Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Mon May 22 2023 Valid 12Z Thu May 25 2023 - 12Z Mon May 29 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. In this blocky pattern, recent trends show a new closed low forms between the Great Lakes and New England from late week into the weekend. The presence of this upper low enhances the frontal characteristics along the East Coast and increases the chance of interaction with another upper low that is forecast to form in the vicinity of the Southeast/Florida late this week. While uncertainty remains too high to provide the specifics, stream interaction offers increased possibilities that enhanced rainfall just offshore with coastal low genesis could work onshore from the east coast of Florida to the Southeast and possibly farther up/off the East Coast as we head into the Memorial Day weekend. This system is also being monitorred by the National Hurricane Center for any non-extratropical development. 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. However, all bets are off with another upper trough/low is forecast to reach the Pacific Northwest by the Memorial Day weekend. There has been quite a bit of uncertainty in its depth, and latest runs even it's existence with guidance trending eratically different with energy tracks out from the Gulf of Alaska leading to horrendous run to run continuity and predictability. This has not had as large an impact on the location of the West Coast mean trough axis on the ensemble means however, and that general flow regime would allow for a continued moist/wet and unsettled pattern across much of the Intermountain West to the Rockies. Overall, the WPC medium range product suite was primarily derived from a composite blend of loosely clustered guidance of the 18 UTC GFS/GEFS, 12 UTC ECMWF/UKMET/Canadian and a compatible 01 UTC National Blend of Models (NBM) for mid-later this week in a pattern with below average predictability. Switched guidance preference increasingly to a blend of broadly compatible blend of the GEFS/ECMWF ensemble means and NBM by the weekend amid even more rapidly growing forecast spread. Despite uncertainties, this solution offers a reasonably good match wih WPC product continuity for much of the lower 48 and lends well with latest NHC/WPC offshore system collaborations and expectations. That said, strong trends from the newer 00 UTC models, especially the GFS/Canadian, are now favoring a more northward lift up over the East of more ample Southeast closed upper system energies than the ensemble means, now more in line with the 12 and now 00 UTC ECMWF. This further opens the door for a closer offshore front/low and rainfall scenario to monitor. ...Weather/Hazards Highlights... An upper low between the Great Lakes and New England offers a possibility of lingering rain across northern New England into Thursday. Meanwhile well to the south, a slow-moving and southern stream separated upper trough over Southeast this week will favor periods of heavy maritime rain just offshore the Southeast, 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 surface lows with sufficient development. It remains quite run to run uncertain in guidance as to the potential for the interaction of these two main systems that could lead to ejection of the Southeast closed low and the northward track off the East Coast of organized low pressure systems and associated wrapping rains and unsettled maritime conditions. 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 prove slow to spread slowly east with time across the north-central U.S. to the Midwest. 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 central to southern Rockies/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 positions and flow interactions. 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