Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 AM EDT Mon May 17 2021 Valid 12Z Thu May 20 2021 - 12Z Mon May 24 2021 ...Southern Plains Heavy Rain/Convection Threat lingers to mid-later week... ...Hot for the East-Central U.S. and Cold/Unsettled over the West/Rockies... ...Guidance Evaluation and Preferences... The WPC medium range product was primarily derived days 3-5 (Thu-Sat) from a composite blend of reasonably well clustered guidance from the 18 UTC GFS/GEFS mean and 12 UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of Models in a pattern with above normal predictability. Opted for a larger focus on the more run to run consistent ensemble means by days 6/7 amid slowly growing forecast spread/uncertainty. This maintains good WPC continuity. The OO UTC models are more in line days 6/7 as the GFS/ECMWF now better align with holding onto ample western U.S. upper troughing longer than their 12/18 UTC runs. This better matches the ensemble means and this latter period trend seems reasonable considering flow amplification. ...Weather Highlights/Hazards... Models and ensembles still agree that upper flow pattern over the lower 48 states will significantly amplify over the next few days. The upper pattern will be highlighted into midweek by an emerging digging trough into the West and troughing over the south-central Plains increasingly blocked by an amplified and warming ridge building out from the East. Ridge amplification into later week will coincide with substantial warming this week from the Midwest to the especially the East where some record high minimum and maximum values will be possible as temperature anomalies build upwards to 10-20F above normal. Meanwhile, an ongoing but transitional heavy rainfall event should persist through mid-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. Digging of amplified upper troughing over the West will support widespread moderate precipitation from the Pacific Northwest and Sierra through the north-central Intermountain West/Rockies, including some mountain enhanced snows to focus over the Northern Rockies. Ahead of the Pacific front that moves into the West, early week maximum temperatures 10-20F above normal from the Northwest U.S. through the northern Rockies/Plains will abruptly cool with pattern amplification and wavy cold frontal passage to result in 10-20F below normal temperatures centered across the Intermountain West. 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 an anchoring east-central ridge. This conduit will supports period of enhanced convective rains as moisture and instability focus along a slow moving and wavy from over the north-central U.S.. 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