to the entire synoptic pattern across the U.S. A notable trend shown by the deterministic model runs since yesterday is the introduction of the idea of a deeper closed low forming between the Great Lakes and New England near the southern end of an amplified trough. The presence of this upper low would enhance the frontal characteristics along the East Coast and thereby increase the chance of interaction with another upper low that is forecast to form in the vicinity of the Southeast/Florida late this week. Uncertainty remains 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 two upper lows (or the merger of the two) to push onshore from the east coast of Florida to the Southeast and possibly farther up the East Coast as we head into the Memorial Day weekend. Along the West Coast, models have settled into the idea of a closed low moving slowly across California for the latter half of this week. The next upper trough/low is forecast to reach the Pacific Northwest by the Memorial Day weekend with quite a bit of uncertainty in its depth. This has not had a large impact on the location of the West Coast mean trough axis on the ensemble means however. 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 on the 06Z GFS and GEFS mean. This overall solution maintains broadly good WPC product continuity. ...Weather/Hazards Highlights... It remains the case that a slow-moving and southern stream separated upper trough over Southeast for much of this 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. Meanwhile, with the models trending toward the scenario of a deeper upper low between the Great Lakes and New England, deterministic guidance is now indicating the possibility of a slug of heavy rain across northern New England by Thursday on the southeastern periphery of the deepening upper low. The latest deterministic guidance from the 12Z ECMWF remains in favor of this heavy rain area. Given the compact nature of this upper low and the rather sharp front that the upper low is interacting with, a marginal risk of flash flooding has been introduced on Day 4 across interior northern New England. This area, as well as all along the East Coast, will need to be monitored for additional model adjustments in the coming days. 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 this 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 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. 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 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