Extended Forecast Discussion NWS Weather Prediction Center College Park MD 317 PM EDT Wed May 24 2023 Valid 12Z Sat May 27 2023 - 12Z Wed May 31 2023 ...Coastal low offers 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... Model guidance has converged the past couple of days on showing a Rex Block type pattern in the East, as an upper low develops atop the Southeast over the weekend while upper ridging potentially cuts off an upper high for a time to its north within a broader southwest to northeast oriented ridge. This pattern will help direct a surface low in the western Atlantic westward into the Carolinas early next week, along with the potential for heavy rain. Low pressure may linger into the western Atlantic in some form through the first half of next week. The surface low(s) still seem most likely to stay frontal/extratropical, but with perhaps a nonzero chance for something subtropical to come out of especially the first low for a time, the potential for which the National Hurricane Center is tracking. There remain some model differences with the track and timing of the surface low, though recent runs are at least are more agreeable for the first low to move inland. The GFS was really the first model to take this type of track with other models converging toward it, but now is among the faster/farther north solutions with the low track, with ECMWF/UKMET/CMC a little more agreeable. These differences are generally within reason for the forecast lead time but affect the location and distribution of heaviest QPF. Meanwhile in the West, periods of troughing are likely. More specifically, a trough axis atop the Great Basin over the weekend looks to reform a bit west over the West Coast states into next week as upstream energy may dig troughing there as the initial energy lifts into the north-central U.S./south-central Canada. Models are showing better clustering with this evolution overall, albeit with differences with potential for a closed upper low within the trough early next week. UKMET runs are perhaps a minor outlier with weaker West Coast troughing at that point. This redeveloped trough may shift slowly east, while some shortwave impulses could be present embedded within central U.S. to Midwest/Great Lakes ridging with low predictability. With more agreeable operational guidance for this forecast than in previous days on the large scale at least, the WPC forecast was able to use a blend of deterministic 00/06Z guidance for much of the forecast period, with the blending process serving to reduce individual variations between models. The latter days had gradual inclusion of the GEFS and ECMWF ensemble means to further temper model differences. This led to generally good continuity from the overnight forecast as well. ...Weather/Hazards Highlights... The low pressure system off the Southeast coast late week is likely to move inland early next week, spreading widespread rain and thunderstorms to the Carolinas and vicinity over the Memorial Day holiday weekend. While the exact low track and timing and thus the precipitation forecast is still somewhat uncertain, the potential is there for widespread heavy rain causing multiple instances of flash flooding. Thus this forecast cycle plans to introduce a Slight Risk of excessive rainfall for Day 4/Saturday from the coastal Carolinas inland toward the Piedmont toward the southern Appalachians. There are some model differences with the evolution by Sunday and for now just a Marginal Risk remains outlined for Day 5 in northern parts of the Southeast/southern Mid-Atlantic, but if model guidance starts to align better on the placement for the heaviest rain, an embedded Slight Risk could be needed in future forecast cycles. Areas of the Southeast to Mid-Atlantic could continue to see rain into Memorial Day and Tuesday but with perhaps lowering amounts. Additionally, beach hazards with potential for gusty winds and unsettled maritime conditions including rip currents are likely this Memorial Day weekend. With the upper trough/low and the cloudiness and rain chances, temperatures are likely to be cooler than normal on average through the weekend across the Southeast, with highs possibly staying as cool as the 60s, with a general moderation toward normal next week. Meanwhile farther west, scattered showers and thunderstorms are likely across the Great Basin and northern/central Rockies as enhanced moisture pools near multiple rounds of frontal systems through the weekend and into next week. The High Plains can also expect showers and storms with moisture/instability pooling ahead of a general dryline pattern that will be long-lasting. Periodic energetic impulses aloft could help support lift for these storms in moist environments with precipitable water values over the 75th, if not 90th, percentile. Marginal Risks are in place for these regions in the weekend EROs with threats for localized heavy downpours that could be slow-moving to cause flooding issues. The exact details and local focus will be dependent on shortwave impulses and smaller-scale boundaries that have little predictability at this forecast lead time though. Precipitation may temper heat locally, but overall expect generally warmer than average temperatures for the northern tier of the country by around 10-15 degrees. The Midwest/Great Lakes region could reach 15-20F above average by the middle of next week with highs in the 90s under the lingering upper high/ridge. Tate 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