Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM EDT Wed May 24 2023 Valid 12Z Sat May 27 2023 - 12Z Wed May 31 2023 ...Coastal low offers lingering heavy rain and coastal/maritime threats for the Southeast/Mid-Atlantic... ...Broad multi-day thunderstorm and local heavy rain threat from the Rockies to Plains... ...Pattern Overview and Guidance/Predictability Assessment... An warming upper ridge/high will spread from the central U.S. to atop the Midwest and northern Mid-Atlantic over for the Memorial holiday weekend as part of a Rex Block, additionally rebuilding into next week with mean troughing anchoring over the West Coast and Southeast, resulting in little overall motion to the entire synoptic pattern across the U.S. through next midweek. Within this blocky pattern, there have been considerable differences from model to model and run to run earlier this week, but recent model guidance seems to be stabilizing. Confidence has increased for a closed/cutoff Southeast upper low to develop and linger as an upper high cuts off for a time north of it within a broader southwest to northeast oriented ridge. A surface low looks to develop offshore of the Southeast coast. The surface evolution still offers differences with position and ultimate ejection. Duration, proximity and any reformations offer coastal/maritime threats and suggest heaviest rains will spread inland into the Carolinas/Mid-Atlantic. The surface low(s) still seems most likely to stay frontal/extratropical, but with perhaps a nonzero chance for something subtropical to come out of it for a time. Out west, upper troughing remains likely to remain a focus through this forecast period, but then various upstream energies from the Pacific and western Canada still show considerable variability in their phasing/interaction potential. However, recent guidance has become better clustered with flow evolution. Regardless of the details, broad areas from the Interior West/Rockies to the Plains are likely to see continued unsettled conditions with broad convective developments/local downpours as anomalous moisture and impulses channels between the upper trough position and the downstream upper ridge. Overall, the WPC medium range product suite was primarily derived for all areas from best clustered guidance from the 18 UTC GFS/GEFS mean and the 12 UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of Models. This maintains max WPC product continuity and the latest 00 UTC guidance versions overall remain in line for the most part with this forecast plan, at least as predictability increases to more average levels through the upcoming holiday weekend. ...Weather/Hazards Highlights... Showers and storms are likely over southern Florida into the holiday weekend with the potential for a developing surface low system, and rain is likely to spread farther north across parts of the Eastern Seaboard dependent on the low track. This forecast update continues to show that the heaviest rain is likely across the coastal Carolinas heading into the weekend, then lingering. Marginal Risks Excessive Rainfall Outlook (ERO)risk areas are in place for the Days 4-5 (Saturday-Sunday) given the potential for heavy rain. One of the more uncertain parts of the forecast is that guidance varies on how far inland to spread the precipitation. If recent trends continue, heavier rain may be seen into interior portions of the Carolinas/Mid-Atlantic as well. Ample moisture may be further directed into the Mid-Atlantic this weekend on the east/north sides of the Southeast upper low forming. Amounts and placement remain uncertain at this point though. Beach hazards for the upcoming Memorial Day weekend may not be ruled out either considering this pattern, with potential for gusty winds and unsettled maritime conditions. Showers and storms may form farther inland across the Tennessee Valley and vicinity too. With the upper trough/low and the potential cloudiness and rain chances, temperatures are likely to be cooler than normal on average through the weekend across much of the East. Otherwise, the aforementioned main upper ridge position slow translation should support summertime temperatures with values upwards to 10-15 degrees above normal into late 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 Rockies and High Plains as well with moisture/instability pooling ahead of a general dryline pattern with southern stream impulses under favorable upper difluence and instability. 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. Since moisture levels will be above average and moisture should be pooling ahead of slow-moving fronts/boundaries, plan to have broad ERO Marginal Risks for the weekend EROs across much of the High Plains and back toward the northern Rockies for some potential for flash flooding with some downpours of heavy rain that could be slow-moving. The details and local focus will be dependent on the ultimate shape of the still quite uncertain later week upper trough positions and flow interactions, as well as smaller-scale boundaries that have little predictability at this forecast lead time. 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