Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Tue May 18 2021 Valid 12Z Fri May 21 2021 - 12Z Tue May 25 2021 ...Southern Plains Heavy Rain/Convection Threat lingers into late week... ...East-Central U.S. Heat... ...Cold and Unsettled West with Northern Rockies Snows... ...Guidance Evaluation and Preferences... The WPC medium-range product suite was derived days 3-5 (Fri-Sun) from a composite of well clustered guidance of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models. Added 18 UTC GEFS and 12 UTC ECMWF ensemble means days 6/7 as forecast spread increases. WPC continuity is well maintained and is in line with newer 00 UTC guidance. ...Weather Highlights/Hazards... The lower 48 pattern will significantly amplify as highlighted by troughing over the West, with lead south-central Plains troughing blocked by an amplified ridge out from the East. Ridge amplification offers substantial warming for the Midwest and the East. Some record high minimum and maximum values will be possible, with temperature anomalies 10-20F above normal. Meanwhile, an ongoing heavy rainfall event should persist into late week with continued but slowly diminishing focus over the Southern Plains. WPC QPF shows this trend, but blocky flow and deep pooled Gulf moisture suggests some downpours may produce more hefty local amounts and runoff issues. Amplified upper troughing over the West favors some moderate precipitation for the north-central Intermountain West/Rockies, including an enhanced snow focus over the northern Rockies. This occurs as post-frontal conditions cool to 10-20F below normal across the Intermountain West/northern Rockies. Downstream, ejecting western trough impulses and some energy lift from the southern Plains will shear northward over the central U.S. on the western periphery of the east-central U.S. ridge. This conduit will support periods of enhanced convective rains as moisture and instability are focused along a slow-moving and wavy front across the north-central states. Some activity may spill over the Great Lakes to the backdoor front tempered Northeast. Schichtel 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