Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EDT Thu May 21 2020 Valid 12Z Sun May 24 2020 - 12Z Thu May 28 2020 ...Overview and Weather/Hazard Highlights... While systems in the beginning of the medium range period appear more progressive than previous days, models suggest this may be short lived as another blocky pattern should set up along/south of 40N early next week. The extended range synoptic pattern features initial amplified troughing pushing through the Rockies, with northern stream energy proceeding north and east into central Canada, while a southern stream possibly cut off low settles over the central and southern Plains early next week. The latest guidance continues to indicate a threat for at least locally heavy rainfall and thunderstorms across portions of the southern and central Plains Sunday into Memorial Day as return moisture ahead of the broad Central U.S. troughing streams northward out of the Gulf of Mexico. By Tuesday, the better moisture should drop southward towards the western Gulf Coast as the surface front weakens over Texas. Model uncertainty remains high, and so the details of this setup are unresolved at this time frame, but a couple of heavy rain areas do continue to be highlighted on our day 3-7 Hazards Outlook. Partially responsible for the slow to move trough in the Central U.S., strong ridging should build throughout the period across both the Southwest and Eastern states. Temperatures along the Eastern Seaboard should be near normal through the period, with the better chance for above normal temperatures shifting from the Midwest into the interior Northeast. Out West, temperatures should rise to above or much above normal by Tuesday/Wednesday with anomalies +10 to +15 possible. Daytime highs near or in the low 100s is possible over much of the inland deserts, approaching 110 in the favored warmer locations. Some areas could approach record highs over the Great Basin. ...Guidance/Predictability Assessment... Model agreement was pretty good to start the period, with uncertainties looming after Monday on the evolution of a potential cut off low in the central/southern Plains. By days 6 and 7, the bulk of the guidance (and ensemble means) suggests the energy/low should begin to lift slowly northward as it gets absorbed by energy in the northern stream. The 18z GFS/May 20 seems out of phase keeping this energy completely separate from the northern stream and even dropping it southward. The 12z/May 20 ECMWF is the model most in line with the ensemble means at this point, while the 12z/May 20 CMC is ok placement wise, but much stronger. Regardless, model run to run consistency remains poor with any one model, especially in the details, and so a trend towards increased ensemble mean weighting is preferred for days 6-7, with smaller contributions from the deterministic models (mostly ECMWF) for some definition. This approach also fits well with previous WPC continuity. Santorelli Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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