Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Mon Apr 26 2021 Valid 12Z Thu Apr 29 2021 - 12Z Mon May 03 2021 ...Heavy rain threat Lower Mississippi/Tennessee Valleys and Central Gulf Coast mid-later week... ...Guidance/Predictability Assessment and Preferences... The larger scale pattern evolution is reasonably well represented by guidance despite split flow and uncertain progression days 3-7. However, recent models and ensembles have been offering much less than stellar run to run continuity through medium range time scales with the timing and interactions of small to mid scale systems in both the northern and southern stream flows. This has led to erratic system trends often contrary to a particular known bias of a given guidance source. Slower solutions are consistent with initially amplified ridging over the West Coast, but embedded system wavelength spacing and stream interactions tends to favor progression. Given uncertainty and with an eye toward best possible WPC product continuity, opted to take a middle of the road approach with a blend of the 18 UTC GFS/GEFS mean and 12 UTC ECMWF ensemble mean that are near the center of the full envelope of recent model solutions. Newer 00 UTC guidance does not show a strong common trend with the GFS/GEFS faster, the ECMWF/UKMET slower and with only a modest change from the Canadian. ...Weather/Hazards Highlights... A heavy rainfall event is expected to already be in progress by Thursday from eastern TX to the Lower MS/TN Valleys as Gulf moisture and instability pools near a wavy surface front, albeit with uncertain main southern stream upper trough approach. There is still a wide range of possible outcomes in guidance, from locally enhanced rainfall settling longer over Texas and the central Gulf Coast region to a faster progression across The South with a drier trend over the Plains. Most of the Northwest will be dry Thursday as the best flow of moisture focuses on western British Columbia/Vancouver island, with light amounts of precipitation extending into the Olympics. The approach/arrival of a Pacific shortwave and leading front will spread mostly light to moderate precipitation from the Pacific Northwest into northern Rockies late this week through the weekend. The strong upper ridge building into the West past midweek will spread much above normal temperatures with 15-20+F above normal anomalies Thursday into Saturday across much of the West and High Plains. Some locations may reach daily records for highs/warm lows. The approaching Pacific trough aloft and leading cold front will start a pronounced cooling trend back toward climatology by next weekend. 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