Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Thu May 20 2021 Valid 12Z Sun May 23 2021 - 12Z Thu May 27 2021 18Z Update: The 12Z deterministic model guidance suite is in above average agreement through early next week. By Wednesday, the GFS becomes more amplified with the northern stream trough crossing the northern Plains and northern Great Lakes, whereas the CMC and ECMWF have a more broad solution. More noteworthy differences become evident over the eastern Pacific and West Coast region by the end of the forecast period, and the CMC is out of phase with the GEFS/EC means with a trough approaching the West Coast by Thursday. The WPC medium range forecast was primarily derived from a UKMET/GFS/ECMWF/EC blend through Tuesday, and then slightly more of the EC/GEFS means while still maintaining some of the operational GFS/ECMWF. The previous discussion is appended below. /Hamrick ...Guidance Evaluation and Preferences... The WPC medium-range product suite was derived days 3-5 (Sun-Tue) 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 southern stream troughing from short range time scales and localized threat for slow moving convection with heavy downpours will gradually weaken this weekend into early next week. Meanwhile, an amplified trough over the West and dynamic ejection favors some moderate precipitation for the north-central Intermountain West/Rockies/High Plains including elevation snows as enhanced by upslope, especially for the Northern Rockies. This occurs as regional post-frontal conditions cool to 10-20F below normal. Downstream, the focus for precipitation episodes is expected to increasingly shift toward the north-central states next week, with scattered strong thunderstorms down through the Plains. This occurs as ample ejecting western U.S. trough energies interact with wavy boundaries. Energies will spill activity across the Midwest/Great Lakes over the top of an amplified east-central U.S. upper ridge and drive backdoor cold fronts from the Northeast to the Mid-Atlantic. Upper ridge transition and backdoor fronts suggests a transition of lead much above normal temperatures from the Midwest and the Mid-Atlantic to the South/Southeast where some record high minimum and maximum values will be possible with temperature anomalies 10-20F above normal. Upstream, less certain upper troughing with mainly modest amplitude/precipitation may dig renewed over the Northwest/West early-mid next week. Guidance has not been as stellar with run to run variance for these features. 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