Extended Forecast Discussion NWS Weather Prediction Center College Park MD 322 PM EDT Fri May 28 2021 Valid 12Z Mon May 31 2021 - 12Z Fri Jun 04 2021 ...Record highs likely in parts of the West next week... ...Heavy rain potential over portions of the Plains... ...Overview... The medium range period will begin Sunday with a possibly closed mid-upper low over the eastern U.S., with a surface low pressure/frontal system in the western Atlantic spreading rain and cool temperatures to parts of the East Coast. Meanwhile, periods of troughing should affect the north-central U.S. southwestward toward the Four Corners/Desert Southwest, leading to several chances of rain and cooler than normal temperatures with abundant cloud cover in central and southern portions of the Plains for the first half of the week, shifting east as the week progresses. An upper ridge along the West Coast will cause warm/hot and potentially record-breaking temperatures for the Pacific Northwest and northern California into the Great Basin. ...Guidance Evaluation and Preferences... The 00Z/06Z guidance cycle was in generally good agreement with the pattern described above, but with typical differences in the lower predictability details of the pattern. Most guidance shows the eastern upper low as being closed on Sunday before rejoining with the main flow by Monday, but the 06Z GFS was the exception. Nevertheless, a multi-model deterministic blend was able to be used for the early part of the medium range period, including the 00Z GFS/ECMWF/CMC/UKMET, which also led to good continuity with previous WPC forecasts. By Tuesday through Thursday, shortwaves moving through the central CONUS trough had some spread amongst guidance, and there was model variability with the potential for energy to split off and form a separate upper low around Baja California or not--which also affects the timing of the main trough shifting eastward. Better agreement was seen with the upper ridge affecting the West and then troughing sneaking into the Pacific Northwest by Thursday. For the latter part of the forecast period, trended toward the GEFS and EC ensemble means to reduce reliance on any particular model's smaller scale features. ...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. Rain and scattered thunderstorms are expected to spread eastward into the Mississippi and Ohio/Tennessee Valleys, but with less certainty on any areas of focused heavy rainfall totals. These areas may welcome at least some of the rainfall as little has been observed in the past week or two. Along the East Coast, the offshore frontal boundary and low pressure is likely to bring showers to the region early in the week. 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 in the Northeast. Meanwhile, the West Coast should see little to no precipitation given the ridge, but coastal Washington may see some scattered showers as the upper trough and cold front move in next Wednesday into Thursday. 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 the West Coast states. As the cold front comes ashore, the warm temperatures will push through the Great Basin to the Divide next Wed-Thu. 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 through much of next week. Tate/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 Hazards: - Heavy rain across portions of the Southern Plains, Mon-Wed, May 31-Jun 2. - Flooding possible across portions of the Central Plains, the Southern Plains, and the Northern Great Basin. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Southern Plains, and the Northern Rockies. - Flooding likely across portions of the Central Plains and the Middle Mississippi Valley. - Much above normal temperatures across portions of California, the Northern/Central Great Basin, the Pacific Northwest, and the Northern Rockies, Mon-Thu, May 31-Jun 3. 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