Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Fri Aug 12 2022 Valid 12Z Mon Aug 15 2022 - 12Z Fri Aug 19 2022 ...Monsoonal moisture/rainfall threat continues for the Southwest and Intermountain West/Rockies... ...Excessive heat threat across the South-Central U.S. and Northwest next week... ...Overview... An amplified upper flow pattern for next week will feature troughs over the northeast Pacific and east-central U.S. sandwiched around a ridge over the West. The ridge will build northward through the Northwest and western Canada, leading to a threat of excessive heat, as the northeast Pacific trough shifts slowly eastward toward the coastline. The ridge and embedded shortwave inpulses will also continue to support widespread monsoonal moisture streaming underneath into a wet and unsettled Southwest, Intermountain West, and Rockies through next week. The downstream digging of shortwaves/height falls to the lee of the ridge will periodically reinforce upper diffluent flow over a hot south-central U.S. before feeding into a slow to dislodge east-northeast U.S. closed low. This low along with modest coastal low development should support multiple days of rainfall in the Northeast and allow a break from summer temperatures down through roughly the eastern half of the nation as a cooling front settles far down through the east-central U.S. to the South to provide a multi-day focus for convection with locally heavy rains. ...Guidance Evaluation/Predictability Assessment... The mid-larger scale pattern evolution for next week seems well depicted by guidance through medium range time scales. Model forecasts of flow embedded systems and affects overall seem decently clustered Monday/Tuesday to provide max detail consistent with an amplified pattern with above normal predictability. A composite of the GFS/ECMWF/UKMET and Canadian seems to provide a good forecast basis along with the 01 UTC National Blend of Models (NBM). Growing small-mid scale system variance mid-later next week led to transition to a blend of the GEFS/ECMWF ensemble means along with most compatible model guidance of the 12 UTC GFS/ECMWF along with the NBM. The 12 UTC GEFS was more in line with this solution than the less amplified patterns of the 18 UTC GFS and 12 UTC Canadian at these longer time frames. The latest 00 UTC GFS/Canadian have trended more amplified and in line with the aforementioned prefered composite, bolstering forecast confidence. NBM QPF forecasts seems reasonable and consistent with the pattern (if not details) so no significant forecaster modifications were applied for the WPC forecast package. ...Weather/Hazards Highlights... Portions 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. Collaborated WPC experimental Slight Risks of excessive rainfall are in place for the Central Rockies for early next week, where there is a viable guidance signal for heavy convective rainfall rooted by digging shortwave energy/upper diffluence and terrain lift. The heavy rain focus looks to shift slowly southward mid-later next week into the Southern Rockies. There are increasing chances for organized clusters of convection with heavy downpours to spread through parts of the Central/Southern Plains and mid-lower Mississippi Valley as the week progresses. Elsewhere, the Gulf Coast and into Florida could see scattered storms with a moist airmass and frontal boundaries. Then as a surface low is currently forecast to track across the Ohio Valley through the Mid-Atlantic to the Northeast, orgainzed rains are forecast for those areas. There's increasing confidence in a multi-day rain event occurring for much of the coastal Northeast, from New Jersey up to Maine through mid-week under closed upper low influence and with near shore track of a coastal frontal low. 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. 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, 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