Extended Forecast Discussion NWS Weather Prediction Center College Park MD 341 PM EDT Wed Sep 06 2023 Valid 12Z Sat Sep 09 2023 - 12Z Wed Sep 13 2023 ...Hazardous Heat from the south-central U.S to the Southwest... 19Z Update: The 12Z model guidance suite is in good overall agreement on the synoptic scale through the upcoming weekend across the U.S., and differences first become apparent across the Pacific Northwest early next week with the timing of the next storm system. There are also some minor differences with the strength and longitudinal placement of the trough over the Great Lakes and Ohio Valley region going into the middle of the week. Overall, a multi-deterministic blend works well as a starting point in the forecast process through early Monday, followed by up to 30% of the ensemble means by next Wednesday. /Hamrick ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a broad composite blend of reasonably well clustered mid-larger scale guidance of the 18 UTC GFS/GEFS mean and the 12 UTC ECMWF/UKMET/ECMWF ensemble mean along with the 01 UTC National Blend of Models in an overall pattern with seemingly above normal predictability. The blend tends to mitigate much of the less predictable smaller scale variances consistent with uncertainty. WPC product continuity is well maintained in this manner and latest guidance from the 00 UTC model cycle generally remains consistent. ...Weather/Hazards Highlights... Amplified upper troughing will settle over the East through the weekend into early next week as anchored by an upper low over Hudson Bay and as a series of weaker impulses ride over the top then to the lee of a transitioning upper ridge as it strengthens over the South and expands westward into the Southwest. With this ridge in place, hot late summer temperatures will linger across the south-central U.S. and the Southwest through this weekend. High dewpoints and humidity especially near the western Gulf Coast will lead to dangerous heat indices as high as 115 degrees. Above normal temperatures by about 10-15 degrees should persist into the weekend broadly across the south-central U.S. and into the Southwest. Many daily record high minimum and maximum temperatures are likely to be set, with perhaps even a few monthly records, in what has proven to be a sweltering summer. Upper-level energy trending southward should help to reduce the strength of the ridge and begin to bring temperatures to still hot but more reasonable levels next week. Meanwhile in this pattern, the northern Mid-Atlantic and the Northeast offer a main area for precipitation to focus, given higher precipitable water values than average and instability pooling ahead of the upper trough and wavy surface frontal system. While rain amounts should be mainly moderate, the ingredients will be in place for localized heavier rain totals especially because storms may be slow-moving and relatively parallel to the front, allowing for potential training of storms. The WPC Day 4/5 Excessive Rainfall Outlooks (EROs) denote Marginal risk areas for this weekend from the northern Mid-Atlantic into the Northeast as convection looks to have a relatively better focus near/ahead of the front and favorable right entrance region upper jet support. The Marginal Risks encompass lower FFGs mainly from Pennsylvania into New York and includes favored terrain of the Poconos and Catskills to the Adirondacks. Rain chances should gradually decrease into next week along the Eastern Seaboard as the fronts push east off the coast. For the 19Z update, the Day 4 Marginal Risk area is planned to be extended to the southwest to basically cover the spine of the Appalachians from western NC to Vermont, where a better deterministic model signal exists in the 12Z guidance for patchy areas of heavy QPF maxima. For Day 5, the existing Marginal Risk is planned to cover most of New England except for the coastal areas. Upstream, a series of shortwaves look to work from the Pacific through the West/Northwest and inland across the Intermountain West, albeit in a pattern with limited precipitation this weekend into early next week. Convection may start to become more widespread and better organized in the northern Rockies/Plains by Saturday before working down more earnestly through the Plains Sunday, where some heavy rain totals are expected locally. A guidance signal is growing for this convective potential over the central Plains for the WPC Day 5/Sunday ERO where a "Marginal" risk area is shown. Forecast uncertainty is greater, but it seems the main rainfall focus in this pattern may shift more to the southern Plains and possibly the Upper Midwest early next week with system emergence and flow translation. Schichtel/Hamrick 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 Hazards: - Heavy rain across portions of the Central Plains and the Southern Plains, Sun, Sep 10. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Central Appalachians, Sat, Sep 9. - Flooding occurring or imminent across portions of the Southeast and the Mid-Atlantic. - Flooding likely across portions of the Southeast. - Hazardous heat across portions of the Southern Rockies, the Southern Plains, the Lower Mississippi Valley, California, and the Southwest, Sat-Sun, Sep 9-Sep 10. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw