Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Aug 11 2022 Valid 12Z Sun Aug 14 2022 - 12Z Thu Aug 18 2022 ...Monsoonal moisture/rainfall threat continues for the Southwest and Intermountain West/Rockies... ...Excessive heat threat across the Plains, with warming temperatures in the Northwest next week... ...Overview... The medium range period will begin Sunday with a continued trough-ridge-trough pattern across the lower 48. As next week progresses, there should be a trend toward the upper high/ridge expanding into the West, leading to well above average temperatures in the Northwest. The ridge aloft with embedded shortwave energy will also continue to support monsoonal moisture streaming into the Southwest, Intermountain West, and Rockies through the week, with rain eventually spreading into portions of the Plains and Mississippi Valley. Meanwhile the trough across the East may deepen and close off an upper low around midweek, which could cause multiple days of rainfall in the Northeast, but dependent on the position of the upper low and the associated surface low/fronts, which remain uncertain at this point. ...Guidance Evaluation/Predictability Assessment... Model guidance remains agreeable for an upper high over the central/southern High Plains to slowly retrograde and expand ridging influence into the western U.S., overtaking the early week weak troughing there. Some small-scale shortwaves embedded within the broad ridge show some variability as they move through the interior West, but nothing too significant on the large scale. Farther east, models have persistently indicated troughing across the eastern third of the U.S. through much of next week. But the details remain inconsistent from model cycle to model cycle, particularly by Tuesday-Thursday. In general, guidance indicates that an upper low could close off within the trough around midweek but with differing placement. The 12/18Z model cycle that was available for the WPC forecast showed an upper low center farther south than previous runs, as well as a deeper surface low. This allowed more more QPF farther inland in the Northeast compared to the previous cycle as deep moisture wraps around the low. WPC's forecast trended upward from continuity but not as high as the NBM given the run-to-run uncertainty. The newer 00Z models have generally backed off a bit, showing a weaker surface low and a farther north upper low, leading to less rainfall overall, though still indicating some heavy amounts in New England (less in New York and the Mid-Atlantic). The WPC forecast used a multi-model deterministic blend for the early part of the medium range forecast, incorporating some GEFS and EC ensemble means into the blend days 5-7, so as not to go all in for the southern trend of the upper low in the 12/18Z cycle. ...Weather/Hazards Highlights... Parts of the West have seen persistent monsoonal moisture over the past couple of months, and next week will continue that pattern as anomalously high moisture streams into the Southwest, Great Basin, and Rockies in conjunction with small-scale shortwave energy to produce rain and thunderstorms, particularly in the afternoon and evening hours. Slight Risks of excessive rainfall are in place for portions of the Desert Southwest on Sunday and the Central Rockies on Monday, where current forecasts show the best chances for heavy rainfall. Heavy rain amounts look to continue through midweek in the Central Rockies. There are increasing chances for rainfall to spread into the central and southern parts of the Plains and Mississippi Valley as the week progresses. Elsewhere, the Gulf Coast and into Florida could see scattered storms with a moist airmass and a couple of frontal boundaries. Then as a surface low is currently forecast to track across the Ohio Valley through the Mid-Atlantic toward the Northeast, rain is forecast for those areas, but the track of the surface low (and upper low) have varied quite a bit in model guidance so there is low confidence in rainfall placement and amounts. Very warm to hot temperatures around 10-15F above average, though mainly not to record-breaking levels, are forecast across the Central Plains Sunday into Monday, with a gradually moderating trend Tuesday onward yielding below normal highs. As the upper ridge expands into the Northwest, this will warm up temperatures there and cause highs 10-20F above normal for Tuesday-Thursday. Meanwhile, below normal highs are expected in the east-central U.S. behind the cold front, with the most persistent cooler than average weather in the Ohio and Tennessee Valleys toward the Mid-Atlantic. Given the rainfall and cloudiness over the Southwest and Great Basin into the Rockies, highs are likely to be several degrees below normal there through much of next week. Tate 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml