Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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 Eastern Seaboard... ...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. 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. First, the 00Z/06Z model cycle trended toward a different evolution of energy within the eastern trough compared to previous cycles. Namely, at the start of the period Thursday-Friday, an upper low present over southeastern Canada now looks to shift northeast away from the forecast domain (though with varied timing--GFS suite faster). But weaker energies west of this low may dive south and are the ones to spin up an upper low in the Southeast by the weekend and lingering into early next week--at least this is what the bulk of more recent guidance has. In previous model cycles the original southeastern Canada low seemed to be the one to linger over the Northeast and possibly direct moisture there. CMC runs from 00Z and the newer 12Z run maintain this original low longer, but the GFS/ECMWF/UKMET are showing this newer trend. If this upper low forms and spins over the Southeast, this could produce a surface low near the Southeast Coast and direct moisture into the Mid-Atlantic region that could be long-lasting late this week into early next week. The WPC forecast trended up with QPF in the Mid-Atlantic region though not as high as some model guidance, given this model trend is pretty new. This evolution makes it less likely an associated surface low will have any tropical characteristics, and this is consistent with the cyclone phase space diagrams from multiple models showing an asymmetric cold-core low. The 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. With the upper low potentially setting up over the Southeast, the ridge to its west may break off an upper high to the low's north early next week. Meanwhile in the West, there are some differences late week with the potential for a low to close off within the trough or not, but the axes of the trough are generally similar. By this weekend and beyond though, models diverge quite a bit as differences in energy tracking out from the Gulf of Alaska leading to horrendous run to run continuity and predictability. In general though, additional troughing is likely over the West, as shown by the more agreeable ensemble means, with some unsettled weather continuing. Did think the 00Z ECMWF may have been an outlier by early next week with holding a strong upper low offshore, and the 12Z ECMWF has trended away from that solution and closer to other models/ensembles. Given these model differences, the WPC forecast utilized a multi-model mainly deterministic blend early in the period, but transitioned fairly quickly to an ensemble mean-heavy blend by days 6-7 considering the increasing model spread and uncertainty. The incoming 12Z model cycle is generally following the 00Z/06Z model trends in the East by showing a Southeast upper low, adding a bit of confidence in this pattern, but prefer to see another model cycle or two before feeling comfortable with too many forecast changes. ...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 recent forecast keeps heavy rainfall offshore of the Carolinas on Thursday with some potential for heavy amounts to reach the North Carolina coast on Friday. Then ample moisture may be 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 15-20 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 central to southern Rockies/Plains as well with moisture/instability pooling ahead of a general dryline pattern 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. Since moisture levels will be above average and moisture should be pooling ahead of slow-moving fronts/boundaries, plan to introduce large Marginal Risks in the Days 4-5 (Thursday-Friday) ERO 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. Tate/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