Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Aug 13 2022 Valid 12Z Tue Aug 16 2022 - 12Z Sat Aug 20 2022 ...Threat of a nor'easter increasing across New England Wednesday-Thursday as a tropical disturbance likely moves west into northern Mexico... ...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 earnest over interior California and broadly across the Northwest next week... ...Overview... The threat of a nor'easter appears to be increasing across New England next Wednesday-Thursday as models are in increasing agreement for cyclogenesis to occur off the New England coast under a slow-moving upper trough/low. Meanwhile, 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... Model solutions have continued to indicate the greatest uncertainty/variability to be focused across the northeastern portion of the country where a broad and amplified upper trough/low is forecast to linger as the medium-range period begins next Tuesday. In particular, recent model runs have indicated an increasing threat of a low pressure system to form and intensify rapidly ahead of the upper trough/low and impact portions of New England midweek. The GFS has been trending toward a more amplified solution with each new run since yesterday, taking the track of the cyclone closer and closer to, and eventually over New England with this morning's run. The ECMWF has been the weakest and farthest east guidance but the most recent run (12Z) made an abrupt switch to indicate a rather robust nor'easter for mid-August to be centered near southeastern New England next Thursday. This scenario is supported by the 12Z UKmet, which made a similar abrupt switch toward a much amplified solution as well. The Canadian model, which has been in between the GFS and ECMWF solutions, continues to maintain a northeastward track just off the New England coast. Elsewhere across the mid-South, there continues to be west-to-east timing differences regarding the potentially heavy rainfall axis ahead of a frontal wave as it moves across the mid to lower Mississippi Valley by midweek. Thereafter, the GEFS tends to keep the rain axis farther north across the Deep South than the EC/EC mean. Meanwhile, models are generally in good agreement for the heavy rains associated with a tropical disturbance to move westward and largely into northern Mexico by next Tuesday. The WPC medium-range forecasts were composed based on a consensus of 40% from the 06Z GFS/GEFS, 40% from the 00Z EC/EC mean, and 20% from the 00Z CMC/CMC mean, together with the 13Z NBM, transitioning to mostly a consensus of the ensemble means by Day 7. ...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. Kong/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