Extended Forecast Discussion NWS Weather Prediction Center College Park MD 440 PM EDT Thu Jul 29 2021 Valid 12Z Sun Aug 01 2021 - 12Z Thu Aug 05 2021 ...Widespread Monsoonal Moisture and Rainfall Pattern over the West... ...Heavy Convection/Rainfall Threat for The South and The Southeast/Mid-Atlantic... 19Z Update: The 12Z model suite is in very good synoptic scale agreement through at least Tuesday with a trough anchored in place across the east-central U.S. and an upper ridge over the Rockies and Intermountain West. More differences become evident across southern Canada by Wednesday and beyond, with the GFS indicating more of a split flow pattern with the eastern trough, and the CMC showing a more robust upper ridge extending into south-central Canada where the GFS/ECMWF have northern stream shortwave energy passing through that region. Heavy rainfall still remains to be an issue for portions of the southeastern U.S. next week as a stalled frontal boundary and favorable upper level support remain in place there, with the ridge over the Atlantic halting eastward progress of the trough over the eastern U.S. Otherwise, the forecast remains close to the previous version, and the overnight forecast 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 periodic reinforcement of 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, record warm minimum temperature records will be possible west of the Rockies over a warmed Northwest and elsewhere with monsoonal clouds and moisture. Meanwhile, uncertain shortwaves rounding the upper ridge will dig downstream into an eastern U.S. mean upper trough. Favorable support aloft may help to produce strong convection and locally heavy downpours/runoff issues as moisture/instability pool near wavy fronts slowing on the southern trough periphery, especially with training/repeat cells. Enhanced waves will lift on the eastern periphery of the upper trough over the Southeast/Mid-Atlantic to favor additional heavy rainfall. The pattern favors modestly below normal post-frontal temperatures farther northward over the central and eastern U.S. while The South/Southeast will in contrast be quite steamy with widespread record warm low temperatures possible along/south of the main front. ...Guidance/Predictability Evaluation... The medium range pattern offers above average predictability into early next week before introduction of closer to normal forecast spread and uncertainty. The WPC medium range product suite was derived from a blend of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian Days 3/4 (Sunday/Monday) along with the 01 UTC National Blend of Models. Transitioned WPC blend weights Tuesday-next Thursday from the models to the ensemble means. Placed highest weighting for this latter period on the 12 UTC ECMWF and ECMWF ensemble mean that maintain pattern amplitude on the higher end of the full envelope of solutions. This seems reasonable considering the generally blocky nature of the flow and WPC product continuity. The 00 UTC GFS/UKMET/Canadian have now trended in this direction. 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 Hazards: - Heavy rain across portions of the Northern Rockies, northern Great Basin, Central Rockies, Central and Southern High Plains, Southern Plains, Mid-South, and the interior Northeast, Sun, Aug 1. - Heavy rain across portions of the Southeast and southern Mid-Atlantic, Sun-Thu, Aug 1-Aug 5. - Heavy rain across portions of the the Southern Appalachians, Southeast, Tennessee Valley, Lower Mississippi Valley, and the Southern Plains, Mon-Tue, Aug 2-Aug 3. - Heavy rain across portions of the Southeast, Mon-Thu, Aug 2-Aug 5. - Excessive heat across portions of the Southern Plains and Lower Mississippi Valley, Sun, Aug 1.