Extended Forecast Discussion NWS Weather Prediction Center College Park MD 218 PM EDT Wed May 19 2021 Valid 12Z Sat May 22 2021 - 12Z Wed May 26 2021 18Z Update: The deterministic guidance remains in above average agreement on the synoptic scale pattern through early next week, so a general blend of the CMC/UKMET/GFS/ECMWF sufficed as a good starting point. By the end of the forecast period, more appreciable differences emerge over the eastern Pacific, and this leads to a little more forecast uncertainty for the West Coast region, and the GFS becomes stronger with the shortwave crossing the northern Plains and northern Great Lakes region by Wednesday. In terms of the temperature forecast for the East Coast region, highs were slightly lowered from the NBM to account for the possibility of a back door cold front, since the upper ridge axis will likely be centered to the west over the Ohio Valley and Deep South. The previous discussion is appended below. /Hamrick ...Guidance Evaluation and Preferences... The WPC medium-range product suite was derived days 3-5 (Sat-Mon) from a composite of well clustered guidance of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models in a pattern with above normal predictability. By days 6-7, the GEFS/ECENS means were added to the blend consistent with slowly increasing forecast spread. WPC continuity is well maintained and is in line with latest 00 UTC guidance. ...Weather Highlights/Hazards... Lingering south-central Plains troughing and an increasingly localized threat for slow moving convection/heavy downpours will gradually weaken this weekend into early next week, blocked by an amplified downstream ridge. Ridge transition supports much above normal temperatures from the Midwest to the Mid-Atlantic and Southeast where some record high minimum and maximum values will be possible with temperature anomalies 10-20F above normal. An amplified trough over the West this weekend favors some moderate precipitation for the north-central Intermountain West/Rockies to include some high elevation snows enhanced by upslope. This occurs as post-frontal conditions cool to 10-20F below normal across the Intermountain West and Northern Rockies. The focus for precipitation episodes is expected to increasingly shift toward the north-central states into next week, with more scattered strong thunderstorms down through the Plains. This occurs as ample ejecting western U.S. trough energies interact with wavy boundaries. Energies will spill some activity over the downstream ridge to the Midwest/Great Lakes and drive backdoor cold fronts from the Northeast to the Mid-Atlantic. Upstream, less certain upper troughing with modest amplitude/precipitation may dig renewed over the Northwest/West early-mid next week. Schichtel/Hamrick 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