Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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... Model guidance for the medium-range forecast period (12 Z Sunday - 12 Z Thursday) begins with a Rex Block pattern over the eastern CONUS as a closed ridge settles in over the Great Lakes while a cutoff low lingers over the Southeast. An associated low pressure system at the surface offshore of the Southeastern U.S. looks to track northwestward into 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. Clustering solutions and run-to-run consistency suggest most of the deterministic (GFS/ECMWF/CMC) and ensemble mean (GEFS/ECens) guidance has a good handle on the pattern evolution throughout the period in a broad sense, with most differences related to smaller-scale shortwaves and other such features that have expectedly lower predictability. However, it is worth noting that the rainfall forecast for the low pressure system over the Carolinas remains a bit more uncertain. Most of the guidance, with the exception of the 18Z GFS, agrees upon the location, but the amounts do vary a bit. Also, subtle differences in height rises over the central/eastern CONUS look to have an impact on forecast high temperatures next week. The 12Z ECMWF is noticeably warmer with temperatures than the 18Z GFS. Otherwise, more broadly, one notable difference early in the forecast period is weaker mean troughing over the West in the 12Z UKMET. This lends well to following a general model blend of the 06Z GFS and 12Z CMC/EMCWF, similar to the prior WPC forecast, but with the UKMET not included given the noted large-scale differences. A contribution from the 18Z GEFS and 12Z ECens means is added during the late period as the deterministic model solutions begin to diverge more, specifically with respect to the degree of the noted height rises over the central/eastern CONUS as well as with respect to the ridging over the Northern Rockies. The 18Z GFS is more aggressive compared to the 12Z ECMWF/CMC and the ensemble means with this progression. The run-to-run consistency in the guidance and similar guidance blend to the prior WPC forecast keeps this forecast update very similar. ...Weather/Hazards Highlights... A low pressure system off the Southeast coast is forecast to track inland over the Carolinas this weekend, bringing heavy rain along with gusty winds and beach related hazards to the coast into Memorial Day. While more significant impacts look to occur on Saturday, just prior to the current forecast period, Day 4/5 Marginal Risks of Excessive Rainfall are in place over the region as widespread, locally heavy rainfall should continue to lead to at least isolated instances of flash flooding. This is especially true for locations that see rain over multiple days contributing to wetter soils. 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. Moist, southeasterly upslope flow will remain in place over the western Plains/High Plains, leading to daily rounds of thunderstorm development and subsequent heavy rainfall chances across the region. A Marginal Risk of Excessive Rainfall is in place Sunday as moisture remains anomalously high over the region and weak flow aloft leads to slow storm motions. 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. 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