Extended Forecast Discussion NWS Weather Prediction Center College Park MD 147 PM EDT Wed Jul 28 2021 Valid 12Z Sat Jul 31 2021 - 12Z Wed Aug 4 2021 ...Widespread Monsoonal Moisture and Rainfall Pattern over the West... ...Heavy Convection/Rainfall Threat for the South/Southeast/Southern Mid-Atlantic... 18Z Update: The western U.S. ridge and the downstream trough over the central/eastern U.S. is expected to remain anchored in place through the medium range period, with the Atlantic upper ridge blocking eastward progression. There is some hint in the guidance that an upper low could develop over portions of the Ohio Valley by the middle of next week, mainly with the operational GFS and ECMWF, as some flow separation in the trough could develop. This could have some ramifications on the extent of the potential heavy rainfall across the Southeast and the southern Appalachians. One of the main changes with the QPF is for higher totals across portions of the Carolinas, owing to the block pattern developing and a quasi-stationary front over this region. Otherwise, the guidance is in above average agreement on most of the synoptic scale aspects, with the greatest differences evident across the northeast Pacific and the Hudson Bay region by the end of the forecast period. The previous discussion is appended below for reference. /Hamrick ...Pattern Overview with Weather Highlights/Hazards... A closed upper high slid over the south-central U.S. will gradually weaken this weekend as ridging builds over the Rockies, Intermountain West and western Canada into next week. Ridge amplitude will support an unseasonably deep and persistent eastern Canada/U.S. upper trough. Impulses and moisture lifting around the ridge and on the eastern periphery of eastern Pacific upper troughing will present widespread monsoonal rainfall with local runoff issues. Best chances are from the Southwest/CA to the central Great Basin/Rockies where there will also be increasing coverage of daytime highs 5-15F below normal. On the other hand, the Pacific Northwest may see up to 10-15F above normal highs Saturday, before a gradual moderating trend as heights aloft decline. Record warm minimum temperature records will be possible west of the Rockies over the Northwest Saturday and elsewhere with monsoonal clouds/moisture. Meanwhile, shortwaves rounding the upper ridge will periodically dig downstream to reinforce an eastern U.S. upper trough. Favorable support aloft may help to produce strong convection and locally heavy downpours as moisture/instability pool near wavy fronts slowing on the southern trough periphery. Rainfall chances shift through the south-central Plains, mid/lower MS and TN Valleys and southern Mid-Atlantic/Southeast. The pattern favors below normal temperatures from the Midwest/Northeast to the Mid-Atlantic where spotty/modest record lows are possible. The Deep South will be steamy with record warm lows possible along and south of a main and wavy front. ...Guidance/Predictability Evaluation... The medium range pattern offers above average predictability. The WPC medium range product suite was derived from best clustered guidance of the 18 UTC GFS/GEFS and 12 UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of Models. Transitioned WPC blend weights increasingly from the models to the ensemble means over time consistent with gradually growing forecast spread. This maintains good WPC product continuity that seems in line with newer 00 UTC guidance. 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