Extended Forecast Discussion NWS Weather Prediction Center College Park MD 207 AM EDT Sat Aug 13 2022 Valid 12Z Tue Aug 16 2022 - 12Z Sat Aug 20 2022 ...Monsoonal moisture/rainfall threat continues for the Southwest and south-central Great Basin/Rockies next week... ...Heavy rain/Flash Flooding possible across the Mid-Lower Mississippi Valley Tuesday-Thursday... ...Excessive heat threat across the south central Plains to refocus in ernest over interior California and broadly across the Northwest next week... ...The National Hurricane Center is monitorring a tropical disturbance over the northwest Gulf of Mexico... ...Overview... The prospect for an amplified upper flow pattern over the nation remains on tap for next week. This pattern 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 impulses will also continue to support widespread monsoonal moisture streaming underneath into a wet and unsettled Southwest and south-central Great Basin/Rockies next week. The downstream digging of shortwaves/height falls to the lee of the ridge will periodically reinforce upper diffluent flow over the 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. The National Hurricane Center (NHC) is also now monitorring a tropical disturbance over the northwest Gulf of Mexico for any signs of development. A main threat by medium range time scales may be for slow tracking and deep tropical moisture fueling heavy rainfall/runoff issues up the Rio Grande Valley and across northern Mexico along with any subsequent moisture influence into the monsoonal fetch into the Southwest, all contingent on uncertain development. ...Guidance Evaluation/Predictability Assessment... While the mid-larger scale pattern evolution for next week seems well depicted by guidance through medium range time scales, flow embedded systems offer increased forecast spread. This combined with uncertainty with the future evolution of a tropical disturbance over the northern Gulf of Mexico lowers overall predictability to no better than average overall. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of the 01 UTC National Blend of Models, the 18 UTC GEFS mean, the 12 UTC ECMWF ensemble mean and closest matching model guidance a la the 12 UTC ECMWF/UKMET for Tuesday/Wednesday. Opted to switch preference to just the NBM and GEFS/ECMWF ensemble means Thursday-next Saturday anid growing detail uncertainties to maximize WPC product continuity. Tagreted forecaster adjustments to 01 UTC NBM QPF guidance were applied to increase rainfall some over NC Tuesday considering available moisture. The NBM looked more significantly too light with convective rainfall over the Upper Midwest/Great Lakes Thursday-next Saturday given dynamic upper support and frontal passage. The 01 UTC NBM along with latest model and ensemble guidance trended less wet with monsoonal flow for the West/Rockies. Hesitance to make such a continuity shift for quite a wet and protracted monsoonal pattern was compensated by blending in some wetter WPC dayshift QPF continuity across the Southwest and south-central Great Basin. ...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. WPC experimental Slight Risks of excessive rainfall are in place for the south-central Rockies for early-mid next week, where there is a viable guidance signal for heavy convective rainfall rooted by digging shortwave energy/upper diffluence and terrain lift. There are increasing chances for organized clusters of convection with heavy downpours to spread through eastern portions of the Central/Southern Plains and especially into the mid-lower Mississippi Valley as next week progresses. There is a WPC slight risk of excessive rainfall is in effect for central-southern Missouri and northern Arkansas Tuesday into Wednesday. Elsewhere, the Gulf Coast and into Florida could see scattered storms with a moist airmass and frontal boundaries. A wavy surface front expected to track across the Mid-Atlantic to the Northeast to focus some organized rains. A multi-day rain event may occur 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 developing coastal low. There remains some uncertainty with the exact track of this moderate low given connection to a main upper low inland. As the upper ridge expands over the West/Northwest next week, expect a heat wave with daytime and overnight temperature 10-20F above normal. These temperatures are likely to produce some local record values, especially for hot overnight conditions. 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 south-central Great Basin/Rockies, highs are likely to remain several degrees below normal. 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