Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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-late week... ...Developing heat for the East-Central U.S.; Cold/Unsettled with high-elevation snow over the West/Rockies... ...Guidance Evaluation and Preferences... Models and ensembles continue the trend of establishing an amplified upper-level pattern across the U.S. with a deep trough moving into the western U.S. and a warm ridge building over the eastern U.S. Downstream from the deep upper trough, there continues to be a general model trend for a faster northern stream flow across the northern tier states into southeastern Canada. Meanwhile, the strengthening of the deep-layer warm high in the East will continue to keep the stream of Gulf moisture open for the Plains states especially for eastern to southern Texas on Friday, followed by a shift in the rain axis toward the southern High Plains during the weekend. There is also the tendency for the rain to become more diurnally-driven with time. Farther north, the deep upper trough will keep a good chance of wet snow for the higher elevation of the northern Rockies especially on Thursday to Friday. By early next week, models have recently trended toward the formation of an occluded low over the northern Plains as the upper trough exits the northern Rockies. The WPC medium-range forecast package was based on the consensus of the 00Z ECMWF/00Z EC mean, the 06Z GFS/GEFS mean, as well as some contribution from the 00Z CMC and CMC mean. Good WPC continuity was maintained. ...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 late this week will coincide with substantial warming from the Midwest towards and especially in the East where some record high minimum and maximum values will be possible, with temperature anomalies 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 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 support periods of enhanced convective rains as moisture and instability are focused along a slow-moving and wavy front across the north-central U.S. Kong/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