Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Thu Aug 29 2024 Valid 12Z Sun Sep 01 2024 - 12Z Thu Sep 05 2024 ...Overview... Initially, an upper trough will be crossing the Great Lakes/Northeast and ridging will be in place across the central U.S. and much of the Southwest. The upper trough will gradually move east and heights will rise in it's wake as the central U.S. ridge moves into the Midwest. Meanwhile, a smaller upper trough/low will approach the West Coast and move into the Northwest. This feature should begin to weaken as is moves across the Northern Rockies into the High Plains, and ridging is expected to build back into the Northwest mid-next week. This upper-level pattern is expected to push a couple of frontal boundaries into the south-central and southeastern U.S., which will likely result in a period of rainy/stormy weather. A persistent surface trough and embedded waves near the western Gulf Coast should provide a focus for some of the heavier rainfall. Precipitation may also accompany the upper low in the Northwest and northern Plains, but this feature will be relatively weak, leading to lighter rainfall totals. ...Guidance/Predictability Assessment... Model guidance has come into fairly good agreement on the pattern through the first half of the period. The latest runs of the ECMWF and GFS both show the upper low moving into the Northwest early then gradually progressing towards the northern Plains through mid-next week. There is still some uncertainty with how the energy from the upper low will interact with northern stream energy, which will have impacts on surface fronts and sensible weather as the system moves past the Rockies. There is also some uncertainty surrounding the evolution of a southern stream wave that is forecast to move into the southern Plains, and there is a large amount of uncertainty concerning the progression of this feature. By late in the period this leads to a range of solutions from a progressive southern tier shortwave that brings a lot of moisture through the southeastern quadrant of the lower 48 and perhaps extending even a little northward to greater northern stream dominance over the East and a much more suppressed rainfall pattern. WPCs afternoon forecast blend consisted of the 06Z/00Z operational model composite through the first half of the period, and ensemble means were introduced to the blend for the second half of the period, making up 40% of the blend for days 6 and 7. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks covering the Sunday through Monday night period focus on three primary regions of interest, all with Marginal Risk areas on both days. One area stretches from the southern Mid-Atlantic into the Appalachians where a couple of fronts will be sagging southward over the course of the period, with associated instability and moderately anomalous moisture. Meanwhile rainfall should continue along parts of the Louisiana/Texas coast, with prior days of rainfall providing wet ground conditions heading into the Days 4-5 period. There is still uncertainty in the coverage of showers and thunderstorms over the southern High Plains and vicinity, but there is still enough of a signal to favor maintenance of the Marginal Risk. Latest model trends have been showing increasing QPF in this area on Day 5/Monday into Monday night, which may be the better focus for heavy rain/flash flooding. Precipitation coverage and totals across the southern tier after early Tuesday remain uncertain due to significant differences in upper flow details. Some areas of locally heavy rainfall appear likely given the fronts settling over the South helping to serve as a focus, but with very low confidence in resolving the details of coverage and totals. Expect the Northwest to see scattered rainfall early next week as the eastern Pacific upper low ejects inland. Some scattered rainfall could reach the northern Plains by midweek or so. Upper ridging will support an area of well above normal temperatures drifting from the Northwest into the northern High PLains Sunday through Tuesday, with some locations seeing highs 10-15 degrees above normal. This warm air should quickly moderate farther eastward. A return of upper ridging over the West Coast states by Wednesday/Thursday should promote above normal temperatures with anomalies for highs again reaching 10-15 degrees above average at some locations. Below normal highs over the central U.S. on Sunday will expand toward the East Coast early next week, suppressing what heat lingers over the Southeast to start the week. Most of the central/eastern U.S. should trend closer to normal by next Thursday but the southern Mid-Atlantic/Southeast may remain below normal for highs. Dolan/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