Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Sun Aug 30 2020 Valid 12Z Wed Sep 02 2020 - 12Z Sun Sep 06 2020 ...High heat rebuilds over the West next week... ...Weather/Hazard Highlights... Some heavy rain potential should linger through midweek from portions of northern Texas through the south-central Plains and Mid-MS River Valley as a southern stream upper support combines with a wavy/stalled front. The ensemble probabilities for excessive rainfall are still present and some runoff issues are possible, particularly in the wet wake of Laura and an additional short range rainfall threat for some of these areas. The weather pattern for much of next week will feature a renewed upper level ridge over the western U.S. and troughing over the central/eastern U.S.. Above normal heat will rebuild over much of the West and in particular, the desert Southwest will have daily highs upwards to 110-120F. Conversely, the passage of a strong cold front mid-later next week will usher in cool/drier Canadian high pressure and an early taste of autumn weather for much of the central to eastern states. This is supported by a series of amplified northern stream upper troughs that dig from Canada. By next weekend, there remains an unusually strong guidance signal for additional flow amplification across the Pacific/Alaska that works downstream into Canada and the lower 48 states. This seems related to a downstream ripple effect from the concurrent northward forecast track of Typhoon Maysak from the west tropical Pacific to inland northeast Asia. This could result in amplified upper trough digging into the north-central U.S.. This would result in colder temperature profiles than shown in the NBM. WPC guidance shows potent post-frontal high pressure into the region, but probably underestimates cooling that in particular may allow a risk of northern Rockies snows given flow support in this scenario. ...Guidance/Predictability Assessment... Guidance is overall well clustered through medium range time scales, bolstering forecast confidence to above normal levels days 4-7. The WPC medium range product suite was derived from a 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian model blend Wed-Fri. This composite is well supported by GEFS/NAEFS/ECMWF ensemble means, the National Blend of Models and WPC continuity. Prefer a solution closer to the 00/12 UTC ECMWF and 00 UTC Canadian and ECMWF ensemble mean that offer a more amplified pattern next weekend than recent GEFS/GEFS/NAEFS runs or the latest 12 UTC Canadian. Schichtel 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