Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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. Then there is increasing 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 southern High Plains upper weakness that eventually drifts eastward, leading to greater rainfall over Texas than a number of other solutions, while the 00Z GFS drops weak northern stream energy into the southern Plains to suppress rainfall more than the mean of guidance. 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. In contrast, the new 00Z ECMWF has trended faster with its evolving weak shortwave so that by midweek its QPF over the eastern half of the country is more suppressed than prior runs. There are also meaningful differences with a potential upper feature near the west-central Gulf Coast, with no coherent themes aside from a somewhat better defined surface reflection along the western Gulf Coast. Elsewhere, latest GFS runs stray a bit to the fast side with the vigorous shortwave crossing the Northeast while guidance spread rapidly increases by next Wednesday for what northern stream flow will look like immediately behind the Pacific upper low that ejects inland. The new 00Z GFS is on the amplified side of the spread with its trough reaching into the Northwest by midweek. The updated forecast started with an operational model composite for the weekend and then split the 18Z GFS with the GEFS mean to account for trough progression over the Northeast. By late in the period the blend reached a half model/half ensemble mean (GEFS/ECens) weight to reflect the most common ideas of the large scale pattern. ...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. Some guidance is raising the potential for some enhanced totals over northern New England but so far with minimal instability. The Southwest/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. The Marginal Risk area over this region is unchanged but with potential for future adjustment depending on guidance trends/clustering. Meanwhile, guidance still shows the potential for convection along parts of the Louisiana/Texas coast. The Day 4 ERO has introduced a Marginal Risk area based on this persistence and expected rainfall from prior days, though confidence in specifics is still not very high. During Day 5 Sunday, the only risk area initially depicted is a Marginal 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. There will be other possibilities for locally heavy rainfall across the southern tier but guidance signals appear too diffuse for depicting any risk areas at this time. 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 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. 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