Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Thu May 27 2021 Valid 12Z Sun May 30 2021 - 12Z Thu Jun 03 2021 ...Record highs likely in parts of the West next week... ...Heavy rain potential over portions of the Plains... ...Overview... Upper troughing initially in the Southwest will slowly shear eastward into Texas/lower Mississippi Valley while ridging asserts itself into the Northwest. A system in the Atlantic will lift northeastward and spread some rain over parts of the Northeast Sun-Mon before exiting on Tuesday. Temperatures will be cool in much of the East to start before moderating, but very warm/hot in the Northwest into the Great Basin where record heat is likely for some locations. Areas in the central and southern Plains will see several chances of rain and cooler than normal temperatures with abundant cloud cover. ...Guidance Evaluation and Preferences... Through the 12Z/18Z guidance cycle, the 18Z GFS and 12Z ECMWF/Canadian were in generally good agreement through the period with the GEFS/ECMWF ensemble means. This maintained an area of low pressure along the frontal boundary just off the East Coast late Sun into early Mon, not as far west as some earlier GFS runs and not as far east as the ensembles were several cycles ago. In the Northwest, GFS and Canadian were a bit quicker to bring in troughing compared to the ECMWF and most ensembles, but upstream pattern of a modest jet across the northern Pacific certainly deems that plausible. For now, consensus solution toward the ensemble means kept good continuity. Over the Plains, somewhat messy upper pattern with lower amplitude waves interacting with the stationary frontal boundary (initially two, then focusing around one) that should trend toward a less active period later in the week next week. Again a consensus served well due to lower predictability of the smaller wavelength features, contingent partially on how the southwestern extension of the Southwest upper low evolves. ...Weather Highlights/Hazards... The main focus for rainfall during the period will be over the central/southern Plains along the west end of the wavy frontal boundary and ahead of a weaker northern stream cold front. High pressure that builds in behind the latter front could lead to some areas of low level upslope flow that would help to enhance convective development over the High Plains. Some areas could see several inches of rainfall over the multi-day period, perhaps adding to flooding concerns over areas that have seen above normal rainfall recently. Others may welcome at least some of the rainfall as little has been observed in the past week for some areas. The West Coast should see little to no precipitation given the ridge, but coastal Washington may see at least some scattered showers as the upper trough and cold front move in next Wednesday. Along the East Coast, the offshore frontal boundary and low pressure is likely to bring showers to the region. Rainfall amounts will depend on the evolution of the system, but the heaviest rain amounts still look to be offshore. Nevertheless, at least some light rainfall is forecast for areas east of the Appalachians with higher chances along I-95 from NJ into New England. Cool temperatures in the Northeast/Mid-Atlantic Sunday will moderate through the rest of the week as skies clear and high pressure moves offshore. The Pacific Northwest can expect temperatures 10-25F above normal by Memorial Day into next Tue-Wed. This could approach or exceed record levels for parts of WA/OR/ID and northern CA. As the cold front comes ashore, the warm temperatures will push through the Great Basin to the Divide. The cloudy/stormy pattern setting up over the central/southern Plains and eastern slopes of the Rockies should bring highs down to 10-15F below normal by next Mon-Wed, sinking into Texas by next Thursday while moderating. Fracasso 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 - Much above normal temperatures across portions of California, the Great Basin, the Pacific Northwest, and the Northern Rockies , Mon-Wed, May 31-Jun 2. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml