Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Thu May 25 2023 Valid 12Z Sun May 28 2023 - 12Z Thu Jun 01 2023 ...Coastal low offers heavy rain and coastal/maritime threats for the Southeast/Mid-Atlantic... ...Broad multi-day thunderstorm and local heavy rain threat continues for the Plains... ...Pattern Overview and Guidance/Predictability Assessment... Initially, much of the eastern half of the lower 48 is in a Rex Block pattern (as a closed ridge settles in over the Great Lakes while a cutoff low lingers over the Southeast) and with a trough over the West. A low pressure system is expected to lift northward near the Southeast coast which will spread heavy rain and wind to the Carolinas. The National Hurricane Center has been monitoring the forecast for this system but keeps the chances of any tropical development very low. Mean troughing will be in place over portions of the West, centered on the West Coast, through at least the mid-period, with weak flow aloft over the center of the country with the northern stream shunted northward into Canada. Subtle shortwaves traverse through a weak southern stream ridge over the Southern Plains, which will likely help promote scattered precipitation chances across the area. Later in the period, the cutoff low over the Southeast slowly translates northeastward up the East Coast. The mean troughing over the West Coast shifts a bit westward as the northern stream amplifies northwestward with a ridge building over the Northern Rockies. The latest guidance had a fair amount clustering during the first few days of the extended period and the overall handling of how each system/feature evolves. The WPC forecast was comprised of deterministic GFS/ECMWF/CMC/UKMET, ensemble mean (GEFS/ECens) guidance and the NBM. ...Weather/Hazards Highlights... Heavy rain, gusty winds and beach related hazards can be expected this Holiday Weekend as a low pressure system off the Southeast coast tracks inland over the Carolinas. Rain prior to the extended period will likely raise soil saturation and increase sensitivity. With rain likely to continue into early next week the Day 4/5 Marginal Risks of Excessive Rainfall where maintained for the potential for at least isolated instances of flash flooding. There is a degree of uncertainty with both the exact track of the low, and thus footprint of heavy rainfall, as well as with the expected amounts, and it is possible that a greater threat of flash flooding may continue from Saturday into Sunday. To the north, upper-level ridging over the Great Lakes will push high temperatures well above average by as much as 15-20 degrees from the Northern Plains through the Great Lakes and into the interior Northeast next week, eventually spreading further eastward into the Northeast and Mid-Atlantic. Frontal activity along the northern stream may bring the chance for some isolated to scattered storms, particularly for the Northern Plains/Upper Midwest, but conditions should otherwise remain dry. Depending on the evolution of broader height rises over the central/eastern CONUS, temperatures could trend even warmer more broadly over the eastern half of the country. Daily rounds of showers and thunderstorms are expected to fire up across the High Plains/Plains within the moist, southeasterly upslope flow. Some of these storms may have the potential to produce moderate to heavy rainfall over the region. A Marginal Risk of Excessive Rainfall is already in effect for Sunday as moisture remains anomalously high over the region and weak flow aloft leads to slow storm motions. A Marginal Risk was raised for Day 5 over portions of Texas and Oklahoma where there will still be the potential for higher rainfall intensity/totals given the abundant moisture and forcing over the region. The subtle height rises more broadly over the central/eastern CONUS and the tempering of low-level jet activity overnight should decrease moisture transport and storm coverage/intensity, bringing down the threat of locally heavy rainfall next week. Mean troughing will remain over the West at the start of the period, with the subsequent stormy pattern over the Northern Rockies/Great Basin this week continuing into the weekend and next week. Storm coverage and rainfall amounts should trend downward as troughing aloft begins to shift west and ridging builds northwestward over the Northern Rockies, but an isolated instance or two of flash flooding in terrain sensitive areas or burn scars will remain possible. High temperatures across most of the West will remain around average before a warming trend over the Northwest as the ridging builds in over the region. Coastal California will be cooler as a cutoff low sits over the coast, with onshore flow in place. Campbell/Putnam 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