Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue May 23 2023 Valid 12Z Fri May 26 2023 - 12Z Tue May 30 2023 ...Watching coastal front/low for the possibility of bringing heavy rain to the Eastern Seaboard... ...Pattern Overview and Guidance/Predictability Assessment... Guidance generally agrees that a warming upper ridge will shift very slowly eastward over the Plains before nudging toward the Mississippi Valley/Midwest by the weekend. Meanwhile, amplified troughs may prove tough to dislodge along both the West and the East Coasts, resulting in little overall motion to the entire synoptic pattern across the U.S. through early next week. Within this blocky pattern, there have been considerable differences from model to model and run to run over the past couple of days. Upstream of the ridge over the west-central U.S., differences linger late week into the weekend with the potential/positioning for a low to close off within the mean trough, but the axes of the trough are generally similar. Forecast spread increases from the weekend into early next week, but run to run continuity has improved to a degree compared to the past few days. While predictability is still only below to near average with embedded systems, the larger scale pattern with lingering channeled moisture between the upper trough and downstream upper ridge should favor a protracted wet and unsettled period. Quite a bit of this widespread convective activity will also spill over the Rockies and dig into the Plains as aided by ejecting impulses and overall favorable upper diffluence and instability. Downstream of the ridge over the East, two main forecast camps have emerged in guidance. Recent runs of the Canadian/NAVGEM and Canadian ensembles allow upper trough/low energy to linger into the holiday weekend over the Northeast and eventually interact more with the Southeast closed upper trough to subsequently affect the evolution of a weather focusing coastal low. This is in stark contrast to recent runs of the GFS/ECMWF/UKMET and GEFS/ECMWF ensembles ejects into the Canadian Maritimes Friday and holds a slow moving closed upper low back in separated southern stream flow over the Southeast/southern Mid-Atlantic. The peristence of this feature would favor a surface low near the Southeast Coast and direct moisture into the Mid-Atlantic region that could be long-lasting through the holdiday weekend. Rainfall amounts are trending upwards and creeping inland. This evolution makes it less likely an associated surface low will have tropical characteristics as per NHC, and this is consistent with the cyclone phase space diagrams from multiple models showing an asymmetric cold-core low. A separate surface low that the National Hurricane Center is tracking for any possible tropical/subtropical development looks to eject from the Bahamas north and northeast away from the lower 48 by Thursday-Friday. Overall, the WPC medium range product suite was primarily derived from a composite blend of best clustered guidance of the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/UKMET/ECMWF ensemble mean and a compatible 01 UTC National Blend of Models (NBM) through the forecast period in a pattern with below average predictability. Despite uncertainties, this solution offers a reasonably good match with WPC product continuity for much of the lower 48 and fits best with latest NHC/WPC offshore system collaborations and expectations. The 00 UTC guidance cycle maintained prior trends. ...Weather/Hazards Highlights... Showers and storms are likely over Florida late this week with the potential for a developing surface low system. Rain may spread up parts of the Eastern Seaboard dependent on the low track. The latest forecast offers increasing potential for heavy amounts to reach the coastal Carolinas by Friday, then lingering. The WPC experimental Day 4/5 Excessive Rainfall Outlook (ERO) plans to add a "marginal" risk area to address. Then ample moisture may be further directed into the Mid-Atlantic this weekend on the east/north sides of the potential 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 during the period 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 diffluence 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 maintain and extend broad "marginal" risks for the new Days 4-5 (Friday-Saturday) ERO periods 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. 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