Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Wed May 26 2021 Valid 12Z Sat May 29 2021 - 12Z Wed Jun 02 2021 ...Record highs possible in the Pacific Northwest/Northern California next week... ...Heavy rain potential over portions of the Plains... ...Overview... The medium range period will begin with a shortwave upper trough moving across the Great Lakes region over the weekend, with a surface frontal system pushing off the East Coast and sinking well southward over the eastern half of the country into the northern Gulf of Mexico. The western end of this front should linger over the Plains while another front moves in from the west/northwest as rounds of upper troughing move through the southwestern to central U.S., helping to promote episodes of showers and storms that may contain locally heavy rainfall. Farther west, upper ridging is likely to build into the Pacific Northwest, leading to abundant warmth there by next week, while the eastern U.S. cools down through the weekend into next week after a recent warm spell. ...Guidance Evaluation and Preferences... The 00Z/06Z model cycle was in generally good agreement with the pattern described above, but with differences in details of the flow. By Sunday into Monday, some differences arose with whether the Great Lakes energy cut off from the main flow in eastern Canada along with some minor timing issues as the shortwave lifts, but not too unreasonable. It appears guidance may be clustering on a slower solution for the surface front/low given the shortwave track compared to previous forecasts. With the central to southwestern U.S. trough, once again there are some questions with stream separation and some potential for closing off mid-level lows around northern Mexico (with the 00Z ECMWF and CMC in favor). Upstream ridging into the Pacific Northwest (at least +2 sigma) may be squeezed by next Wednesday as a Pacific front edges into Washington. Overall, through early next week, a multi-model deterministic blend was able to be used, with somewhat increasing weight on the ensemble means by the end of the period, which helped smooth out differences between individual models. ...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. It will take additional time to monitor potential overlap of heavy rainfall potential with areas that have seen above normal rainfall over the past week or so. The West Coast should see little to no precipitation given the ridge, but areas near and east of the Great Basin to the Divide may see at least scattered showers as the upper trough moves in. 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 current expectations are for slower frontal movement compared to previous forecasts; thus, rainfall forecasts were increased onshore as the front lingers, but the heaviest rain amounts still look to be offshore. Temperatures will be cool in the Great Lakes/Northeast/Mid-Atlantic Saturday with northeast winds over most areas. Expect readings to be 10-15F below normal in general (many areas only in the 60s). Though the cold front may dip into the Gulf of Mexico by Sunday, this may only cool temperatures in the Southeast to a few degrees below normal (low 80s but dew points in the 50s to low 60s). The Pacific Northwest can expect a warming trend with 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. On the other hand 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. 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 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