Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM EDT Wed Aug 28 2024 Valid 12Z Sat Aug 31 2024 - 12Z Wed Sep 04 2024 ...Overview... Models and ensembles show moderately progressive northern stream flow, with a portion of an initial western U.S. ridge eventually reaching the northern Plains while reinforcing trough energy downstream crosses the Great Lakes/Northeast, with its departure next week yielding weaker troughing by the middle of next week. Remaining western U.S. ridging will likely settle into the southern half of the region by that time. An eastern Pacific upper low should eject across the Northwest early next week, but with increasing uncertainty over details of incoming Pacific flow that could interact with it. Meanwhile, guidance has been showing a lot of difficulty in resolving lower-predictability shortwave features from the southern Rockies eastward, leading to significant differences in forecast rainfall across the southern half of the country. The forecast pattern evolution will support some well above normal temperatures crossing the Northwest while a decent portion of the central/eastern U.S. will see multiple days with below normal highs. ...Guidance/Predictability Assessment... Overall the dynamical and machine learning (ML) guidance has maintained good agreement and continuity for most aspects of the forecast into the weekend. But there is notable spread and variability for some aspects of the forecast--leading to significant differences in rainfall coverage/amounts across the southern half of the lower 48. In particular, the dynamical and ML models disagree among each other for lower-predictability shortwave details between the southern Rockies and east-central U.S. leading to significant QPF differences. For example recent ECMWF runs have been favoring a southern High Plains upper weakness that eventually drifts eastward, while GFS runs remain suppressed/farther south than other guidance, but they seem to be the outlier, as non-NCEP guidance including the EC-based AI/ML models show notable convection into much of Texas. ML models differ as well but overall seem to show a general theme of an upper weakness reaching somewhere over the Plains/Mississippi Valley by next Wednesday. Eventually the southern stream differences lead to Southeast QPF differences, and also complicating matters is the potential for energies along the west-central Gulf Coast. Recent model guidance has shown reasonable agreement with Northeast troughing. But in the eastern Pacific to Northwest, guidance spread increases by Monday-Wednesday, at first to the north of and then interacting with the upper low feature moving into northern California. The newer 12Z models have shown a marked shift to showing less northern troughing than the previous cycles. This keeps the upper low feature moving northeast separate from the northern flow that has flipped to ridging for a notable pattern change for the northwestern U.S./southwestern Canada. This will have to be monitored to determine if this trend remains in future cycles. The WPC forecast used a multi-model deterministic blend for the early to mid-period, with the blending process taking care of smaller scale differences. As spread increased later in the period, the proportion of ensemble means reached half by Day 7. ...Weather/Hazards Highlights... On Day 4/Saturday, continue to expect locally heavy rain to affect parts of the Northeast into the Mid-Atlantic and back into the Appalachians plus perhaps the Tennessee/Ohio Valleys as a leading front progresses into this region, with moisture and instability supporting maintenance of a Marginal Risk area in the Excessive Rainfall Outlook. A trend toward slightly slower frontal timing led to adjusting the ERO to cover northern New England as well. By Day 5/Sunday, a Marginal remains in place over parts of the southern Mid-Atlantic/Appalachians as moisture/instability may be sufficient to produce locally heavy rainfall along the wavy front reaching the area. The front gradually tracking south into the workweek should lead to storms across the Southeast. The southern Rockies/southern High Plains should also continue to see thunderstorms into Saturday but with declining confidence in specifics due to differences in potential shortwave energy. As mentioned above, favored the more agreeable non-NCEP guidance instead of the southern/suppressed GFS. Thus have Marginal Risks outlooked for Days 4 and 5 for western parts of Texas into southeast New Mexico, where above normal moisture is in place and there may be some energy aloft. For Day 4/Saturday, western parts of the previously issued Marginal were trimmed out from the Southwest-- though convection is likely to exist across the Mogollon Rim in particular, there is likely less than a 5 percent chance (the Marginal threshold) of flash flooding. Meanwhile, guidance still shows the potential for convection along parts of the Louisiana/Texas coast. The Day 4 ERO has maintained a Marginal Risk area based on this persistence and expected rainfall from prior days, and a Marginal is introduced for Day 5/Sunday as well. Confidence in specifics like how much rain will fall offshore/onshore is still not very high. After Sunday, uncertain details of southern tier shortwave energy continue to make it difficult to resolve specifics of the rainfall forecast in terms of northward extent and magnitude/location of highest totals. The fronts settling into the southern tier should provide some focus at times though, while trailing high pressure promotes a drying trend farther north. The Northwest may see scattered rainfall early next week as an eastern Pacific upper low ejects inland. Upper ridging tracking across the Northwest into the northern High Plains will produce well above normal temperatures over that part of the country during the period, with some locations seeing greater than plus 10F anomalies on one or more days. Meanwhile there will be some lingering heat over parts of the southern half or so of the East into the weekend but with highs only up to 5-10F above normal. Otherwise, a decent portion of the central/eastern U.S. will tend to see near to below normal highs, with coolest anomalies (some pockets of highs at least 10F below normal) over the Plains Sunday-Tuesday and into the East Monday-Wednesday. Tate/Rausch 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw