Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Sun May 16 2021 Valid 12Z Wed May 19 2021 - 12Z Sun May 23 2021 ...Heavy rain threat for the Southern Plains/Lower Mississippi Valley this week... ...Guidance Evaluation and Preferences... The WPC medium range product suite was primarily derived days 3-5 (Wed-Fri) from a composite blend of reasonably well clustered mid-larger scale guidance of the 18 UTC GFS/GEFS and 12 UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of Models. Smaller scale system differences and convective rainfall focus details remain less certain and growing forecast spread through medium scales next weekend prompted usage of mainly just the ensemble means and NBM. By then, the 18/00 UTC GFS runs move to the more progressive side of solutions and the 12/00 UTC ECMWF the less progressive side of the solution envelope. The aforementioned WPC forecast plan focusing on the ensemble means maintains good WPC product continuity near the envelope composite. This seems prudent given the recent model run-run fluctuations at these longer time scales. ...Weather Highlights/Hazards... Deep layered moisture from the Gulf of Mexico will return into an emerging and slow moving central U.S. systems this week. This will occur in downstream response to the slow ejection and passage of a southern stream upper-level trough/low energies forecast to exit the southern Rockies to lift up over the Plains. Expect a multi-day period with a threat of significant rainfall along with periods of increased instability and strong to severe convection. Locations and magnitude of heaviest rainfall will likely vary on a day-to-day basis and some specifics may take until well into the short range to be resolved, but the guidance signal for a wet pattern mainly over the south-central U.S. has been consistent. Meanwhile, deep moisture and vorticity underneath a building upper ridge over the East may support some heavy downpours for southern Florida and into the Gulf of Mexico. Ridge amplification will coincide with substantial warming this week from the Midwest to the especially the East where some record high minimum and maximum values are possible as temperature anomalies build upwards to 10-20F above normal. Mid-later next week 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. Ahead of the Pacific front that moves into the West, early week maximum temperatures 10-20F above normal from the Northwest U.S. though the northern Rockies/Plains will abruptly cool with pattern amplification and wavy cold frontal passage that may result in 10-20F below normal temperatures centered across the Intermountain West. 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